ADC081C021,ADC081C027
ADC081C021/ADC081C027 I 2 C-Compatible, 8-Bit Analog-to-Digital Converter
with Alert Function
Literature Number: SNAS447B
ADC081C021/ADC081C027
February 5, 2009
I2C-Compatible, 8-Bit Analog-to-Digital Converter with
Alert Function
General Description
The ADC081C021 is a low-power, monolithic, 8-bit,
analog-to-digital converter (ADC) that operates from a +2.7
to 5.5V supply. The converter is based on a successive ap-
proximation register architecture with an internal track-and-
hold circuit that can handle input frequencies up to 11MHz.
The ADC081C021 operates from a single supply which also
serves as the reference. The device features an
I2C-compatible serial interface that operates in all three speed
modes, including high speed mode (3.4MHz).
The ADC's Alert feature provides an interrupt that is activated
when the analog input violates a programmable upper or low-
er limit value. The device features an automatic conversion
mode, which frees up the controller and I2C interface. In this
mode, the ADC continuously monitors the analog input for an
"out-of-range" condition and provides an interrupt if the mea-
sured voltage goes out-of-range.
The ADC081C021 comes in two packages: a small TSOT-6
package with an alert output, and an MSOP-8 package with
an alert output and two address selection inputs. The
ADC081C027 comes in a small TSOT-6 package with an ad-
dress selection input. The ADC081C027 provides three pin-
selectable addresses while the MSOP-8 version of the
ADC081C021 provides nine pin-selectable addresses. Pin-
compatible alternatives to the TSOT-6 options are available
with additional address options.
Normal power consumption using a +3V or +5V supply is
0.26mW or 0.78mW, respectively. The automatic power-
down feature reduces the power consumption to less than
1µW while not converting. Operation over the industrial tem-
perature range of −40°C to +105°C is guaranteed. Their low
power consumption and small packages make this family of
ADCs an excellent choice for use in battery operated equip-
ment.
The ADC081C021 and ADC081C027 are part of a family of
pin-compatible ADCs that also provide 10- and 12-bit resolu-
tion. For 10-bit ADCs see the ADC101C021 and
ADC101C027. For 12-bit ADCs see the ADC121C021 and
ADC121C027.
Features
■I2C-Compatible 2-wire Interface which supports standard
(100kHz), fast (400kHz), and high speed (3.4MHz) modes
■Extended power supply range (+2.7V to +5.5V)
■Up to nine pin-selectable chip addresses (MSOP-8 only)
■Out-of-range Alert Function
■Automatic Power-down mode while not converting
■Very small TSOT-6 and MSOP-8 packages
■±8kV HBM ESD protection (SDA, SCL)
Key Specifications
■Resolution 8 bits; no missing codes
■Conversion Time 1µs (typ)
■INL & DNL ±0.2 LSB (max)
■Throughput Rate 188.9ksps (max)
■Power Consumption (at 22ksps)
—3V Supply 0.26 mW (typ)
—5V Supply 0.78 mW (typ)
Applications
■System Monitoring
■Peak Detection
■Portable Instruments
■Medical Instruments
■Test Equipment
Pin-Compatible Alternatives
All devices are fully pin and function compatible.
Resolution TSOT-6 (Alert only)
and MSOP-8
TSOT-6 (Addr only)
12-bit ADC121C021 ADC121C027
10-bit ADC101C021 ADC101C027
8-bit ADC081C021 ADC081C027
Connection Diagrams
30052101 30052102 30052110
I2C® is a registered trademark of Phillips Corporation.
© 2009 National Semiconductor Corporation 300521 www.national.com
ADC081C021/ADC081C027 I2C-Compatible, 8-Bit Analog-to-Digital Converter with Alert
Ordering Information
Order Code Option Package Top Mark
ADC081C021CIMK Alert pin TSOT-6 X34C
ADC081C021CIMKX Alert pin TSOT-6 Tape-and-Reel X34C
ADC081C027CIMK Address pin TSOT-6 X35C
ADC081C027CIMKX Address pin TSOT-6 Tape-and-Reel X35C
ADC081C021CIMM Alert and Address pins MSOP-8 X36C
ADC081C021CIMMX Alert and Address pins MSOP-8 X36C
ADC081C021EB
Shipped with the ADC081C021
(TSOT-6). Also compatible with
the ADC081C027 option.
Please order samples.
Evaluation Board
Block Diagram
30052103
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ADC081C021/ADC081C027
Pin Descriptions
Symbol Type Equivalent Circuit Description
VASupply Power and unbufferred reference voltage. VA must be free
of noise and decoupled to GND.
GND Ground Ground for all on-chip circuitry.
VIN Analog Input See Figure 4 Analog input. This signal can range from GND to VA.
ALERT Digital Output
Alert output. Can be configured as active high or active
low. This is an open drain data line that must be pulled to
the supply (VA) with an external pull-up resistor.
SCL Digital Input
Serial Clock Input. SCL is used together with SDA to
control the transfer of data in and out of the device. This is
an open drain data line that must be pulled to the supply
(VA) with an external pull-up resistor. This pin's extended
ESD tolerance( 8kV HBM) allows extension of the I2C bus
across multiple boards without extra ESD protection.
SDA Digital
Input/Output
Serial Data bi-directional connection. Data is clocked into
or out of the internal 16-bit register with SCL. This is an
open drain data line that must be pulled to the supply
(VA) with an external pull-up resistor. This pin's extended
ESD tolerance( 8kV HBM) allows extension of the I2C bus
across multiple boards without extra ESD protection.
ADDR0
Digital Input,
three levels
Tri-level Address Selection Input. Sets Bits A0 & A1 of the
7-bit slave address. (see Table 1)
ADDR1 Tri-level Address Selection Input. Sets Bits A2 & A3 of the
7-bit slave address. (see Table 1)
Package Pinouts
VAGND VIN ALERT SCL SDA ADR0 ADR1
ADC081C021
TSOT-6 1 2 3 4 5 6 N/A N/A
ADC081C027
TSOT-6 1 2 3 N/A 5 6 4 N/A
ADC081C021
MSOP-8 57421836
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ADC081C021/ADC081C027
Absolute Maximum Ratings
(Notes 1, 2)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage, VA-0.3V to +6.5V
Voltage on any Analog Input Pin to
GND −0.3V to (VA +0.3V)
Voltage on any Digital Input Pin to
GND −0.3V to 6.5V
Input Current at Any Pin (Note 3) ±15 mA
Package Input Current (Note 3) ±20 mA
Power Dissipation at TA = 25°C See (Note 4)
ESD Susceptibility (Note 5)
VA, GND, VIN, ALERT,
ADR pins:
Human Body Model
Machine Model
Charged Device Model
(CDM)
SDA, SCL pins:
Human Body Model
Machine Model
2500V
250V
1250V
8000V
400V
Junction Temperature +150°C
Storage Temperature −65°C to +150°C
Operating Ratings (Notes 1, 2)
Operating Temperature Range −40°C ≤ TA ≤ +105°C
Supply Voltage, VA+2.7V to 5.5V
Analog Input Voltage, VIN 0V to VA
Digital Input Voltage (Note 7) 0V to 5.5V
Sample Rate up to 188.9 ksps
Package Thermal Resistances
Package θJA
6-Lead TSOT 250°C/W
8-Lead MSOP 200°C/W
Soldering process must comply with National
Semiconductor's Reflow Temperature Profile specifications.
Refer to www.national.com/packaging. (Note 6)
Electrical Characteristics
The following specifications apply for VA = +2.7V to +5.5V, GND = 0V, fSCL up to 3.4MHz, fIN = 1kHz for fSCL up to 400kHz,
fIN = 10kHz for fSCL = 3.4MHz unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25°C
unless otherwise noted.
Symbol Parameter Conditions Typical
(Note 9)
Limits
(Note 9) Units (Limits)
STATIC CONVERTER CHARACTERISTICS
Resolution with No Missing Codes 8Bits
INL Integral Non-Linearity (End Point
Method)
VA = +2.7V to +3.6V ±0.04 ±0.2 LSB (max)
VA = +2.7V to +5.5V. fSCL up to 400kHz
(Note 13) ±0.1 ±0.25 LSB (max)
DNL Differential Non-Linearity
VA = +2.7V to +3.6V +0.04 ±0.2 LSB (max)
VA = +2.7V to +5.5V. fSCL up to 400kHz
(Note 13) ±0.08 ±0.25 LSB (max)
VOFF Offset Error
VA = +2.7V to +3.6V +0.26 ±0.5 LSB (max)
VA = +2.7V to +5.5V. fSCL up to 400kHz
(Note 13) +0.25 ±0.5 LSB (max)
GE Gain Error -0.01 ±0.4 LSB (max)
DYNAMIC CONVERTER CHARACTERISTICS
ENOB Effective Number of Bits 7.98 7.8 Bits (min)
SNR Signal-to-Noise Ratio 49.8 49 dB (min)
THD Total Harmonic Distortion −70.6 −64 dB (max)
SINAD Signal-to-Noise Plus Distortion Ratio 49.8 49 dB (min)
SFDR Spurious-Free Dynamic Range -68.8 65 dB (min)
IMD Intermodulation Distortion, Second
Order Terms (IMD2)
VA = +3.0V,
fa = 1.035 kHz, fb = 1.135 kHz −75.5 dB
Intermodulation Distortion, Third
Order Terms (IMD3)
VA = +3.0V,
fa = 1.035 kHz, fb = 1.135 kHz −71.8 dB
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ADC081C021/ADC081C027
Symbol Parameter Conditions Typical
(Note 9)
Limits
(Note 9) Units (Limits)
FPBW Full Power Bandwidth (−3dB) VA = +3.0V 8 MHz
VA = +5.0V 11 MHz
ANALOG INPUT CHARACTERISTICS
VIN Input Range 0 to VAV
IDCL DC Leakage Current (Note 10) ±1 µA (max)
CINA Input Capacitance Track Mode 30 pF
Hold Mode 3 pF
SERIAL INTERFACE INPUT CHARACTERISTICS (SCL, SDA)
VIH Input High Voltage 0.7 x VAV (min)
VIL Input Low Voltage 0.3 x VAV (max)
IIN Input Current (Note 10) ±1 µA (max)
CIN Input Pin Capacitance 3 pF
VHYST Input Hysteresis 0.1 x VAV (min)
ADDRESS SELECTION INPUT CHARACTERISTICS (ADDR)
VIH Input High Voltage VA - 0.5V V (min)
VIL Input Low Voltage 0.5 V (max)
IIN Input Current (Note 10) ±1 µA (max)
LOGIC OUTPUT CHARACTERISTICS, OPEN-DRAIN (SDA, ALERT)
VOL Output Low Voltage ISINK = 3 mA 0.4 V (max)
ISINK = 6 mA 0.6 V (max)
IOZ
High-Impedence Output
Leakage Current (Note 10) ±1 µA (max)
Output Coding Straight (Natural) Binary
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ADC081C021/ADC081C027
Symbol Parameter Conditions Typical
(Note 9)
Limits
(Note 9) Units (Limits)
POWER REQUIREMENTS
VA
Supply Voltage Minimum 2.7 V (min)
Supply Voltage Maximum 5.5 V (max)
Continuous Operation Mode -- 2-wire interface active.
INSupply Current
fSCL=400kHz VA = 2.7V to 3.6V 0.08 0.14 mA (max)
VA = 4.5V to 5.5V 0.16 0.30 mA (max)
fSCL=3.4MHz VA = 2.7V to 3.6V 0.37 0.55 mA (max)
VA = 4.5V to 5.5V 0.74 0.99 mA (max)
PNPower Consumption
fSCL=400kHz VA = 3.0V 0.26 mW
VA = 5.0V 0.78 mW
fSCL=3.4MHz VA = 3.0V 1.22 mW
VA = 5.0V 3.67 mW
Automatic Conversion Mode -- 2-wire interface stopped and quiet (SCL = SDA = VA). fSAMPLE = TCONVERT * 32
IASupply Current VA = 2.7V to 3.6V 0.41 0.59 mA (max)
VA = 4.5V to 5.5V 0.78 1.2 mA (max)
PAPower Consumption VA = 3.0V 1.35 mW
VA = 5.0V 3.91 mW
Power Down Mode (PD1) -- 2-wire interface stopped and quiet. (SCL = SDA = VA).(Note 10)
IPD1 Supply Current 0.1 0.2 µA (max)
PPD1 Power Consumption 0.5 0.9 µW (max)
Power Down Mode (PD2) -- 2-wire interface active. Master communicating with a different device on the bus.
IPD2 Supply Current
fSCL=400kHz VA = 2.7V to 3.6V 13 45 µA (max)
VA = 4.5V to 5.5V 27 80 µA (max)
fSCL=3.4MHz VA = 2.7V to 3.6V 89 150 µA (max)
VA = 4.5V to 5.5V 168 250 µA (max)
PPD2 Power Consumption
fSCL=400kHz VA = 3.0V 0.04 mW
VA = 5.0V 0.14 mW
fSCL=3.4MHz VA = 3.0V 0.29 mW
VA = 5.0V 0.84 mW
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ADC081C021/ADC081C027
A.C. and Timing Characteristics
The following specifications apply for VA = +2.7V to +5.5V. Boldface limits apply for TMIN ≤ TA ≤ TMAX and all other limits are at
TA = 25°C, unless otherwise specified.
Symbol Parameter Conditions (Note 12) Typical
(Note 9)
Limits
(Notes 9,
12)
Units
(Limits)
CONVERSION RATE
Conversion Time 1 µs
fCONV Conversion Rate
fSCL = 100kHz 5.56 ksps
fSCL = 400kHz 22.2 ksps
fSCL = 1.7MHz 94.4 ksps
fSCL = 3.4MHz 188.9 ksps
DIGITAL TIMING SPECS (SCL, SDA)
fSCL Serial Clock Frequency
Standard Mode
Fast Mode
High Speed Mode, Cb = 100pF
High Speed Mode, Cb = 400pF
100
400
3.4
1.7
kHz (max)
kHz (max)
MHz (max)
MHz (max)
tLOW SCL Low Time
Standard Mode
Fast Mode
High Speed Mode, Cb = 100pF
High Speed Mode, Cb = 400pF
4.7
1.3
160
320
us (min)
us (min)
ns (min)
ns (min)
tHIGH SCL High Time
Standard Mode
Fast Mode
High Speed Mode, Cb = 100pF
High Speed Mode, Cb = 400pF
4.0
0.6
60
120
us (min)
us (min)
ns (min)
ns (min)
tSU;DAT Data Setup Time
Standard Mode
Fast Mode
High Speed Mode
250
100
10
ns (min)
ns (min)
ns (min)
tHD;DAT Data Hold Time
Standard Mode (Note 14) 0
3.45
us (min)
us (max)
Fast Mode (Note 14) 0
0.9
us (min)
us (max)
High Speed Mode, Cb = 100pF 0
70
ns (min)
ns (max)
High Speed Mode, Cb = 400pF 0
150
ns (min)
ns (max)
tSU;STA
Setup time for a start or a repeated
start condition
Standard Mode
Fast Mode
High Speed Mode
4.7
0.6
160
us (min)
us (min)
ns (min)
tHD;STA
Hold time for a start or a repeated start
condition
Standard Mode
Fast Mode
High Speed Mode
4.0
0.6
160
us (min)
us (min)
ns (min)
tBUF
Bus free time between a stop and start
condition
Standard Mode
Fast Mode 4.7
1.3
us (min)
us (min)
tSU;STO Setup time for a stop condition
Standard Mode
Fast Mode
High Speed Mode
4.0
0.6
160
us (min)
us (min)
ns (min)
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ADC081C021/ADC081C027
Symbol Parameter Conditions (Note 12) Typical
(Note 9)
Limits
(Notes 9,
12)
Units
(Limits)
trDA Rise time of SDA signal
Standard Mode 1000 ns (max)
Fast Mode 20+0.1Cb
300
ns (min)
ns (max)
High Speed Mode, Cb = 100pF 10
80
ns (min)
ns (max)
High Speed Mode, Cb = 400pF 20
160
ns (min)
ns (max)
tfDA Fall time of SDA signal
Standard Mode 250 ns (max)
Fast Mode 20+0.1Cb
250
ns (min)
ns (max)
High Speed Mode, Cb = 100pF 10
80
ns (min)
ns (max)
High Speed Mode, Cb = 400pF 20
160
ns (min)
ns (max)
trCL Rise time of SCL signal
Standard Mode 1000 ns (max)
Fast Mode 20+0.1Cb
300
ns (min)
ns (max)
High Speed Mode, Cb = 100pF 10
40
ns (min)
ns (max)
High Speed Mode, Cb = 400pF 20
80
ns (min)
ns (max)
trCL1
Rise time of SCL signal after a
repeated start condition and after an
acknowledge bit.
Standard Mode 1000 ns (max)
Fast Mode 20+0.1Cb
300
ns (min)
ns (max)
High Speed Mode, Cb = 100pF 10
80
ns (min)
ns (max)
High Speed Mode, Cb = 400pF 20
160
ns (min)
ns (max)
tfCL Fall time of a SCL signal
Standard Mode 300 ns (max)
Fast Mode 20+0.1Cb
300
ns (min)
ns (max)
High Speed Mode, Cb = 100pF 10
40
ns (min)
ns (max)
High Speed Mode, Cb = 400pF 20
80
ns (min)
ns (max)
Cb
Capacitive load for each bus line (SCL
and SDA) 400 pF (max)
tSP
Pulse Width of spike suppressed
(Note 11)
Fast Mode
High Speed Mode 50
10
ns (max)
ns (max)
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed
specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test
conditions. Operation of the device beyond the maximum Operating Ratings is not recommended.
Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified.
Note 3: When the input voltage at any pin exceeds 5.5V or is less than GND, the current at that pin should be limited per the Absolute Maximum Ratings. The
maximum package input current rating limits the number of pins that can safely exceed the power supplies.
Note 4: The absolute maximum junction temperature (TJmax) for this device is 150°C. The maximum allowable power dissipation is dictated by TJmax, the
junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax − TA) / θJA. The values
for maximum power dissipation will be reached only when the device is operated in a severe fault condition (e.g., when input or output pins are driven beyond
the operating ratings, or the power supply polarity is reversed).
Note 5: Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is a 220 pF capacitor discharged through 0 Ω. Charged
device model simulates a pin slowly acquiring charge (such as from a device sliding down the feeder in an automated assembler) then rapidly being discharged.
Note 6: Reflow temperature profiles are different for lead-free packages.
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ADC081C021/ADC081C027
Note 7: The inputs are protected as shown below. Input voltage magnitudes up to 5.5V, regardless of VA, will not cause errors in the conversion result. For
example, if VA is 3V, the digital input pins can be driven with a 5V logic device.
30052104
Note 8: To guarantee accuracy, it is required that VA be well bypassed and free of noise.
Note 9: Typical figures are at TJ = 25°C, and represent most likely parametric norms. Test limits are guaranteed to National's AOQL (Average Outgoing Quality
Level).
Note 10: This parameter is guaranteed by design and/or characterization and is not tested in production.
Note 11: Spike suppression filtering on SCL and SDA will suppress spikes that are less than 50ns for standard and fast modes, and less than 10ns for hs-mode.
Note 12: Cb refers to the capacitance of one bus line. Cb is expressed in pF units.
Note 13: The ADC will meet Minimum/Maximum specifications for fSCL up to 3.4MHz and VA = 2.7V to 3.6V when operating in the Quiet Interface Mode (Section
1.11).
Note 14: The ADC081C021 will provide a minimum data hold time of 300ns to comply with the I2C Specification.
Timing Diagrams
30052160
FIGURE 1. Serial Timing Diagram
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ADC081C021/ADC081C027
Specification Definitions
ACQUISITION TIME is the time required for the ADC to ac-
quire the input voltage. During this time, the hold capacitor is
charged by the input voltage.
APERTURE DELAY is the time between the start of a con-
version and the time when the input signal is internally ac-
quired or held for conversion.
CONVERSION TIME is the time required, after the input volt-
age is acquired, for the ADC to convert the input voltage to a
digital word.
DIFFERENTIAL NON-LINEARITY (DNL) is the measure of
the maximum deviation from the ideal step size of 1 LSB.
EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE
BITS) is another method of specifying Signal-to-Noise and
Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02
and says that the converter is equivalent to a perfect ADC of
this (ENOB) number of bits.
FULL POWER BANDWIDTH is a measure of the frequency
at which the reconstructed output fundamental drops 3 dB
below its low frequency value for a full scale input.
GAIN ERROR is the deviation of the last code transition
(111...110) to (111...111) from the ideal (VREF - 1.5 LSB), after
adjusting for offset error.
INTEGRAL NON-LINEARITY (INL) is a measure of the de-
viation of each individual code from a line drawn from negative
full scale (½ LSB below the first code transition) through pos-
itive full scale (½ LSB above the last code transition). The
deviation of any given code from this straight line is measured
from the center of that code value.
INTERMODULATION DISTORTION (IMD) is the creation of
additional spectral components as a result of two sinusoidal
frequencies being applied to an individual ADC input at the
same time. It is defined as the ratio of the power in both the
second and third order intermodulation products to the power
in one of the original frequencies. Second order products are
fa ± fb, where fa and fb are the two sine wave input frequencies.
Third order products are (2fa ± fb ) and (fa ± 2fb). IMD is usually
expressed in dB.
MISSING CODES are those output codes that will never ap-
pear at the ADC output. The ADC081C021 is guaranteed not
to have any missing codes.
OFFSET ERROR is the deviation of the first code transition
(000...000) to (000...001) from the ideal (i.e. GND + 0.5 LSB).
SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in
dB, of the rms value of the input signal to the rms value of the
sum of all other spectral components below one-half the sam-
pling frequency, not including harmonics or d.c.
SIGNAL TO NOISE PLUS DISTORTION (S/N+D or
SINAD) Is the ratio, expressed in dB, of the rms value of the
input signal to the rms value of all of the other spectral com-
ponents below half the clock frequency, including harmonics
but excluding d.c.
SPURIOUS FREE DYNAMIC RANGE (SFDR) is the differ-
ence, expressed in dB, between the desired signal amplitude
to the amplitude of the peak spurious spectral component,
where a spurious spectral component is any signal present in
the output spectrum that is not present at the input and may
or may not be a harmonic.
TOTAL HARMONIC DISTORTION (THD) is the ratio, ex-
pressed in dBc, of the rms total of the first n harmonic com-
ponents at the output to the rms level of the input signal
frequency as seen at the output. THD is calculated as
where Af1 is the RMS power of the input frequency at the out-
put and Af2 through Afn are the RMS power in the first n
harmonic frequencies.
THROUGHPUT TIME is the minimum time required between
the start of two successive conversions. It is the acquisition
time plus the conversion time.
LEAST SIGNIFICANT BIT (LSB) is the bit that has the small-
est value or weight of all bits in a word. This value is
LSB = VA / 2n
where VA is the supply voltage for this product, and "n" is the
resolution in bits, which is 8 for the ADC081C021.
MOST SIGNIFICANT BIT (MSB) is the bit that has the largest
value or weight of all bits in a word. Its value is 1/2 of VA.
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ADC081C021/ADC081C027
Typical Performance Characteristics fSCL = 400kHz, fSAMPLE = 22ksps, fIN = 1kHz, VA = 5.0V, TA = +25°
C, unless otherwise stated.
INL vs. Code - VA=3V
30052122
DNL vs. Code - VA=3V
30052123
INL vs. Code - VA=5V
30052124
DNL vs. Code - VA=5V
30052125
INL vs. Supply
30052126
DNL vs. Supply
30052127
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ADC081C021/ADC081C027
ENOB vs. Supply
30052128
SINAD vs. Supply
30052129
FFT Plot - VA=3V
30052130
FFT Plot - VA=3V
30052131
Offset Error vs. Temperature
30052132
Gain Error vs. Temperature
30052133
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ADC081C021/ADC081C027
Continuous Operation Supply Current vs. VA
30052134
Automatic Conversion Supply Current vs. VA
30052135
Power Down (PD1) Supply Current vs. VA
30052136
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ADC081C021/ADC081C027
1.0 Functional Description
The ADC081C021 and the ADC081C027 are successive-ap-
proximation analog-to-digital converters designed around a
charge-redistribution digital-to-analog converter. Unless oth-
erwise stated, references to the ADC081C021 in this section
will apply to both the ADC081C021 and the ADC081C027.
1.1 CONVERTER OPERATION
Simplified schematics of the ADC081C021 in both track and
hold operation are shown in Figure 2 and Figure 3 respec-
tively. In Figure 2, the ADC081C021 is in track mode. SW1
connects the sampling capacitor to the analog input channel
and SW2 equalizes the comparator inputs. The ADC is in this
state for approximately 0.4µs at the beginning of every con-
version cycle, which begins at the ACK fall of SDA. Conver-
sions occur when the conversion result register is read and
when the ADC is in automatic conversion mode. (see Section
1.9 AUTOMATIC CONVERSION MODE).
Figure 3 shows the ADC081C021 in hold mode. SW1 con-
nects the sampling capacitor to ground and SW2 unbalances
the comparator. The control logic then instructs the charge-
redistribution DAC to add or subtract fixed amounts of charge
to or from the sampling capacitor until the comparator is bal-
anced. When the comparator is balanced, the digital word
supplied to the DAC is also the digital representation of the
analog input voltage. This digital word is stored in the con-
version result register and read via the 2-wire interface.
In the Normal (non-Automatic) Conversion mode, a new con-
version is started after the previous conversion result is read.
In the Automatic Mode, conversions are started at set inter-
vals, as determined by bits D7 through D5 of the Configuration
Register. The intent of the Automatic mode is to provide a
"watchdog" function to ensure that the input voltage remains
within the limits set in the Alert Limit Registers. The minimum
and maximum conversion results can then be read from the
Lowest Conversion Register and the Highest Conversion
Register, as described in Section 1.6 INTERNAL REGIS-
TERS.
30052165
FIGURE 2. ADC081C021 in Track Mode
30052166
FIGURE 3. ADC081C021 in Hold Mode
1.2 ANALOG INPUT
An equivalent circuit for the input of the ADC081C021 is
shown in Figure 4. The diodes provide ESD protection for the
analog input. The operating range for the analog input is 0 V
to VA. Going beyond this range will cause the ESD diodes to
conduct and result in erratic operation. For this reason, these
diodes should NOT be used to clamp the input signal.
The capacitor C1 in Figure 4 has a typical value of 3 pF and
is mainly the package pin capacitance. Resistor R1 is the on
resistance (RON) of the multiplexer and track / hold switch and
is typically 500Ω. Capacitor C2 is the ADC081C021 sampling
capacitor, and is typically 30 pF. The ADC081C021 will de-
liver best performance when driven by a low-impedance
source (less than 100Ω). This is especially important when
using the ADC081C021 to sample dynamic signals. A buffer
amplifier may be necessary to limit source impedance. Use a
precision op-amp to maximize circuit performance. Also im-
portant when sampling dynamic signals is a band-pass or low-
pass filter to reduce noise at the input.
30052167
FIGURE 4. Equivalent Input Circuit
The analog input is sampled for eight internal clock cycles, or
for typically 400 ns, after the fall of SDA for acknowledgement.
This time could be as long as about 530 ns. The sampling
switch opens and the conversion begins this time after the fall
of ACK. This time are typical at room temperature and may
vary with temperature.
1.3 ADC TRANSFER FUNCTION
The output format of the ADC081C021 is straight binary.
Code transitions occur midway between successive integer
LSB values. The LSB width for the ADC081C021 is VA / 256.
The ideal transfer characteristic is shown in Figure 5. The
transition from an output code of 0000 0000 0000 to a code
of 0000 0000 0001 is at 1/2 LSB, or a voltage of VA / 512.
Other code transitions occur at intervals of 1 LSB.
30052168
FIGURE 5. Ideal Transfer Characteristic
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ADC081C021/ADC081C027
1.4 REFERENCE VOLTAGE
The ADC081C021 uses the supply (VA) as the reference, so
VA must be treated as a reference. The analog-to-digital con-
version will only be as precise as the reference (VA), so the
supply voltage should be free of noise. The reference should
be driven by a low output impedance voltage source.
The Applications section provides recommended ways to pro-
vide the supply voltage appropriately. Refer to Section 2.1
TYPICAL APPLICATION CIRCUIT for details.
1.5 POWER-ON RESET
An internal power-on reset (POR) occurs when the supply
voltage transitions above the power-on reset threshold. Each
of the registers contains a defined value upon POR and this
data remains there until any of the following occurs:
•The first conversion is completed, causing the Conversion
Result and Status registers to be updated.
•A different data word is written to a writeable register.
•The ADC is powered down.
The internal registers will lose their contents if the supply volt-
age goes below 2.4V. Should this happen, it is important that
the VA supply be lowered to a maximum of 200mV before the
supply is raised again to properly reset the device and ensure
that the ADC performs as specified.
1.6 INTERNAL REGISTERS
The ADC081C021 has 8 internal data registers and one ad-
dress pointer. The registers provide additional ADC functions
such as storing minimum and maximum conversion results,
setting alert threshold levels, and storing data to configure the
operation of the device. Figure 6 shows all of the registers and
their corresponding address pointer values. All of the regis-
ters are read/write capable except the conversion result reg-
ister which is read-only.
30052169
FIGURE 6. Register Structure
1.6.1 Address Pointer Register
The address pointer determines which of the data registers is
accessed by the I2C interface. The first data byte of every
write operation is stored in the address pointer register. This
value selects the register that the following data bytes will be
written to or read from. Only the three LSBs of this register
are variable. The other bits must always be written to as zeros.
After a power-on reset, the pointer register defaults to all ze-
ros (conversion result register).
Default Value: 00h
P7 P6 P5 P4 P3 P2 P1 P0
0 0 0 0 0 Register Select
P2 P1 P0 REGISTER
0 0 0 Conversion Result (read only)
0 0 1 Alert Status (read/write)
0 1 0 Configuration (read/write)
0 1 1 Low Limit (read/write)
1 0 0 High Limit (read/write)
1 0 1 Hysteresis (read/write)
1 1 0 Lowest Conversion (read/write)
1 1 1 Highest Conversion (read/write)
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ADC081C021/ADC081C027
1.6.2 Conversion Result Register
This register holds the result of the most recent conversion. In the normal mode, a new conversion is started whenever this register
is read. The conversion result data is in straight binary format with the MSB at D11.
Pointer Address 00h (Read Only)
Default Value: 0000h
D15 D14 D13 D12 D11 D10 D9 D8
Alert Flag Reserved Conversion Result [7:4]
D7 D6 D5 D4 D3 D2 D1 D0
Conversion Result [3:0] Reserved
Bits Name Description
15 Alert Flag This bit indicates when an alert condition has occurred. When the Alert Bit Enable is set in the
Configuration Register, this bit will be high if either alert flag is set in the Alert Status Register.
Otherwise, this bit is a zero. The I2C controller will typically read the Alert Status register and
other data registers to determine the source of the alert.
14:12 Reserved Always reads zeros.
11:4 Conversion Result The Analog-to-Digital conversion result. The Conversion result data is a 8-bit data word in straight
binary format. The MSB is D11.
3:0 Reserved Always reads zeros.
1.6.3 Alert Status Register
This register indicates if a high or a low threshold has been violated. The bits of this register are active high. That is, a high indicates
that the respective limit has been violated.
Pointer Address 01h (Read/Write)
Default Value: 00h
D7 D6 D5 D4 D3 D2 D1 D0
Reserved Over Range
Alert
Under Range
Alert
Bits Name Description
7:2 Reserved Always reads zeros. Zeros must be written to these bits.
1 Over Range
Alert Flag
Bit is set to 1 when the measured voltage exceeds the VHIGH limit stored in the programmable
VHIGH limit register. Flag is reset to 0 when one of the following two conditions is met: (1) The
controller writes a one to this bit. (2) The measured voltage decreases below the programmed
VHIGH limit minus the programmed VHYST value (See Figure 9) . The alert will only self-clear if the
Alert Hold bit is cleared in the Configuration register. If the Alert Hold bit is set, the only way to
clear an over range alert is to write a one to this bit.
0 Under Range
Alert Flag
Bit is set to 1 when the measured voltage falls below the VLOW limit stored in the programmable
VLOW limit register. Flag is reset to 0 when one of the following two conditions is met: (1) The
controller writes a one to this bit. (2) The measured voltage increases above the programmed
VLOW limit plus the programmed VHYST value. The alert will only self-clear if the Alert Hold bit is
cleared in the Configuration register. If the Alert Hold bit is set, the only way to clear an under
range alert is to write a one to this bit.
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ADC081C021/ADC081C027
1.6.4 Configuration Register
Pointer Address 02h (Read/Write)
Default Value: 00h
D7 D6 D5 D4 D3 D2 D1 D0
Cycle Time [2:0] Alert
Hold
Alert
Flag
Enable
Alert
Pin
Enable
0 Polarity
Cycle Time[2:0] Conversion
Interval
Typical
fconvert
(ksps)
D7 D6 D5
0 0 0 Mode Disabled 0
0 0 1 Tconvert x 32 27
0 1 0 Tconvert x 64 13.5
0 1 1 Tconvert x 128 6.7
1 0 0 Tconvert x 256 3.4
1 0 1 Tconvert x 512 1.7
1 1 0 Tconvert x 1024 0.9
1 1 1 Tconvert x 2048 0.4
Bits Name Description
7:5 Cycle Time Configures Automatic Conversion mode. When these bits are set to zeros, the automatic
conversion mode is disabled. This is the case at power-up.
When these bits are set to a non-zero value, the ADC will begin operating in automatic conversion
mode. (See Section 1.9 AUTOMATIC CONVERSION MODE). The Cycle Time table shows how
different values provide various conversion intervals.
4 Alert Hold 0: Alerts will self-clear when the measured voltage moves within the limits by more than the
hysteresis register value.
1: Alerts will not self-clear and are only cleared when a one is written to the alert high flag or the
alert low flag in the Alert Status register.
3 Alert Flag Enable 0: Disables alert status bit [D15] in the Conversion Result register.
1: Enables alert status bit [D15] in the Conversion Result register.
2 Alert Pin Enable 0: Disables the ALERT output pin. The ALERT output will TRI-STATE when the pin is disabled.
1: Enables the ALERT output pin.
*This bit does not apply to the ADC081C027.
1 Reserved Always reads zeros. Zeros must be written to these bits.
0 Polarity This bit configures the active level polarity of the ALERT output pin.
0: Sets the ALERT pin to active low.
1: Sets the ALERT pin to active high.
*This bit does not apply to the ADC081C027.
1.6.5 VLOW -- Alert Limit Register - Under Range
This register holds the lower limit threshold used to determine the alert condition. If the conversion moves lower than this limit, a
VLOW alert is generated.
Pointer Address 03h (Read/Write)
Default Value: 0000h
D15 D14 D13 D12 D11 D10 D9 D8
Reserved VLOW Limit [7:4]
D7 D6 D5 D4 D3 D2 D1 D0
VLOW Limit [3:0] Reserved
Bits Name Description
15:12 Reserved Always reads zeros. Zeros must be written to these bits.
11:4 VLOW Limit Sets the lower limit threshold used to determine the alert condition. If the conversion moves lower
than this limit, a VLOW alert is generated.
3:0 Reserved Always reads zeros. Zeros must be written to these bits.
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ADC081C021/ADC081C027
1.6.6 VHIGH -- Alert Limit Register - Over Range
This register holds the upper limit threshold used to determine the alert condition. If the conversion moves higher than this limit, a
VHIGH alert is generated.
Pointer Address 04h (Read/Write)
Default Value: 0FFFh
D15 D14 D13 D12 D11 D10 D9 D8
Reserved VHIGH Limit [7:4]
D7 D6 D5 D4 D3 D2 D1 D0
VHIGH Limit [3:0] Reserved
Bits Name Description
15:12 Reserved Always reads zeros. Zeros must be written to these bits.
11:4 VHIGH Limit Sets the upper limit threshold used to determine the alert condition. If the conversion moves
higher than this limit, a VHIGH alert is generated.
3:0 Reserved Always reads zeros. Zeros must be written to these bits.
1.6.7 VHYST -- Alert Hysteresis Register
This register holds the hysteresis value used to determine the alert condition. After a VHIGH or VLOW alert occurs, the conversion
result must move within the VHIGH or VLOW limit by more than this value to clear the alert condition. Note: If the Alert Hold bit is set
in the configuration register, alert conditions will not self-clear.
Pointer Address 05h (Read/Write)
Default Value: 0000h
D15 D14 D13 D12 D11 D10 D9 D8
Reserved Hysteresis [7:4]
D7 D6 D5 D4 D3 D2 D1 D0
Hysteresis [3:0] Reserved
Bits Name Description
15:12 Reserved Always reads zeros. Zeros must be written to these bits.
11:4 Hysteresis Hysteresis value. D11 is MSB.
3:0 Reserved Always reads zeros. Zeros must be written to these bits.
1.6.8 VMIN -- Lowest Conversion Register
This register holds the Lowest Conversion result when in the automatic conversion mode. Each conversion result is compared
against the contents of this register. If the value is lower, it becomes the lowest conversion and replaces the current value. If the
value is higher, the register contents remain unchanged. The lowest conversion value can be cleared at any time by writing 0FFFh
to this register. The value of this register will update automatically when the automatic conversion mode is enabled, but is NOT
updated in the normal mode.
Pointer Address 06h (Read/Write)
Default Value: 0FFFh
D15 D14 D13 D12 D11 D10 D9 D8
Reserved Lowest Conversion [7:4]
D7 D6 D5 D4 D3 D2 D1 D0
Lowest Conversion [3:0] Reserved
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ADC081C021/ADC081C027
Bits Name Description
15:12 Reserved Always reads zeros. Zeros must be written to these bits.
11:4 Lowest Conversion Lowest conversion result data. D11 is MSB.
3:0 Reserved Always reads zeros. Zeros must be written to these bits.
1.6.9 VMAX -- Highest Conversion Register
This register holds the Highest Conversion result when in the Automatic mode. Each conversion result is compared against the
contents of this register. If the value is higher, it replaces the previous value. If the value is lower, the register contents remain
unchanged. The highest conversion value can be cleared at any time by writing 0000h to this register. The value of this register
will update automatically when the automatic conversion mode is enabled, but is NOT updated in the normal mode.
Pointer Address 07h (Read/Write)
Default Value: 0000h
D15 D14 D13 D12 D11 D10 D9 D8
Reserved Highest Conversion [7:4]
D7 D6 D5 D4 D3 D2 D1 D0
Highest Conversion [3:0] Reserved
Bits Name Description
15:12 Reserved Always reads zeros. Zeros must be written to these bits.
11:4 Highest Conversion Highest conversion result data. D11 is MSB.
3:0 Reserved Always reads zeros. Zeros must be written to these bits.
1.7 SERIAL INTERFACE
The I2C-compatible interface operates in all three speed
modes. Standard mode (100kHz) and Fast mode (400kHz)
are functionally the same and will be referred to as Standard-
Fast mode in this document. High-Speed mode (3.4MHz) is
an extension of Standard-Fast mode and will be referred to
as Hs-mode in this document.
The following diagrams describe the timing relationships of
the clock (SCL) and data (SDA) signals. Pull-up resistors or
current sources are required on the SCL and SDA busses to
pull them high when they are not being driven low. A logic zero
is transmitted by driving the output low. A logic high is trans-
mitted by releasing the output and allowing it to be pulled-up
externally. The appropriate pull-up resistor values will depend
upon the total bus capacitance and operating speed. The AD-
C081C021 offers extended ESD tolerance (8kV HBM) for the
I2C bus pins (SCL & SDA) allowing extension of the bus
across multiple boards without extra ESD protection.
1.7.1 Basic I2C Protocol
The I2C interface is bi-directional and allows multiple devices
to operate on the same bus. The bus consists of master de-
vices and slave devices which can communicate back and
forth over the I2C interface. Master devices control the bus
and are typically microcontrollers, FPGAs, DSPs, or other
digital controllers. Slave devices are controlled by a master
and are typically peripheral devices such as the
ADC081C021. To support multiple devices on the same bus,
each slave has a unique hardware address which is referred
to as the "slave address." To communicate with a particular
device on the bus, the controller (master) sends the slave ad-
dress and listens for a response from the slave. This response
is referred to as an acknowledge bit. If a slave on the bus is
addressed correctly, it acknowledges (ACKs) the master by
driving the SDA bus low. If the address doesn't match a
device's slave address, it not-acknowledges (NACKs) the
master by letting SDA be pulled high. ACKs also occur on the
bus when data is being transmitted. When the master is writ-
ing data, the slave ACKs after every data byte is successfully
received. When the master is reading data, the master ACKs
after every data byte is received to let the slave know it wants
to receive another data byte. When the master wants to stop
reading, it NACKs after the last data byte and creates a stop
condition on the bus.
All communication on the bus begins with either a Start con-
dition or a Repeated Start condition. The protocol for starting
the bus varies between Standard-Fast mode and Hs-mode.
In Standard-Fast mode, the master generates a
Start condition by driving SDA from high to low while SCL is
high. In Hs-mode, starting the bus is more complicated.
Please refer to Section 1.7.3 High-Speed (Hs) Mode for the
full details of a Hs-mode Start condition.
A Repeated Start is generated to address a different device
or register, or to switch between read and write modes. The
master generates a Repeated Start condition by driving SDA
low while SCL is high. Following the Repeated Start, the mas-
ter sends out the slave address and a read/write bit as shown
in Figure 7. The bus continues to operate in the same speed
mode as before the Repeated Start condition.
All communication on the bus ends with a Stop condition. In
either Standard-Fast mode or Hs-Mode, a Stop condition oc-
curs when SDA is pulled high while SCL is high. After a Stop
condition, the bus remains idle until a master generates an-
other Start condition.
Please refer to the Philips I2C® Specification (Version 2.1
Jan, 2000) for a detailed description of the serial interface.
19 www.national.com
ADC081C021/ADC081C027
30052111
FIGURE 7. Basic Operation.
1.7.2 Standard-Fast Mode
In Standard-Fast mode, the master generates a start condi-
tion by driving SDA from high to low while SCL is high. The
start condition is always followed by a 7-bit slave address and
a Read/Write bit. After these 8 bits have been transmitted by
the master, SDA is released by the master and the
ADC081C021 either ACKs or NACKs the address. If the slave
address matches, the ADC081C021 ACKs the master. If the
address doesn't match, the ADC081C021 NACKs the master.
For a write operation, the master follows the ACK by sending
the 8-bit register address pointer to the ADC. Then the
ADC081C021 ACKs the transfer by driving SDA low. Next,
the master sends the upper 8-bits to the ADC081C021. Then
the ADC081C021 ACKs the transfer by driving SDA low. For
a single byte transfer, the master should generate a stop con-
dition at this point. For a 2-byte write operation, the lower 8-
bits are sent by the master. The ADC081C021 then ACKs the
transfer, and the master either sends another pair of data
bytes, generates a Repeated Start condition to read or write
another register, or generates a Stop condition to end com-
munication.
A read operation can take place either of two ways:
If the address pointer is pre-set before the read operation, the
desired register can be read immediately following the slave
address. In this case, the upper 8-bits of the register, set by
the pre-set address pointer, are sent out by the ADC. For a
single byte read operation, the Master sends a NACK to the
ADC and generates a Stop condition to end communication
after receiving 8-bits of data. For a 2-Byte read operation, the
Master continues the transmission by sending an ACK to the
ADC. Then, the ADC sends out the lower 8-bits of the ADC
register. At this point, the master either sends; an ACK to re-
ceive more data or, a NACK followed by a Stop or Repeated
Start. If the master sends an ACK, the ADC sends the next
upper data byte, and the read cycle repeats.
If the ADC081S021 address pointer needs to be set, the
master needs to write to the device and set the address point-
er before reading from the desired register. This type of read
requires a start, the slave address, a write bit, the address
pointer, a Repeated Start (if appropriate), the slave address,
and a read bit (refer to Figure 12). Following this sequence,
the ADC sends out the upper 8-bits of the register. For a single
byte read operation, the Master must then send a NACK to
the ADC and generate a Stop condition to end communica-
tion. For a 2-Byte write operation, the Master sends an ACK
to the ADC. Then, the ADC sends out the lower 8-bits of the
ADC register. At this point, the master sends either an ACK
to receive more data, or a NACK followed by a Stop or Re-
peated Start. If the master sends an ACK, the ADC sends
another pair of data bytes, and the read cycle will repeat. The
number of data words that can be read is unlimited.
1.7.3 High-Speed (Hs) Mode
For Hs-mode, the sequence of events to begin communica-
tion differs slightly from Standard-Fast mode. Figure 8 de-
scribes this in further detail. Initially, the bus begins running
in Standard-Fast mode. The master generates a
Start condition and sends the 8-bit Hs master code
(00001XXX) to the ADC081C021. Next, the ADC081C021
responds with a NACK. Once the SCL line has been pulled to
a high level, the master switches to Hs-mode by increasing
the bus speed and generating a second Repeated Start con-
dition (driving SDA low while SCL is pulled high). At this point,
the master sends the slave address to the ADC081C021, and
communication continues as shown above in the "Basic Op-
eration" Diagram (see Figure 7).
When the master generates a Repeated Start condition while
in Hs-mode, the bus stays in Hs-mode awaiting the slave ad-
dress from the master. The bus continues to run in Hs-mode
until a Stop condition is generated by the master. When the
master generates a Stop condition on the bus, the bus must
be started in Standard-Fast mode again before increasing the
bus speed and switching to Hs-mode.
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ADC081C021/ADC081C027
30052112
FIGURE 8. Beginning Hs-Mode Communication
1.7.4 I2C Slave (Hardware) Address
The ADC has a seven-bit hardware address which is also referred to as a slave address. For the MSOP-8 version of the AD-
C081C021, this address is configured by the ADR0 and ADR1 addres selection inputs. For the ADC081C027, the address is
configured by the ADR0 address selection input. ADR0 and ADR1 can be grounded, left floating, or tied to VA. If desired, ADR0
can be set to VA/2 rather than left floating. The state of these inputs sets the hardware address that the ADC responds to on the
I2C bus (see Table 1). For the TSOT-6 version of the ADC081C021, the hardware address is not pin-configurable and is set to
1010100. The diagrams in Section 1.10 COMMUNICATING WITH THE ADC081C021 describe how the I2C controller should
address the ADC via the I2C interface.
Pin compatible alternatives that provide additional address options to the TSOT-6 version of the ADC081C021 and the AD-
C081C027 are available.
TABLE 1. Slave Addresses
Slave Address
[A6 - A0]
ADC081C027
(TSOT-6)
ADC081C021
(TSOT-6)
ADC081C021
(MSOP-8)
ADR0 ALERT ADR1 ADR0
1010000 Floating ----------------- Floating Floating
1010001 GND ----------------- Floating GND
1010010 VA----------------- Floating VA
1010100 ----------------- Single Address GND Floating
1010101 ----------------- ----------------- GND GND
1010110 ----------------- ----------------- GND VA
1011000 ----------------- ----------------- VAFloating
1011001 ----------------- ----------------- VAGND
1011010 ----------------- ----------------- VAVA
1.8 ALERT FUNCTION
The ALERT function is an "out-of-range" indicator. At the end
of every conversion, the measured voltage is compared to the
values in the VHIGH and VLOW registers. If the measured volt-
age exceeds the value stored in VHIGH or falls below the value
stored in VLOW, an alert condition occurs. The Alert condition
is indicated in up to three places. First, the alert condition al-
ways causes either or both of the alert flags in the Alert Status
register to go high. If the measured voltage exceeds the
VHIGH limit, the Over Range Alert Flag is set. If the measured
voltage falls below the VLOW limit, the Under Range Alert Flag
is set. Second, if the Alert Flag Enable bit is set in the Con-
figuration register, the alert condition also sets the MSB of the
Conversion Result register. Third, if the Alert Pin Enable bit is
set in the Configuration register, the ALERT output becomes
active (see Figure 9). The ALERT output (ADC081S021 only)
can be configured as an active high or active low output via
the Polarity bit in the Configuration register. If the Polarity bit
is cleared, the ALERT output is configured as active low. If
the Polarity bit is set, the ALERT output is configured as active
high.
The Over Range Alert condition is cleared when one of the
following two conditions is met:
1. The controller writes a one to the Over Range Alert Flag
bit.
2. The measured voltage goes below the programmed
VHIGH limit minus the programmed VHYST value and the
Alert Hold bit is cleared in the Configuration register. (see
Figure 9). If the Alert Hold bit is set, the alert condition
persists and only clears when a one is written to the Over
Range Alert Flag bit.
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ADC081C021/ADC081C027
The Under Range Alert condition is cleared when one of the
following two conditions is met:
1. The controller writes a one to the Under Range Alert Flag
bit.
2. The measured voltage goes above the programmed
VLOW limit plus the programmed VHYST value and the
Alert Hold bit is cleared in the Configuration register. If
the Alert Hold bit is set, the alert condition persists and
only clears when a one is written to the Under Range
Alert Flag bit.
If the alert condition has been cleared by writing a one to the
alert flag while the measured voltage still violates the VHIGH
or VLOW limits, an alert condition will occur again after the
completion of the next conversion (see Figure 10).
Alert conditions only occur if the input voltage exceeds the
VHIGH limit or falls below the VLOW limit at the sample-hold
instant. The input voltage can exceed the VHIGH limit or fall
below the VLOW limit briefly between conversions without
causing an alert condition.
30052174
FIGURE 9. Alert condition cleared when measured
voltage crosses VHIGH - VHYST
30052175
FIGURE 10. Alert condition cleared by writing a "1" to the
Alert Flag.
1.9 AUTOMATIC CONVERSION MODE
The automatic conversion mode configures the ADC to con-
tinually perform conversions without receiving "read" instruc-
tions from the controller over the I2C interface. The mode is
activated by writing a non-zero value into the Cycle Time bits
- D[7:5] - of the Configuration register (see Section 1.6.4 Con-
figuration Register). Once the ADC081C021 enters this
mode, the internal oscillator is always enabled. The ADC's
control logic samples the input at the sample rate set by the
cycle time bits. Although the conversion result is not trans-
mitted by the 2-wire interface, it is stored in the conversion
result register and updates the various status registers of the
device.
In automatic conversion mode, the out-of-range alert function
is active and updates after every conversion. The ADC can
operate independently of the controller in automatic conver-
sion mode. When the input signal goes "out-of-range", an
alert signal is sent to the controller. The controller can then
read the status registers and determine the source of the alert
condition. Also, comparison and updating of the VMIN and
VMAX registers occurs after every conversion in automatic
conversion mode. The controller can occasionally read the
VMIN and/or VMAX registers to determine the sampled input
extremes. These register values persist until the user resets
the VMIN and VMAX registers. These two features are useful in
system monitoring, peak detection, and sensing applications.
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ADC081C021/ADC081C027
1.10 COMMUNICATING WITH THE ADC081C021
The ADC081C021's data registers are selected by the ad-
dress pointer (see Section 1.6.1 Address Pointer Register).
To read/write a specific data register, the pointer must be set
to that register's address. The pointer is always written at the
beginning of a write operation. When the pointer needs to be
updated for a read cycle, a write operation must precede the
read operation to set the pointer address correctly. On the
other hand, if the pointer is preset correctly, a read operation
can occur without writing the address pointer register. The
following timing diagrams describe the various read and write
operations supported by the ADC.
1.10.1 Reading from a 2-Byte ADC Register
The following diagrams indicate the sequence of actions re-
quired for a 2-Byte read from an ADC081C021 Register.
30052163
FIGURE 11. (a) Typical Read from a 2-Byte ADC Register with Preset Pointer
30052170
FIGURE 12. (b) Typical Pointer Set Followed by Immediate Read of a 2-Byte ADC Register
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ADC081C021/ADC081C027
1.10.2 Reading from a 1-Byte ADC Register
The following diagrams indicate the sequence of actions re-
quired for a single Byte read from an ADC081C021 Register.
30052171
FIGURE 13. (a) Typical Read from a 1-Byte ADC Register with Preset Pointer
30052172
FIGURE 14. (b) Typical Pointer Set Followed by Immediate Read of a 1-Byte ADC Register
1.10.3 Writing to an ADC Register
The following diagrams indicate the sequence of actions re-
quired for writing to an ADC081C021 Register.
30052164
FIGURE 15. (a) Typical Write to a 1-Byte ADC Register
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ADC081C021/ADC081C027
30052173
FIGURE 16. (b) Typical Write to a 2-Byte ADC Register
1.11 QUIET INTERFACE MODE
To improve performance at High Speed, operate the ADC in
Quiet Interface Mode. This mode provides improved INL and
DNL performance in I2C Hs-Mode (3.4MHz). The Quiet Inter-
face mode provides a maximum throughput rate of 162ksps.
Figure 17 describes how to read the conversion result register
in this mode. Basically, the Master needs to release SCL for
at least 1µs before the MSB of every upper data byte. The
diagram assumes that the address pointer register is set to
its default value.
Quiet Interface mode will only improve INL and DNL perfor-
mance in Hs-Mode. Standard and Fast mode performance is
unaffected by the Quiet Interface mode.
30052176
FIGURE 17. Reading in Quiet Interface Mode
25 www.national.com
ADC081C021/ADC081C027
2.0 Applications Information
2.1 TYPICAL APPLICATION CIRCUIT
A typical application circuit is shown in Figure 18. The analog
supply is bypassed with a capacitor network located close to
the ADC081C021. The ADC uses the analog supply (VA) as
its reference voltage, so it is very important that VA be kept as
clean as possible. Due to the low power requirements of the
ADC081C021, it is possible to use a precision reference as a
power supply.
The bus pull-up resistors (RP) should be powered by the
controller's supply. It is important that the pull-up resistors are
pulled to the same voltage potential as VA. This will ensure
that the logic levels of all devices on the bus are compatible.
If the controller's supply is noisy, an appropriate bypass ca-
pacitor should be added between the controller's supply pin
and the pull-up resistors. For Hs-mode applications, this by-
pass capacitance will improve the accuracy of the ADC.
The value of the pull-up resistors (RP) depends upon the
characteristics of each particular I2C bus. The I2C specifica-
tion describes how to choose an appropriate value. As a
general rule-of-thumb, we suggest using a 1kΩ resistor for
Hs-mode bus configurations and a 5kΩ resistor for Standard
or Fast Mode bus configurations. Depending upon the bus
capacitance, these values may or may not be sufficient to
meet the timing requirements of the I2C bus specification.
Please see the I2C specification for further information.
30052120
FIGURE 18. Typical Application Circuit
2.2 BUFFERED INPUT
A buffered input application circuit is shown in Figure 19. The
analog input is buffered by a National Semiconductor
LMP7731. The non-inverting amplifier configuration provides
a buffered gain stage for a single ended source. This appli-
cation circuit is good for single-ended sensor interface. The
input must have a DC bias level that keeps the ADC input
signal from swinging below GND or above the supply (+5V in
this case).
The LM4132, with its 0.05% accuracy over temperature, is an
excellent choice as a reference source for the ADC081C021.
30052121
FIGURE 19. Buffered Input Circuit
www.national.com 26
ADC081C021/ADC081C027
2.3 INTELLIGENT BATTERY MONITOR
The ADC081C021 is easily used as an intelligent battery
monitor. The simple circuit shown in Figure 20, uses the
ADC081C021, the LP2980 fixed reference, and a resistor di-
vider to implement an intelligent battery monitor with a window
supervisory feature. The window supervisory feature is im-
plemented by the "out of range" alert function. When the
battery is recharging, the Over Range Alert will indicate that
the charging cycle is complete (see Figure 21). When the
battery is nearing depletion, the Under Range Alert will indi-
cate that the battery is low (see Figure 22).
30052177
FIGURE 20. Intelligent Battery Monitor Circuit
30052178
FIGURE 21. Recharge Cycle
30052179
FIGURE 22. Discharge Cycle
In addition to the window supervisory feature, the
ADC081C021 will allow the controller to read the battery volt-
age at any time during operation.
The accurate voltage reading and the alert feature will allow
a controller to improve the efficiency of a battery-powered
device. During the discharge cycle, the controller can switch
to a low-battery mode, safely suspend operation, or report a
precise battery level to the user. During the recharge cycle,
the controller can implement an intelligent recharge cycle,
decreasing the charge rate when the battery charge nears
capacity.
2.3.1 Trickle Charge Controller
While a battery is discharging, the ADC081C021 can be used
to control a trickle charge to keep the battery near full capacity
(see Figure 23). When the alert output is active, the battery
will recharge. An intelligent recharge cycle will prevent over-
charging and damaging the battery. With a trickle charge, the
battery powered device can be disconnected from the charger
at any time with a full charge.
30052180
FIGURE 23. Trickle Charge
2.4 LAYOUT, GROUNDING, AND BYPASSING
For best accuracy and minimum noise, the printed circuit
board containing the ADC081C021 should have separate
analog and digital areas. The areas are defined by the loca-
tions of the analog and digital power planes. Both of these
planes should be located on the same board layer. A single,
solid ground plane is preferred if digital return current does
not flow through the analog ground area. Frequently a single
ground plane design will utilize a "fencing" technique to pre-
vent the mixing of analog and digital ground currents. Sepa-
rate ground planes should only be utilized when the fencing
technique is inadequate. The separate ground planes must
be connected in one place, preferably near the ADC121C021.
Special care is required to guarantee that signals do not pass
over power plane boundaries. Return currents must always
have a continuous return path below their traces.
The ADC081C021 power supply should be bypassed with a
4.7µF and a 0.1µF capacitor as close as possible to the device
with the 0.1µF right at the device supply pin. The 4.7µF ca-
pacitor should be a tantalum type and the 0.1µF capacitor
should be a low ESL type. The power supply for the
ADC081C021 should only be used for analog circuits.
Avoid crossover of analog and digital signals and keep the
clock and data lines on the component side of the board. The
clock and data lines should have controlled impedances.
27 www.national.com
ADC081C021/ADC081C027
Physical Dimensions inches (millimeters) unless otherwise noted
6-Lead TSOT
Order Numbers ADC081C021CIMK & ADC081C027CIMK
NS Package Number MK06A
8-Lead MSOP
Order Numbers ADC081C021CIMM
NS Package Number MUA08A
www.national.com 28
ADC081C021/ADC081C027
Notes
29 www.national.com
ADC081C021/ADC081C027
Notes
ADC081C021/ADC081C027 I2C-Compatible, 8-Bit Analog-to-Digital Converter with Alert
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