Low Power, Buffered 24-Bit
Sigma-Delta ADC
Data Sheet AD7791
Rev. A Document Feedback
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FEATURES
Power
Supply: 2.5 V to 5.25 V operation
Normal: 75 μA max
Power-down: 1 μA max
RMS noise: 1.1 μV at 9.5 Hz update rate
19.5-bit p-p resolution (22 bits effective resolution)
Integral nonlinearity: 3.5 ppm typical
Simultaneous 50 Hz and 60 Hz rejection
Internal clock oscillator
Rail-to-rail input buffer
VDD monitor channel
Temperature range: –40°C to +105°C
10-lead MSOP
INTERFACE
3-wire serial
SPI®, QSPI™, MICROWIRE™, and DSP compatible
Schmitt trigger on SCLK
APPLICATIONS
Smart transmitters
Battery applications
Portable instrumentation
Sensor measurement
Temperature measurement
Pressure measurement
Weigh scales
4 to 20 mA loops
FUNCTIONAL BLOCK DIAGRAM
DOUT/RDY
DIN
SCLK
CS
04227-0-001
SERIAL
INTERFACE
CLOCK
-
ADC
AIN(–)
AIN(+)
GND REFIN(+) REFIN(–)
AD7791
V
DD
BUF
V
DD
GND
Figure 1.
GENERAL DESCRIPTION
The AD7791 is a low power, complete analog front end for
low frequency measurement applications. It contains a low
noise 24-bit ∑-Δ ADC with one differential input that can be
buffered or unbuffered.
The device operates from an internal clock. Therefore, the user
does not have to supply a clock source to the device. The output
data rate from the part is software programmable and can be
varied from 9.5 Hz to 120 Hz, with the rms noise equal to
1.1 μV at the lower update rate. The internal clock frequency
can be divided by a factor of 2, 4, or 8, which leads to a reduc-
tion in the current consumption. The update rate, cutoff fre-
frequency, and settling time will scale with the clock frequency.
The part operates with a power supply from 2.5 V to 5.25 V.
When operating from a 3 V supply, the power dissipation for
the part is 225 μW maximum. It is housed in a 10-lead MSOP.
AD7791 Data Sheet
Rev. A | Page 2 of 20
TABLE OF CONTENTS
AD7791—Specifications .................................................................. 3
Timing Characteristics, ................................................................... 5
Absolute Maximum Ratings ............................................................ 7
ESD Caution .................................................................................. 7
Pin Configuration and Function Descriptions ............................. 8
Typical Performance Characteristics ............................................. 9
On-chip Registers ........................................................................... 10
Communications Register (RS1, RS0 = 0, 0) .......................... 10
Status Register (RS1, RS0 = 0, 0;
Power-on/Reset = 0x8C) ........................................................... 11
Mode Register (RS1, RS0 = 0, 1;
Power-on/Reset = 0x02) ............................................................ 11
Filter Register (RS1, RS0 = 1, 0;
Power-on/Reset = 0x04) ............................................................ 12
Data Register (RS1, RS0 = 1, 1;
Power-on/Reset = 0x000000) .................................................... 13
ADC Circuit Information .............................................................. 14
Overview ...................................................................................... 14
Noise Performance ..................................................................... 14
Reduced Current Modes ........................................................... 14
Digital Interface .......................................................................... 15
Single Conversion Mode ....................................................... 16
Continuous Conversion Mode ............................................. 16
Continuous Read Mode ........................................................ 17
Circuit Description......................................................................... 18
Analog Input Channel ............................................................... 18
Bipolar/Unipolar Configuration .............................................. 18
Data Output Coding .................................................................. 18
Reference Input ........................................................................... 18
VDD Monitor ................................................................................ 19
Grounding and Layout .............................................................. 19
Outline Dimensions ....................................................................... 20
Ordering Guide .......................................................................... 20
REVISION HISTORY
3/13—Rev. 0 to Rev. A
Moved ESD Caution Section ............................................................ 7
Changes to Figure 13 ....................................................................... 15
Changes to Reference Input Section ............................................. 18
Updated Outline Dimensions ........................................................ 20
Changes to Ordering Guide ........................................................... 20
8/03—Revision 0: Initial Version
Data Sheet AD7791
Rev. A | Page 3 of 20
AD7791—SPECIFICATIONS1
Table 1. (VDD = 2.5 V to 5.25 V; REFIN(+) = 2.5 V; REFIN(–) = GND; GND = 0 V; CDIV1 = CDIV0 = 0;
all specifications TMIN to TMAX, unless otherwise noted.)
Parameter AD7791B Unit Test Conditions/Comments
ADC CHANNEL SPECIFICATION
Output Update Rate 9.5 Hz min nom
120 Hz max nom
ADC CHANNEL
No Missing Codes2 24 Bits min Update Rate
≤
20 Hz
Resolution 19.5 Bits p-p 9.5 Hz Update Rate
Output Noise 1.1 µV rms typ
Integral Nonlinearity ±15 ppm of FSR max 3.5 ppm typ
Offset Error
±3
µV typ
Offset Error Drift vs. Temperature ±10 nV/°C typ
Full-Scale Error3 ±10 µV typ
Gain Drift vs. Temperature ±0.5 ppm/°C typ
Power Supply Rejection 90 dB min 100 dB typ, AIN = 1 V
ANALOG INPUTS
Differential Input Voltage Ranges ±REFIN V nom REFIN = REFIN(+) – REFIN(–);
Absolute AIN Voltage Limits2 GND + 100 mV V min Buffered Mode of Operation
VDD – 100 mV V max
Analog Input Current Buffered Mode of Operation
Average Input Current2 ±1 nA max
Average Input Current Drift ±5 pA/°C typ
Absolute AIN Voltage Limits2 GND – 30 mV V min Unbuffered Mode of Operation
VDD + 30 mV V max
Analog Input Current Unbuffered Mode of Operation
Input current varies with input voltage.
Average Input Current ±400 nA/V typ
Average Input Current Drift ±50 pA/V/°C typ
Normal Mode Rejection2
@ 50 Hz, 60 Hz 65 dB min 73 dB typ, 50 ± 1 Hz, 60 ± 1 Hz, FS[2:0] = 1004
@ 50 Hz 80 dB min 90 dB typ, 50 ± 1 Hz, FS[2:0] = 1014
@ 60 Hz
80
dB min
90 dB typ, 60 ± 1 Hz, FS[2:0] = 011
4
Common Mode Rejection AIN = 1 V
@DC 90 dB min 100 dB typ, FS[2:0] = 1004
@ 50 Hz, 60 Hz2 100 dB min 50 ± 1 Hz (FS[2:0] = 1014), 60 ± 1 Hz (FS[2:0] = 0114)
REFERENCE INPUT
REFIN Voltage 2.5 V nom REFIN = REFIN(+) – REFIN(–)
Reference Voltage Range2 0.1 V min
V DD V max
Absolute REFIN Voltage Limits2 GND – 30 mV V min
VDD + 30 mV V max
Average Reference Input Current 0.5 µA/V typ
Average Reference Input Current Drift ±0.03 nA/V/°C typ
1 Temperature Range –40°C to +105°C.
2 Specification is not production tested but is supported by characterization data at initial product release.
3 Full-scale error applies to both positive and negative full-scale and applies at the factory calibration conditions (VDD = 4 V).
4 FS[2:0] are the three bits used in the filter register to select the output word rate.
AD7791 Data Sheet
Rev. A | Page 4 of 20
SPECIFICATIONS (continued)1
Parameter AD7791B Unit Test Conditions/Comments
REFERENCE INPUT (continued)
Normal Mode Rejection2
@ 50 Hz, 60 Hz 65 dB min 73 dB typ, 50 ± 1 Hz, 60 ± 1 Hz, FS[2:0] = 1004
@ 50 Hz 80 dB min 90 dB typ, 50 ± 1 Hz, FS[2:0] = 1014
@ 60 Hz 80 dB min 90 dB typ, 60 ± 1 Hz, FS[2:0] = 0114
Common Mode Rejection AIN = 1 V
@ DC 100 dB typ FS[2:0] = 1004
@ 50 Hz, 60 Hz 110 dB typ 50 ± 1 Hz (FS[2:0] = 1014), 60 ± 1 Hz (FS[2:0] = 0114)
LOGIC INPUTS
All Inputs Except SCLK2
VINL, Input Low Voltage 0.8 V max VDD = 5 V
0.4 V max VDD = 3 V
VINH, Input High Voltage 2.0 V min VDD = 3 V or 5 V
SCLK Only (Schmitt-Triggered Input)
2
VT(+) 1.4/2 V min/V max VDD = 5 V
VT(–) 0.8/1.4 V min/V max VDD = 5 V
VT(+) – VT(–) 0.3/0.85 V min/V max VDD = 5 V
VT(+) 0.9/2 V min/V max VDD = 3 V
VT(–) 0.4/1.1 V min/V max VDD = 3 V
VT(+) - VT(–) 0.3/0.85 V min/V max VDD = 3 V
Input Currents ±1 µA max VIN = VDD or GND
Input Capacitance 10 pF typ All Digital Inputs
LOGIC OUTPUTS
VOH, Output High Voltage2 VDD – 0.6 V min VDD = 3 V, ISOURCE = 100 µA
VOL, Output Low Voltage2 0.4 V max VDD = 3 V, ISINK = 100 µA
VOH, Output High Voltage2 4 V min VDD = 5 V, ISOURCE = 200 µA
VOL, Output Low Voltage2 0.4 V max VDD = 5 V, ISINK = 1.6 mA
Floating-State Leakage Current
±1
µA max
Floating-State Output Capacitance 10 pF typ
Data Output Coding Offset Binary
POWER REQUIREMENTS5
Power Supply Voltage
VDD – GND 2.5/5.25 V min/max
Power Supply Currents
IDD Current6 75 µA max 65 µA typ, VDD = 3.6 V, Unbuffered Mode
145 µA max 130 µA typ, VDD = 3.6 V, Buffered Mode
80
µA max
73 µA typ, V
DD
= 5.25 V, Unbuffered Mode
160 µA max 145 µA typ, VDD = 5.25 V, Buffered Mode
IDD (Power-Down Mode) 1 µA max
5 Digital inputs equal to VDD or GND.
6 The current consumption can be further reduced by using the ADC in one of the low power modes (see Table 14).
Data Sheet AD7791
Rev. A | Page 5 of 20
TIMING CHARACTERISTICS1, 2
Table 2. (VDD = 2.5 V to 5.25 V; GND = 0 V, REFIN(+) = 2.5 V, REFIN(–) = GND, CDIV1 = CDIV0 = 0, Input Logic 0 = 0 V,
Input Logic 1 = VDD, unless otherwise noted.)
Parameter
Limit at TMIN, TMAX
(B Version) Unit Conditions/Comments
t3 100 ns min SCLK High Pulsewidth
t4 100 ns min SCLK Low Pulsewidth
Read Operation
t1 0 ns min CS Falling Edge to DOUT/RDY Active Time
60 ns max VDD = 4.75 V to 5.25 V
80 ns max VDD = 2.5 V to 3.6 V
t23 0 ns min SCLK Active Edge to Data Valid Delay4
60 ns max VDD = 4.75 V to 5.25 V
80 ns max VDD = 2.5 V to 3.6 V
t55, 6 10 ns min Bus Relinquish Time after CS Inactive Edge
80 ns max
t6 100 ns max SCLK Inactive Edge to CS Inactive Edge
t7 10 ns min SCLK Inactive Edge to DOUT/RDY High
Write Operation
t8 0 ns min CS Falling Edge to SCLK Active Edge Setup Time4
t9 30 ns min Data Valid to SCLK Edge Setup Time
t10 25 ns min Data Valid to SCLK Edge Hold Time
t11 0 ns min CS Rising Edge to SCLK Edge Hold Time
1 Sample tested during initial release to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of VDD) and timed from a voltage level of 1.6 V.
2 See Figure 3 and Figure 4.
3 These numbers are measured with the load circuit of Figure 2 and defined as the time required for the output to cross the VOL or VOH limits.
4 SCLK active edge is falling edge of SCLK.
5 These numbers are derived from the measured time taken by the data output to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then
extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the times quoted in the timing characteristics are the true bus
relinquish times of the part and, as such, are independent of external bus loading capacitances.
6 RDY returns high after a read of the ADC. In single conversion mode and continuous conversion mode, the same data can be read again, if required, while RDY is high,
although care should be taken to ensure that subsequent reads do not occur close to the next output update. In continuous read mode, the digital word can be read
only once.
AD7791 Data Sheet
Rev. A | Page 6 of 20
04227-0-002
I
SINK
(1.6mA WITH V
DD
= 5V,
100A WITH V
DD
= 3V)
I
SOURCE
(200A WITH V
DD
= 5V,
100A WITH V
DD
= 3V)
1.6V
TO OUTPUT
PIN 50pF
Figure 2. Load Circuit for Timing Characterization
t2
t3
t4
t1t6t5
t7
04227-0-003
CS (I)
DOUT/RDY (O)
SCLK (I)
I = INPUT, O = OUTPUT
MSB LSB
Figure 3. Read Cycle Timing Diagram
04227-0-004
I = INPUT, O = OUTPUT
CS (I)
SCLK (I)
DIN (I) MSB LSB
t8
t9t10
t11
Figure 4. Write Cycle Timing Diagram
Data Sheet AD7791
Rev. A | Page 7 of 20
ABSOLUTE MAXIMUM RATINGS
Table 3. (TA= 25°C, unless otherwise noted.)
Parameter Rating
VDD to GND –0.3 V to +7 V
Analog Input Voltage to GND –0.3 V to VDD + 0.3 V
Reference Input Voltage to GND –0.3 V to VDD + 0.3 V
Total AIN/REFIN Current (Indefinite) 30 mA
Digital Input Voltage to GND –0.3 V to VDD + 0.3 V
Digital Output Voltage to GND –0.3 V to VDD + 0.3 V
Operating Temperature Range –40°C to +105°C
Storage Temperature Range –65°C to +150°C
Maximum Junction Temperature 150°C
MSOP
θJA Thermal Impedance 206°C/W
θJC Thermal Impedance 44°C/W
Lead Temperature, Soldering (10 sec) 300°C
IR Reflow, Peak Temperature 220°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those listed in the operational sections
of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
AD7791 Data Sheet
Rev. A | Page 8 of 20
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
04227-0-005
AD7791
TOP VIEW
(Not to Scale)
SCLK 1
CS 2
AIN(+) 3
AIN(–) 4
REF(+) 5
DIN
DOUT/RDY
VDD
GND
REF(–)
10
9
8
7
6
Figure 5. Pin Configuration
Table 4. Pin Function Descriptions
Pin
No. Mnemonic Function
1 SCLK Serial Clock Input for Data Transfers to and
from the ADC. The SCLK has a Schmitt-
triggered input, making the interface suita-
ble for opto-isolated applications. The serial
clock can be continuous with all data
transmitted in a continuous train of pulses.
Alternatively, it can be a noncontinuous
clock with the information being trans-
mitted to or from the ADC in smaller
batches of data.
2 CS Chip Select Input. This is an active low logic
input used to select the ADC. CS can be
used to select the ADC in systems with
more than one device on the serial bus or as
a frame synchronization signal in communi-
cating with the device. CS can be hardwired
low, allowing the ADC to operate in 3-wire
mode with SCLK, DIN, and DOUT used to
interface with the device.
3 AIN(+) Analog Input. AIN(+) is the positive terminal
of the fully differential analog input.
4 AIN(–) Analog Input. AIN(–) is the negative termi-
nal of the fully differential analog input.
5 REFIN(+) Positive Reference Input. REFIN(+) can lie
anywhere between VDD and GND + 0.1 V.
The nominal reference voltage (REFIN(+) –
REFIN(–)) is 2.5 V, but the part functions with
a reference from 0.1 V to VDD.
Pin
No. Mnemonic Function
6 REFIN(–) Negative Reference Input. This reference
input can lie anywhere between GND and
VDD – 0.1 V.
7
GND
Ground Reference Point.
8 VDD Supply Voltage, 2.5 V to 5.25 V.
9 DOUT/RDY Serial Data Output/Data Ready Output.
DOUT/RDY serves a dual pur
pose . It functions
as a serial data output pin to access the out
put
shift register of the ADC. The output shift reg-
ister can contain data from any of the
on-chip data or control registers. In addition,
DOUT/RDY operates as a data ready pin,
going low to indicate the completion of a
conversion. If the data is not read after the
conversion, the pin will go high before the
next update occurs.
The DOUT/RDY
falling edge can be used as an
interrupt to a processor, indicating that valid
data is available. With an external serial clock,
the data can be read using the DOUT/RDY
pin.
With CS low, the data/control word informa-
tion is placed on the DOUT/RDY pin on the
SCLK falling edge and is valid on the SCLK
rising edge.
The end of a conversion is also indicated by
the RDY bit in the status register. When CS is
high, the DOUT/RDY pin is three-stated but
the RDY bit remains active.
10 DIN Serial Data Input to the Input Shift Register
on the ADC. Data in this shift register is trans-
ferred to the control registers within
the ADC, the register selection bits of the
communications register identifying the
appropriate register.
Data Sheet AD7791
Rev. A | Page 9 of 20
TYPICAL PERFORMANCE CHARACTERISTICS
04227-0-012
–120
–110
–100
–90
–80
–70
–60
–50
–40
–30
–20
–10
0 40 8020 60 100 120 140
dB
160
0
FREQUENCY (Hz)
Figure 6. Frequency Response with 16.6 Hz Update Rate
04227-0-010
0
20
8388591
OCCURENCE
8388619
40
60
80
100
CODE
V
DD
= 3V
V
REF
= 2.048V
9.5Hz UPDATE RATE
T
A
= 25°C
RMS NOISE = 1µV
Figure 7. Noise Distribution Histogram
(CDIV1 = CDIV0 = 0)
04227-0-011
83885910 200 400 600 800
CODE
1000
8388619
READ NO.
VDD = 3V, VREF = 2.048V, 9.5Hz UPDATE RATE
TA = 25°C, RMS NOISE = 1µV
Figure 8. Typical Noise Plot with 16.6 Hz Update Rate
(CDIV1 = CDIV0 = 0)
04227-0-014
0
1
2
3
4
5
6
7
8
8388592
OCCURENCE
8388616
9
CODE
V
DD
= 3V
V
REF
= 2.048V
1.1875Hz UPDATE RATE
T
A
= 25°C
RMS NOISE = 1.25µF
Figure 9. Noise Histogram for Clock Divide by 8 Mode
(CDIV0 = CDIV1 = 1)
04227-0-013
83885920 20 40 60 80
CODE
100
8388616
READ NO.
VDD = 3V, VREF = 2.048V
1.1875Hz UPDATE RATE
TA = 25°C, RMS NOISE = 1.25µF
Figure 10. Noise Plot in Clock Divide by 8 Mode
(CDIV0 = CDIV1 = 1)
04227-0-015
0
0.5
1.0
1.5
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
RMS NOISE (µV)
5.0
3.0
2.5
2.0
VREF (V)
VDD = 5V
UPDATE RATE = 16.6Hz
TA = 25°C
Figure 11. RMS Noise vs. Reference Voltage
AD7791 Data Sheet
Rev. A | Page 10 of 20
ON-CHIP REGISTERS
The ADC is controlled and configured via a number of on-chip registers, which are described on the following pages. In the following
descriptions, set implies a Logic 1 state and cleared implies a Logic 0 state, unless otherwise stated.
COMMUNICATIONS REGISTER (RS1, RS0 = 0, 0)
The communications register is an 8-bit write-only register. All communications to the part must start with a write operation to the
communications register. The data written to the communications register determines whether the next operation is a read or write oper-
ation, and to which register this operation takes place. For read or write operations, once the subsequent read or write operation to the
selected register is complete, the interface returns to where it expects a write operation to the communications register. This is the default
state of the interface and, on power-up or after a reset, the ADC is in this default state waiting for a write operation to the communica-
tions register. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high returns
the ADC to this default state by resetting the entire part. Table 5 outlines the bit designations for the communications register. CR0
through CR7 indicate the bit location, CR denoting the bits are in the communications register. CR7 denotes the first bit of the data
stream. The number in brackets indicates the power-on/reset default status of that bit.
CR7 CR6 CR5 CR4 CR3 CR2 CR1 CR0
WEN(0) 0(0) RS1(0) RS0(0) R/W(0) CREAD(0) CH1(0) CH0(0)
Table 5. Communications Register Bit Designations
Bit Location Bit Name Description
CR7 WEN Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually
occurs. If a 1 is the first bit written, the part will not clock on to subsequent bits in the register. It will stay
at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits
will be loaded to the communications register.
CR6
0
This bit must be programmed to Logic 0 for correct operation.
CR5–CR4 RS1–RS0 Register Address Bits. These address bits are used to select which of the ADC’s registers are being select-
ed during this serial interface communication. See Table 6.
CR3 R/W A 0 in this bit location indicates that the next operation will be a write to a specified register. A 1 in this
position indicates that the next operation will be a read from the designated register.
CR2 CREAD Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the
serial interface is configured so that the data register can be continuously read, i.e., the contents of the
data register are placed on the DOUT pin automatically when the SCLK pulses are applied. The commu-
nications register does not have to be written to for data reads. To enable continuous read mode, the
instruction 001111XX must be written to the communications register. To exit the continuous read
mode, the instruction 001110XX must be written to the communications register while the RDY pin is
low. While in continuous read mode, the ADC monitors activity on the DIN line so that it can receive the
instruction to exit continuous read mode. Additionally, a reset will occur if 32 consecutive 1s are seen on
DIN. Therefore, DIN should be held low in continuous read mode until an instruction is to be written to
the device.
CR1–CR0 CH1–CH0 These bits are used to select the analog input channel. The differential channel can be selected
(AIN(+)/AIN(–)) or an internal short (AIN(–)/AIN(–)) can be selected. Alternatively, the power supply can
be selected, i.e., the ADC can measure the voltage on the power supply, which is useful for monitoring
power supply variation. The power supply voltage is divided by 5 and then applied to the modulator for
conversion. The ADC uses a 1.17 V ± 5% on-chip reference as the reference source for the analog to digi-
tal conversion. Any change in channel resets the filter and a new conversion is started.
Table 6. Register Selection
RS1 RS0 Register Register Size
0 0 Communications Register
during a Write Operation
8-Bit
0 0 Status Register during a
Read Operation
8-Bit
0
1
Mode Register
8-Bit
1 0 Filter Register 8-Bit
1 1 Data Register 24-Bit
Table 7. Channel Selection
CH1 CH0 Channel
0 0 AIN(+) – AIN(–)
0 1 Reserved
1 0 AIN(–) – AIN(–)
1 1 VDD Monitor
Data Sheet AD7791
Rev. A | Page 11 of 20
STATUS REGISTER (RS1, RS0 = 0, 0; POWER-ON/RESET = 0x8C)
The status register is an 8-bit read-only register. To access the ADC status register, the user must write to the communications register,
select the next operation to be a read, and load bits RS1 and RS0 with 0. Table 8 outlines the bit designations for the status register. SR0
through SR7 indicate the bit locations, SR denoting the bits are in the status register. SR7 denotes the first bit of the data stream. The
number in brackets indicates the power-on/reset default status of that bit.
SR7 SR6 SR5 SR4 SR3 SR2 SR1 SR0
RDY(1) ERR(0) 0(0) 0(0) 1(1) WL(1) CH1(0) CH0(0)
Table 8. Status Register Bit Designations
Bit Location Bit Name Description
SR7 RDY Ready bit for ADC. Cleared when data is written to the ADC data register. The RDY bit is set automatically
after the ADC data register has been read or a period of time before the data register is updated with a
new conversion result to indicate to the user not to read the conversion data. It is also set when the part
is placed in power-down mode. The end of a conversion is indicated by the DOUT/RDY pin also. This pin
can be used as an alternative to the status register for monitoring the ADC for conversion data.
SR6 ERR ADC Error Bit. This bit is written to at the same time as the RDY bit. Set to indicate that the result written
to the ADC data register has been clamped to all 0s or all 1s. Error sources include overrange, under-
range. Cleared by a write operation to start a conversion.
SR5 0 This bit is automatically cleared.
SR4 0 This bit is automatically cleared.
SR3 1 This bit is automatically set.
SR2 1 This bit is automatically set if the device is an AD7791. It can be used to distinguish between the AD7791
and AD7790, in which the bit is cleared.
SR1–SR0 CH1–CH0 These bits indicate which channel is being converted by the ADC.
MODE REGISTER (RS1, RS0 = 0, 1; POWER-ON/RESET = 0x02)
The mode register is an 8-bit register from which data can be read or to which data can be written. This register is used to configure the
ADC for unipolar or bipolar mode, enable or disable the buffer, or place the device into power-down mode. Table 9 outlines the bit desig-
nations for the mode register. MR0 through MR7 indicate the bit locations, MR denoting the bits are in the mode register. MR7 denotes
the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit. Any write to the setup regis-
ter resets the modulator and filter and sets the RDY bit.
MR7 MR6 MR5 MR4 MR3 MR2 MR1 MR0
MD1(0) MD0(0) 0(0) 0(0) BO(0) U/B(0) BUF(1) 0(0)
Table 9. Mode Register Bit Designations
Bit Location Bit Name Description
MR7–MR6 MD1–MD0 Mode Select Bits. These bits select between continuous conversion mode, single conversion mode, and
standby mode. In continuous conversion mode, the ADC continuously performs conversions and places
the result in the data register. RDY goes low when a conversion is complete. The user can read these
conversions by placing the device in continuous read mode whereby the conversions are automatically
placed on the DOUT line when SCLK pulses are applied. Alternatively, the user can instruct the ADC to
output the conversion by writing to the communications register. After power-on, the first conversion is
available after a period 2/ fADC while subsequent conversions are available at a frequency of fADC. In single
conversion mode, the ADC is placed in power-down mode when conversions are not being performed.
When single conversion mode is selected, the ADC powers up and performs a single conversion, which
occurs after a period 2/fADC. The conversion result in placed in the data register, RDY goes low, and the
ADC returns to power-down mode. The conversion remains in the data register and RDY remains active
(low) until the data is read or another conversion is performed. See Table 10.
AD7791 Data Sheet
Rev. A | Page 12 of 20
Bit Location Bit Name Description
MR5–MR4 0 This bit must be programmed with a Logic 0 for correct operation.
MR3
BO
Burnout Current Enable Bit. When this bit is set to 1 by the user, the 100 nA current sources in the signal
path are enabled. When BO = 0, the burnout currents are disabled. The burnout currents can be enabled
only when the buffer is active.
MR2 U/B Unipolar/Bipolar Bit. Set by user to enable unipolar coding, i.e., zero differential input will result in
0x000000 output and a full-scale differential input will result in 0xFFFFFF output. Cleared by the user to
enable bipolar coding. Negative full-scale differential input will result in an output code of 0x000000,
zero differential input will result in an output code of 0x800000, and a positive full-scale differential input
will result in an output code of 0xFFFFFF.
MR1 BUF Configures the ADC for buffered or unbuffered mode of operation. If cleared, the ADC operates in un-
buffered mode, lowering the power consumption of the device. If set, the ADC operates in buffered
mode, allowing the user to place source impedances on the front end without contributing gain errors
to the system.
MR0 0 This bit must be programmed with a Logic 0 for correct operation.
Table 10. Operating Modes
MD1 MD0 Mode
0 0 Continuous Conversion Mode (De-
fault)
0 1 Reserved
1 0 Single Conversion Mode
1 1 Power-Down Mode
FILTER REGISTER (RS1, RS0 = 1, 0; POWER-ON/RESET = 0x04)
The filter register is an 8-bit register from which data can be read or to which data can be written. This register is used to set the output
word rate. Table 11 outlines the bit designations for the filter register. FR0 through FR7 indicate the bit locations, FR denoting the bits are in
the filter register. FR7 denotes the first bit of the data stream. The number in brackets indicates the power-on/reset default status of that bit.
FR7 FR6 FR5 FR4 FR3 FR2 FR1 FR0
0(0)
0(0)
CDIV1(0)
CDIV0(0)
0(0)
FS2(1)
FS1(0)
FS0(0)
Table 11. Filter Register Bit Designations
Bit Location Bit Name Description
FR7–FR6 0 These bits must be programmed with a Logic 0 for correct operation.
FR5–FR4 CLKDIV1–
CDIV0
These bits are used to operate the AD7791 in the lower power modes. The clock is internally divided and
the power is reduced. In the low power modes, the update rates will scale with the clock frequency so
that dividing the clock by 2 causes the update rate to be reduced by a factor of 2 also.
00 Normal Mode
01 Clock Divided by 2
10 Clock Divided by 4
11 Clock Divided by 8
FR3 0 This bit must be programmed with a Logic 0 for correct operation.
FR2–FR0 FS2–FS0
These bits set the output word rate of the ADC. The update rate influences the 50 Hz/60 Hz rejection and
the noise. See Table 12, which shows the allowable update rates when normal power mode is used. In
the low power modes, the update rate is scaled with the clock frequency. For example, if the internal
clock is divided by a factor of 2, the corresponding update rates will be divided by 2 also.
Data Sheet AD7791
Rev. A | Page 13 of 20
Table 12. Update Rates
FS2 FS1 FS0 fADC (Hz) f3dB (Hz) RMS Noise (µV) Rejection
0 0 0 120 28 40 25 dB @ 60 Hz
0 0 1 100 24 25 25 dB @ 50 Hz
0 1 0 33.3 8 3.36
0 1 1 20 4.7 1.6 80 dB @ 60 Hz
1 0 0 16.6 4 1.5 65 dB @ 50 Hz/60 Hz (Default Setting)
1 0 1 16.7 4 1.5 80 dB @ 50 Hz
1 1 0 13.3 3.2 1.2
1 1 1 9.5 2.3 1.1 67 dB @ 50/60 Hz
DATA REGISTER (RS1, RS0 = 1, 1; POWER-ON/RESET = 0x000000)
The conversion result from the ADC is stored in this data register. This is a read-only register. On completion of a read operation from
this register, the RDY bit/pin is set.
AD7791 Data Sheet
Rev. A | Page 14 of 20
ADC CIRCUIT INFORMATION
OVERVIEW
The AD7791 is a low power ADC that incorporates a ∑-Δ mod-
ulator, a buffer and on-chip digital filtering intended for the
measurement of wide dynamic range, low frequency signals
such as those in pressure transducers, weigh scales, and temper-
ature measurement applications.
The part has one differential input that can be buffered or
unbuffered. Buffering the input channel means that the part can
accommodate significant source impedances on the analog
input and that R, C filtering (for noise rejection or RFI reduc-
tion) can be placed on the analog input, if required. The device
requires an external reference of 2.5 nominal. Figure 12 shows
the basic connections required to operate the part.
04227-0-006
IN+
10F0.1F
IN–
OUT–
POWER
SUPPLY
OUT+
REFIN(+)
CS
DOUT/RDY
SCLK
V
DD
GND
AIN(+)
AIN(–)
REFIN(–)
AD7791
MICROCONTROLLER
Figure 12. Basic Connection Diagram
The output rate of the AD7791 (fADC) is user programmable
with the settling time equal to 2 × tADC. Normal mode rejection
is the major function of the digital filter. Table 12 lists the avail-
able output rates from the AD7791. Simultaneous 50 Hz and
60 Hz rejection is optimized when the update rate equals
16.6 Hz as notches are placed at both 50 Hz and 60 Hz with this
update rate (see Figure 6).
NOISE PERFORMANCE
Table 13 shows the output rms noise, rms resolution, and peak-
to-peak resolution (rounded to the nearest 0.5 LSB) for the
different update rates and input ranges for the AD7791. The
numbers given are for the bipolar input range with a reference
of 2.5 V. These numbers are typical and generated with a differ-
ential input voltage of 0 V. The peak-to-peak resolution figures
represent the resolution for which there will be no code flicker
within a six-sigma limit. The output noise comes from two
sources. The first is the electrical noise in the semiconductor
devices (device noise) used in the implementation of the modu-
lator. The second is quantization noise, which is added when
the analog input is converted into the digital domain. The de-
vice noise is at a low level and is independent of frequency. The
quantization noise starts at an even lower level but rises rapidly
with increasing frequency to become the dominant noise
source.
Table 13. Typical Peak-to-Peak Resolution (Effective
Resolution) vs. Update Rate
Update
Rate
Peak-toPeak Resolu-
tion
Effective Resolu-
tion
9.5 19.5 22
13.3 19 21.5
16.7 19 21.5
16.6 19 21.5
20 18.5 21
33.3 17.5 20
100 14.5 17
120 14 16.5
REDUCED CURRENT MODES
The AD7791 has a current consumption of 160 μA maximum
when operated with a 5 V power supply, the buffer enabled, and
the clock operating at its maximum speed. The clock frequency
can be divided by a factor of 2, 4, or 8 before being applied to
the modulator and filter, resulting in a reduction in the current
consumption of the AD7791. Bits CDIV1 and CDIV0 in the filter
register are used to enter these low power modes (see Table 14).
When the internal clock is reduced, the update rate will also be
reduced. For example, if the filter bits are set to give an update
rate of 16.6 Hz when the AD7791 is operated in full power
mode, the update rate will equal 8.3 Hz in divide by 2 mode. In
the low power modes, there may be some degradation in the
ADC performance.
Table 14. Low Power Mode Selection
CDIV[1:0] Clock Typ Current, Buffered (μA) Typ Current, Unbuffered (μA) 50 Hz/60 Hz Rejection (dB)
00 1 146 75 65
10 1/2 87 45 64
10 1/4 56 30 75
11 1/8 41 25 86
Data Sheet AD7791
Rev. A | Page 15 of 20
DIGITAL INTERFACE
As previously outlined, the AD7791’s programmable functions
are controlled using a set of on-chip registers. Data is written to
these registers via the part’s serial interface and read access to
the on-chip registers is also provided by this interface. All
communications with the part must start with a write to the
communications register. After power-on or reset, the device
expects a write to its communications register. The data written
to this register determines whether the next operation is a read
operation or a write operation and also determines to which
register this read or write operation occurs. Therefore, write
access to any of the other registers on the part begins with a
write operation to the communications register followed by a
write to the selected register. A read operation from any other
register (except when continuous read mode is selected) starts
with a write to the communications register followed by a read
operation from the selected register.
The AD7791’s serial interface consists of four signals: CS, DIN,
SCLK, and DOUT/RDY. The DIN line is used to transfer data
into the on-chip registers while DOUT/RDY is used for access-
ing from the on-chip registers. SCLK is the serial clock input for
the device and all data transfers (either on DIN or DOUT/RDY)
occur with respect to the SCLK signal. The DOUT/ RDY pin
operates as a Data Ready signal also, the line going low when a
new data-word is available in the output register. It is reset high
when a read operation from the data register is complete. It also
goes high prior to the updating of the data register to indicate
when not to read from the device to ensure that a data read is
not attempted while the register is being updated. CS is used to
select a device. It can be used to decode the AD7791 in systems
where several components are connected to the serial bus.
Figure 3 and Figure 4 show timing diagrams for interfacing to
the AD7791 with CS being used to decode the part. Figure 3
shows the timing for a read operation from the AD7791’s out-
put
shift register while Figure 4 shows the timing for a write opera-
tion to the input shift register. In all modes except continuous
read mode, it is possible to read the same word from the data
register several times even though the DOUT/RDY line returns
high after the first read operation. However, care must be taken
to ensure that the read operations have been completed before
the next output update occurs. In continuous read mode, the
data register can be read only once.
The serial interface can operate in 3-wire mode by tying CS low.
In this case, the SCLK, DIN, and DOUT/RDY lines are used to
communicate with the AD7791. The end of the conversion can
be monitored using the RDY bit in the status register. This
scheme is suitable for interfacing to microcontrollers. If CS is
required as a decoding signal, it can be generated from a port
pin. For microcontroller interfaces, it is recommended that
SCLK idles high between data transfers.
The AD7791 can be operated with CS being used as a frame
synchronization signal. This scheme is useful for DSP interfac-
es. In this case, the first bit (MSB) is effectively clocked out by
CS since CS would normally occur after the falling edge of
SCLK in DSPs. The SCLK can continue to run between data
transfers, provided the timing numbers are obeyed.
The serial interface can be reset by writing a series of 1s on the
DIN input. If a Logic 1 is written to the AD7791 line for at least
32 serial clock cycles, the serial interface is reset. This ensures
that in 3-wire systems, the interface can be reset to a known
state if the interface gets lost due to a software error or some
glitch in the system. Reset returns the interface to the state in
which it is expecting a write to the communications register.
This operation resets the contents of all registers to their power-
on values.
The AD7791 can be configured to continuously convert or to
perform a single conversion. See Figure 13 through Figure 15.
04227-0-007
DIN
SCLK
DOUT/RDY
CS
0x10 0x380x82
DATA
Figure 13. Single Conversion
AD7791 Data Sheet
Rev. A | Page 16 of 20
Single Conversion Mode
In single conversion mode, the AD7791 is placed in shutdown
mode between conversions. When a single conversion is initiat-
ed by setting MD1 to 1 and MD0 to 0 in the mode register, the
AD7791 powers up, performs a single conversion, and then
returns to shutdown mode. A conversion will require a time
period of 2 × tADC. DOUT/RDY goes low to indicate the com-
pletion of a conversion. When the data-word has been read
from the data register, DOUT/RDY will go high. If CS is low,
DOUT/RDY will remain high until another conversion is initi-
ated and completed. The data register can be read several times,
if required, even when DOUT/ RDY has gone high.
Continuous Conversion Mode
This is the default power-up mode. The AD7791 will continu-
ously convert, the RDY pin in the status register going low each
time a conversion is complete. If CS is low, the DOUT/RDY line
will also go low when a conversion is complete. To read a con-
version, the user can write to the communications register,
indicating that the next operation is a read of the data register.
The digital conversion will be placed on the DOUT/RDY pin as
soon as SCLK pulses are applied to the ADC. DOUT/RDY will
return high when the conversion is read. The user can read this
register additional times, if required. However, the user must
ensure that the data register is not being accessed at the comple-
tion of the next conversion or else the new conversion word will
be lost.
04227-0-009
DIN
SCLK
DOUT/RDY
CS
0x38 0x38
DATA DATA
Figure 14. Continuous Conversion
Data Sheet AD7791
Rev. A | Page 17 of 20
Continuous Read Mode
Rather than write to the communications register each time a
conversion is complete to access the data, the AD7791 can be
placed in continuous read mode. By writing 001111XX to the
communications register, the user needs only to apply the
appropriate number of SCLK cycles to the ADC and the 24-bit
word will automatically be placed on the DOUT/RDY line
when a conversion is complete.
When DOUT/RDY goes low to indicate the end of a conver-
sion, sufficient SCLK cycles must be applied to the ADC and
the data conversion will be placed on the DOUT/RDY line.
When the conversion is read, DOUT/RDY will return high
until the next conversion is available. In this mode, the data can
be read only once. Also, the user must ensure that the data-
word is read
before the next conversion is complete. If the user has not read
the conversion before the completion of the next conversion or
if insufficient serial clocks are applied to the AD7791 to read
the word, the serial output register is reset when the next con-
version is complete and the new conversion is placed in the
output serial register.
To exit the continuous read mode, the instruction 001110XX
must be written to the communications register while the RDY
pin is low. While in the continuous read mode, the ADC moni-
tors activity on the DIN line so that it can receive the
instruction to exit the continuous read mode. Additionally, a
reset will occur if 32 consecutive 1s are seen on DIN. Therefore,
DIN should be held low in continuous read mode until an in-
struction is to be written to the device.
04227-0-008
DIN
SCLK
DOUT/RDY
CS
0x3C
DATA DATA DATA
Figure 15. Continuous Read
AD7791 Data Sheet
Rev. A | Page 18 of 20
CIRCUIT DESCRIPTION
ANALOG INPUT CHANNEL
The AD7791 has one differential analog input channel. This is
connected to the on-chip buffer amplifier when the device is
operated in buffered mode and directly to the modulator when
the device is operated in unbuffered mode. In buffered mode
(the BUF bit in the mode register is set to 1), the input channel
feeds into a high impedance input stage of the buffer amplifier.
Therefore, the input can tolerate significant source impedances
and is tailored for direct connection to external resistive-type
sensors such as strain gauges or resistance temperature detec-
tors (RTDs).
When BUF = 0, the part is operated in unbuffered mode. This
results in a higher analog input current. Note that this
unbuffered input path provides a dynamic load to the driving
source. Therefore, resistor/capacitor combinations on the input
pins can cause dc gain errors, depending on the output
impedance of the source that is driving the ADC input. Table 15
shows the allowable external resistance/capacitance values for
unbuffered mode such that no gain error at the 20-bit level is
introduced.
Table 15. External R-C Combination for No 20-Bit Gain Error
C (pF) R (Ω)
50 16.7K
100 9.6K
500 2.2K
1000 1.1K
5000 160
The absolute input voltage range in buffered mode is restricted
to a range between GND + 100 mV and VDD – 100 mV. Care
must be taken in setting up the common-mode voltage so that
these limits are not exceeded. Otherwise, there will be degrada-
tion in linearity and noise performance.
The absolute input voltage in unbuffered mode includes the
range between GND – 30 mV and VDD + 30 mV as a result of
being unbuffered. The negative absolute input voltage limit does
allow the possibility of monitoring small true bipolar signals
with respect to GND.
BIPOLAR/UNIPOLAR CONFIGURATION
The analog input to the AD7791 can accept either unipolar or
bipolar input voltage ranges. A bipolar input range does not
imply that the part can tolerate negative voltages with respect to
system GND. Unipolar and bipolar signals on the AIN(+) input
are referenced to the voltage on the AIN(–) input. For example,
if AIN(–) is 2.5 V and the ADC is configured for unipolar
mode, the input voltage range on the AIN(+) pin is 2.5 V t o 5 V.
If the ADC is configured for bipolar mode, the analog input
range on the AIN(+) input is 0 V to 5 V. The bipolar/unipolar
option is chosen by programming the B/U bit in the mode
register.
DATA OUTPUT CODING
When the ADC is configured for unipolar operation, the output
code is natural (straight) binary with a zero differential input
voltage resulting in a code of 00...00, a midscale voltage
resulting in a code of 100...000, and a full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
Code = 2N × (AIN/VREF)
When the ADC is configured for bipolar operation, the output
code is offset binary with a negative full-scale voltage resulting
in a code of 000...000, a zero differential input voltage resulting
in a code of 100...000, and a positive full-scale input voltage
resulting in a code of 111...111. The output code for any analog
input voltage can be represented as
Code = 2N – 1 × [(AIN/VREF) + 1]
where AIN is the analog input voltage and N = 24.
REFERENCE INPUT
The AD7791 has a fully differential input capability for the
channel. The common-mode range for these differential inputs
is from GND to VDD. The reference input is unbuffered and,
therefore, excessive R-C source impedances will introduce gain
errors. The reference voltage REFIN (REFIN(+) – REFIN(–)) is
2.5 V nominal, but the AD7791 is functional with reference
voltages from 0.1 V to VDD. In applications where the excitation
(voltage or current) for the transducer on the analog input also
drives the reference voltage for the part, the effect of the low
frequency noise in the excitation source will be removed
because the application is ratiometric. If the AD7791 is used
in a nonratiometric application, a low noise reference should
be used.
Recommended 2.5 V reference voltage sources for the AD7791
include the ADR381 and ADR391, which are low noise, low
power references. In a system that operates from a 2.5 V power
supply, the reference voltage source will require some head-
room. In this case, a 2.048 V reference such as the ADR380 can
be used, requiring only 300 mV of headroom. Also note that the
reference inputs provide a high impedance, dynamic load.
Because the input impedance of each reference input is
dynamic, resistor/ capacitor combinations on these inputs can
cause dc gain errors, depending on the output impedance of the
source that is driving the reference inputs.
Data Sheet AD7791
Rev. A | Page 19 of 20
Reference voltage sources like those recommended above
(e.g., ADR391) will typically have low output impedances and
are, therefore, tolerant to having decoupling capacitors on
REFIN(+) without introducing gain errors in the system.
Deriving the reference input voltage across an external resistor
will mean that the reference input sees a significant external
source impedance. External decoupling on the REFIN pins
would not be recommended in this type of circuit
configuration.
VDD MONITOR
Along with converting external voltages, the analog input chan-
nel can be used to monitor the voltage on the VDD pin. When
the CH1 and CH0 bits in the communications register are set to
1, the voltage on the VDD pin is internally attenuated by 5 and
the resultant voltage is applied to the ∑-∆ modulator using an
internal 1.17 V reference for analog to digital conversion. This
is useful because variations in the power supply voltage can be
monitored.
GROUNDING AND LAYOUT
Since the analog inputs and reference inputs of the ADC are
differential, most of the voltages in the analog modulator are
common-mode voltages. The excellent common-mode rejec-
tion of the part will remove common-mode noise on these
inputs. The digital filter will provide rejection of broadband
noise on the power supply, except at integer multiples of the
modulator sampling frequency. The digital filter also removes
noise from the analog and reference inputs, provided that these
noise sources do not saturate the analog modulator. As a result,
the AD7791 is more immune to noise interference than a con-
ventional high resolution converter. However, because the
resolution of the AD7791 is so high, and the noise levels from
the AD7791 are so low, care must be taken with regard to
grounding and layout.
The printed circuit board that houses the AD7791 should be
designed such that the analog and digital sections are separated
and confined to certain areas of the board. A minimum etch
technique is generally best for ground planes because it gives
the best shielding.
It is recommended that the AD7791’s GND pin be tied to the
AGND plane of the system. In any layout, it is important that
the user keep in mind the flow of currents in the system, ensur-
ing that the return paths for all currents are as close as possible
to the paths the currents took to reach their destinations. Avoid
forcing digital currents to flow through the AGND sections of
the layout.
The AD7791’s ground plane should be allowed to run under the
AD7791 to prevent noise coupling. The power supply lines to
the AD7791 should use as wide a trace as possible to provide
low impedance paths and reduce the effects of glitches on the
power supply line. Fast switching signals such as clocks should
be shielded with digital ground to avoid radiating noise to other
sections of the board, and clock signals should never be run
near the analog inputs. Avoid crossover of digital and analog
signals. Traces on opposite sides of the board should run at
right angles to each other. This will reduce the effects of feed-
through through the board. A microstrip technique is by far the
best, but it is not always possible with a double-sided board. In
this technique, the component side of the board is dedicated to
ground planes, while signals are placed on the solder side.
Good decoupling is important when using high resolution
ADCs. VDD should be decoupled with 10 µF tantalum in parallel
with 0.1 µF capacitors to GND. To achieve the best from these
decoupling components, they should be placed as close as
possible to the device, ideally right up against the device. All
logic chips should be decoupled with 0.1 µF ceramic capacitors
to DGND.
AD7791 Data Sheet
Rev. A | Page 20 of 20
OUTLINE DIMENSIONS
COMPLIANT TO JEDEC STANDARDS MO-187-BA
091709-A
6°
0°
0.70
0.55
0.40
5
10
1
6
0.50 BSC
0.30
0.15
1.10 MAX
3.10
3.00
2.90
COPLANARITY
0.10
0.23
0.13
3.10
3.00
2.90
5.15
4.90
4.65
PIN 1
IDENTIFIER
15° MAX
0.95
0.85
0.75
0.15
0.05
Figure 16. 10-Lead Mini Small Outline Package [MSOP]
(RM-10)
Dimensions shown in millimeters
ORDERING GUIDE
Model1 Temperature Range Package Description Package Option Branding
AD7791BRM –40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COT
AD7791BRMZ –40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 C44
AD7791BRM-REEL –40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 COT
AD7791BRMZ-REEL –40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 C44
EVAL-AD7791EBZ Evaluation Board
1 Z = RoHS Compliant Part.
©2004–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D04227-0-3/13(A)
