Not Recommended for New Designs
This product was manufactured for Maxim by an outside wafer foundry
using a process that is no longer available. It is not recommended for
new designs. The data sheet remains available for existing users.
A Maxim replacement or an industry second-source m a y be available.
Please see the QuickView data sheet for this part or contact technical
support for assistance.
For further information, contact Maxim’s Applications Tech Support.
General Description
The MX536A and MX636 are true RMS-to-DC convert-
ers. They feature low power and are designed to accept
low-level input signals from 0 to 7VRMS for the MX536A
and 0 to 200mVRMS for the MX636. Both devices accept
complex input waveforms containing AC and DC com-
ponents. They can be operated from either a single sup-
ply or dual supplies. Both devices draw less than 1mA
of quiescent supply current, making them ideal for bat-
tery-powered applications.
Input and output offset, positive and negative waveform
symmetry (DC reversal), and full-scale accuracy are
laser trimmed, so that no external trims are required to
achieve full rated accuracy.
________________________Applications
Digital Multimeters
Battery-Powered Instruments
Panel Meters
Process Control
____________________________Features
♦True RMS-to-DC Conversion
♦Computes RMS of AC and DC Signals
♦Wide Response:
2MHz Bandwidth for VRMS > 1V (MX536A)
1MHz Bandwidth for VRMS > 100mV (MX636)
♦Auxiliary dB Output: 60dB Range (MX536A)
50dB Range (MX636)
♦Single- or Dual-Supply Operation
♦Low Power: 1.2mA typ (MX536A)
800µA typ (MX636)
MX536A/MX636
True RMS-to-DC Converters
________________________________________________________________
Maxim Integrated Products
1
14
13
12
11
10
9
8
1
2
3
4
5
6
7
+VS
N.C.
N.C.
N.C.
COMMON
RL
IOUT
VIN
N.C.
-VS
CAV
dB
BUF OUT
BUF IN
MX536A
MX636
DIP
TOP VIEW
MX536A
MX636B
TO-100
8
9
10
1
2
3
456
7
BUF IN
BUF OUT
dB
CAV
-VS
+VS
IOUT
RL
COMMON
VIN
Pin Configurations
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
-VS
+VS
VOUT
CAV
_________Typical Operating Circuits
19-0824; Rev 2; 3/96
PART
MX536AJC/D
MX536AJCWE
MX536AJD 0°C to +70°C
0°C to +70°C
0°C to +70°C
TEMP. RANGE PIN-PACKAGE
Dice**
16 Wide SO
14 Ceramic
Ordering Information
Ordering Information continued at end of data sheet.
*
Maxim reserves the right to ship ceramic packages in lieu of
CERDIP packages.
**
Dice are specified at TA= +25°C.
MX536AJH
MX536AJN 0°C to +70°C
0°C to +70°C 10 TO-100
14 Plastic DIP
MX536AJQ*
MX536AKCWE 0°C to +70°C
0°C to +70°C 14 CERDIP
16 Wide SO
MX536AKD
MX536AKH 0°C to +70°C
0°C to +70°C 14 Ceramic
10 TO-100
MX536AKN 0°C to +70°C 14 Plastic DIP
Pin Configurations continued at end of data sheet. Typical Operating Circuits continued at end of data sheet.
MX536AKQ* 0°C to +70°C 14 CERDIP
MX536ASD -55°C to +125°C 14 Ceramic
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800.
For small orders, phone 408-737-7600 ext. 3468.
MX536A/MX636
True RMS-to-DC Converters
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS—MX536A
(TA= +25°C, +VS= +15V, -VS= -15V, unless otherwise noted.)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage: Dual Supplies (MX536A)............................±18V
(MX636) .............................±12V
Single Supply (MX536A)...........................+36V
(MX636) .............................+24V
Input Voltage (MX536A).......................................................±25V
(MX636).........................................................±12V
Power Dissipation (Package)
Plastic DIP (derate 12mW/°C above +75°C) ...............450mW
Small Outline (derate 10mW/°C above +75°C)............400mW
Ceramic (derate 10mW/°C above +75°C) ...................500mW
TO-100 metal can (derate 7mW/°C above +75°C)......450mW
Output Short-Circuit Duration........................................Indefinite
Operating Temperature Ranges
Commercial (J, K)...............................................0°C to +70°C
Military (S)......................................................-55°C to +125°C
Storage Temperature Range.............................-55°C to +150°C
Lead Temperature (soldering, 10sec)................................300°C
MX536AJ, AS
TMIN to +70°C
+70°C to +125°C
MHz2.3
±3dB Bandwidth 450 kHz
90
120
Bandwidth for 1%
Additional Error (0.09dB) 45 kHz
5
-1.0
Additional Error -0.1 % of
Reading
Specified Accuracy
±2 ±0.1
Total Error, External Trim
(Note 1) mV ±% of
Reading
±3 ±0.3
±2 ±0.2
Total Error, Internal Trim (Note 1) mV ±% of
Reading
±5 ±0.5
VOUT = [avg. (VIN)2] 1/2
Transfer Equation
±0.1
Total Error vs. DC Reversal % of
Reading
±0.2
mV ±% of
Reading/V
±0.1 ±0.01Total Error vs. Supply
Total Error vs. Temperature mV ±% of
Reading/°C
±0.1 ±0.01
±0.05 ±0.005
±0.1 ±0.005
±0.03 ±0.005
UNITS
MIN TYP MAX
PARAMETER
VIN = 1V
VIN = 100mV
VIN = 10mV
MX536AK
VIN = 1V
VIN = 100mV
MX536AJ, AS
VIN = 10mV
Crest Factor = 7
Crest Factor = 3
Crest Factor 1 to 2
MX536AJ
MX536AK
MX536AS
MX536AK
MX536AS
MX536AJ, AS
CONDITIONS
MX536AK
Figure 3 ms/µF CAV
25Averaging Time Constant
CONVERSION ACCURACY
ERROR vs. CREST FACTOR (Note 2)
FREQUENCY RESPONSE (Note 3)
MX536A/MX636
True RMS-to-DC Converters
_______________________________________________________________________________________ 3
UNITS
MIN TYP MAX
CONDITIONSPARAMETER
VRMS
0 to 7
±15V Supplies
Continuous RMS Peak Transient ±20 VPK
0 to 2 VRMS
Input Signal Range ±5V Supplies
Continuous RMS Peak Transient ±7 VPK
Safe Input All Supplies ±25 VPK
Input Resistance 13.33 16.7 20.00 kΩ
MX536AJ, AS 0.8 ±2 mVInput Offset Voltage MX536AK 0.5 ±1
MX536AJ ±1 ±2
MX536AK ±0.5 ±1TA= +25°C MX536AS ±2 mV
MX536AJ, AK ±0.1
TA= TMIN to TMAX MX536AS ±0.2 mV/°C
MX536AJ, AK ±0.1
Offset Voltage
Supply Voltage MX536AS ±0.2 mV/V
±15V Supplies 0 to 11 12.5
Output Voltage Swing ±5V Supplies 0 to 2 V
Source 5 mA
Output Current Sink -130 µA
Short Circuit Current 20 mA
Output Resistance 0.5 Ω
MX536AJ ±0.4 ±0.6 dBMX536AK ±0.2 ±0.3Error VIN = 7mV to 7VRMS,
0dB = 1VRMS MX536AS ±0.5 ±0.6
Scale Factor -3 mV/dB
Scale Factor TC (Uncompensated) 0.33 % of
Reading/°C
ELECTRICAL CHARACTERISTICS—MX536A (continued)
(TA= +25°C, +VS= +15V, -VS= -15V, unless otherwise noted.)
IREF 0dB = 1VRMS 520 80 µA
IREF Range 1 100 µA
IOUT Scale Factor 40 µA/VRMS
IOUT Scale Factor Tolerance ±10 ±20 %
Output Resistance 20 25 30 kΩ
Voltage Compliance -VSto
(+VS- 2.5) V
INPUT CHARACTERISTICS
OUTPUT CHARACTERISTICS
dB OUTPUT
IOUT TERMINAL
MX536A/MX636
True RMS-to-DC Converters
4 _______________________________________________________________________________________
Input Bias Current 20 300 nA
Input Resistance 108Ω
Output Current Source +5 mA
Sink -130 µA
Short-Circuit Current 20 mA
UNITS
MIN TYP MAX
CONDITIONSPARAMETER
Input and Output Voltage Range -VSto
(+VS- 2.5) V
Input Offset Voltage RS= 25kΩ
Small-Signal Bandwidth
±0.5 ±4 mV
1 MHz
Slew Rate (Note 4) 5 V/µs
ELECTRICAL CHARACTERISTICS—MX536A (continued)
(TA= +25°C, +VS= +15V, -VS= -15V, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS—MX636
(TA= +25°C, +VS= +3V, -VS= -5V, unless otherwise noted.)
VIN = 200mV
MHz1.5VIN = 200mV
±3dB Bandwidth 900VIN = 100mV kHz
100VIN = 10mV 130
MX636K
Bandwidth for 1%
Additional Error (0.09dB) 90
VIN = 200mV kHz
14
VIN = 100mV
MX636J
-0.5
VIN = 10mV
Additional Error -0.2 ±% of
Reading
Specified Accuracy
Crest Factor = 6
±0.1 ±0.1
Crest Factor = 3
Total Error, External Trim
(Note 5) mV ±% of
Reading
±0.3 ±0.1
MX636K
±0.2 ±0.5
MX636J
Total Error, Internal Trim
(Notes 5, 6) mV ±% of
Reading
±0.5 ±1.0
Crest Factor 1 to 2
VOUT = [avg. (VIN)2]1/2
Transfer Equation
±0.1
Total Error vs. DC Reversal ±% of
Reading
±0.2
mV ±% of
Reading/V
±0.1 ±0.01
MX636J
Total Error vs. Supply
MX636K
Total Error vs. Temperature
(0°C to +70°C) mV ±% of
Reading/°C
±0.1 ±0.01
±0.1 ±0.005
MX636K
MX636J
UNITSMIN TYP MAXCONDITIONSPARAMETER
BUFFER AMPLIFIER
Figure 3 ms/µF CAV
25Averaging Time Constant
CONVERSION ACCURACY
ERROR vs. CREST FACTOR (Note 3)
FREQUENCY RESPONSE (Notes 6, 8)
MX536A/MX636
True RMS-to-DC Converters
_______________________________________________________________________________________ 5
ELECTRICAL CHARACTERISTICS—MX636 (continued)
(TA= +25°C, +VS= +3V, -VS= -5V, unless otherwise noted.)
IREF
±2.8
248
±2
Input Signal Range
µA
Peak Transient ±5 VPK
Safe Input
IREF Range
±12 VPK
Input Resistance 5.33 6.7 8.00 kΩ
150
MX636J ±0.5 mVInput Offset Voltage MX636K ±0.2
µA
MX636J ±0.5
TA= +25°C MX636K ±0.2 mV
IOUT Scale Factor
TA= TMIN to TMAX ±10
With Supply Voltage ±0.1 µV/°C
100
Offset Voltage
µA/VRMS
IOUT Scale Factor Tolerance
+3V, -5V Supplies 0 to 1
Output Voltage Swing ±5V to ±16.5V Supplies 0 to 1 1.4 V
-20 ±10 +20 %
Output Resistance 8 10 12 kΩ
UNITSMIN TYP MAXCONDITIONSPARAMETER
MX636J ±0.3 ±0.5
Error 7mV ≤VIN ≤300mV MX636K ±0.1 ±0.2
Scale Factor -3 mV/dB
Scale Factor Tempco
Output Resistance
0 to 200
+0.33 %/°C
810 12 kΩ
Voltage Compliance -VSto
(+VS- 2.0) V
Continuous RMS, All Supplies
All Supplies ±5V Supplies
±2.5V Supplies
+3V, -5V Supplies mVRMS
Input and Output Voltage Range -VSto
(+VS- 2) V
RS= 10kΩ±0.8 ±2 mVInput Offset Voltage ±0.5 ±1
Input Current 100 300 nA
Input Resistance 108Ω
Source +5 mA
Output Current Sink -130 µA
Short-Circuit Current 20 mA
Small-Signal Bandwidth 1 MHz
Slew Rate (Note 9) 5 V/µs
0dB = 1VRMS
MX636J
MX636K
-0.033 dB/°C
mV/V
dB
INPUT CHARACTERISTICS
OUTPUT CHARACTERISTICS (Note 5)
dB OUTPUT
IOUT TERMINAL
BUFFER AMPLIFIER
MX536A/MX636
_______________Detailed Description
The MX536A/MX636 uses an implicit method of RMS
computation that overcomes the dynamic range as well
as other limitations inherent in a straightforward compu-
tation of the RMS. The actual computation performed
by the MX536A/MX636 follows the equation:
VRMS = Avg. [VIN2/VRMS]
The input voltage, VIN, applied to the MX536A/MX636 is
processed by an absolute-value/voltage to current con-
verter that produces a unipolar current I1(Figure 1).
This current drives one input of a squarer/divider that
produces a current I4that has a transfer function:
I4= I12
I3
The current I4drives the internal current mirror through
a lowpass filter formed by R1 and an external capaci-
tor, CAV. As long as the time constant of this filter is
greater than the longest period of the input signal, I4is
averaged. The current mirror returns a current, I3, to the
square/divider to complete the circuit. The current I4is
then a function of the average of (I12/I4), which is equal
to I1RMS.
The current mirror also produces a 2 · I4output current,
IOUT, that can be used directly or converted to a volt-
age using resistor R2 and the internal buffer to provide
a low-impedance voltage output. The transfer function
for the MX536A/MX636 is:
VOUT = 2 · R2 · IRMS = VIN
The dB output is obtained by the voltage at the emitter
of Q3, which is proportional to the -log VIN. The emitter
follower Q5 buffers and level shifts this voltage so that
the dB output is zero when the externally set emitter
current for Q5 approximates I3.
Standard Connection
(Figure 2)
The standard RMS connection requires only one exter-
nal component, CAV. In this configuration the
MX536A/MX636 measures the RMS of the AC and DC
levels present at the input, but shows an error for low-
frequency inputs as a function of the CAV filter capaci-
tor. Figure 3 gives practical values of CAV for various
values of averaging error over frequency for the stan-
dard RMS connections (no post filtering). If a 3µF
capacitor is chosen, the additional error at 100Hz will
be 1%. If the DC error can be rejected, a capacitor
should be connected in series with the input, as would
typically be the case in single-supply operation.
The input and output signal ranges are a function of the
supply voltages. Refer to the electrical characteristics for
guaranteed performance. The buffer amplifier can be
used either for lowering the output impedance of the cir-
cuit, or for other applications such as buffering high-
impedance input signals. The MX536A/MX636 can be
used in current output mode by disconnecting the inter-
nal load resistor, RL, from ground. The current output is
available at pin 8 (pin 10 on the “H” package) with a
nominal scale of 40µA/VRMS input for the MX536A and
100µA/VRMS input for the MX636. The output is positive.
True RMS-to-DC Converters
6 _______________________________________________________________________________________
ELECTRICAL CHARACTERISTICS—MX636 (continued)
(TA= +25°C, +VS= +3V, -VS= -5V, unless otherwise noted.)
Rated Performance +3/-5 V
Dual Supplies +2/-2.5 ±16.5 V
Single Supply +5 +24 V
UNITS
MIN TYP MAX
CONDITIONSPARAMETER
Quiescent Current (Note 10) 0.8 1 mA
Note 1: Accuracy is specified for 0 to 7VRMS, DC or 1kHz sine-wave input with the MX536A connected as in Figure 2.
Note 2: Error vs. crest factor is specified as an additional error for 1VRMS rectangular pulse stream, pulse width = 200µs.
Note 3: Input voltages are expressed in volts RMS, and error as % of reading.
Note 4: With 2kΩexternal pull-down resistor.
Note 5: Accuracy is specified for 0 to 200mV, DC or 1kHz sine-wave input. Accuracy is degraded at higher RMS signal levels.
Note 6: Measured at pin 8 of DIP and SO (IOUT), with pin 9 tied to COMMON.
Note 7: Error vs. crest factor is specified as an additional error for 200mVRMS rectangular pulse input, pulse width = 200µs.
Note 8: Input voltages are expressed in volts RMS.
Note 9: With 10kΩexternal pull-down resistor from pin 6 (BUF OUT) to -VS.
Note 10: With BUF input tied to COMMON.
POWER SUPPLY
MX536A/MX636
True RMS-to-DC Converters
_______________________________________________________________________________________ 7
R3
25k
R4
50k
12k
R2
25k
12k
A1 A4
A2
VIN R-1 I1
I3
I4
IREF
Q1
Q2
Q3
A3
Q4 Q5
14
10
9
8
1
3
4
5
6
7
VIN
-VS
IOUT
0.2mA
F.S.
0.4mA
F.S.
+VS
COM
dB
OUT
BUFF
OUT
BUFF
IN
RL
CAV
R1
25k
BUFFER
25k
ABSOLUTE VALUE/
VOLTAGE-CURRENT
CONVERTER
ONE-QUADRANT
SQUARER/DIVIDER
CURRENT MIRROR
MX536A
Figure 1. MX536A Simplified Schematic
14
13
12
11
10
9
8
1
2
3
4
5
6
7
8VOUT
9
10
1
2
3
4
56
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
VIN
-VS
+VS
+VS
-VS
VOUT
CAV
CAV
MX536A
MX636
Figure 2. MX536A/MX636 Standard RMS Connection
MX536A/MX636
High-Accuracy Adjustments
The accuracy of the MX536A/MX636 can be improved
by the addition of external trims as shown in Figure 4.
R4 trims the offset. The input should be grounded and
R4 adjusted to give zero volts output from pin 6. R1 is
trimmed to give the correct value for either a calibrated
DC input or a calibrated AC signal. For example: 200mV
DC input should give 200mV DC output; a ±200mV
peak-to-peak sine-wave should give 141mV DC output.
Single-Supply Operation
Both the MX536A and the MX636 can be used with a
single supply down to +5V (Figure 5). The major limita-
tion of this connection is that only AC signals can be
measured, since the differential input stage must be
biased off ground for proper operation. The load resis-
tor is necessary to provide output sink current. The
input signal is coupled through C2 and the value cho-
sen so that the desired low-frequency break point is
obtained with the input resistance of 16.7kΩfor the
MX536A and 6.7kΩfor the MX636.
Figure 5 shows how to bias pin 10 within the range of
the supply voltage (pin 2 on “H” packages). It is critical
that no extraneous signals are coupled into this pin. A
capacitor connected between pin 10 and ground is
recommended. The common pin requires less than 5µA
of input current, and if the current flowing through resis-
tors R1 and R2 is chosen to be approximately 10 times
the common pin current, or 50µA, the resistor values
can easily be calculated.
Choosing the Averaging Time Constant
Both the MX536A and MX636 compute the RMS value
of AC and DC signals. At low frequencies and DC, the
output tracks the input exactly; at higher frequencies,
the average output approaches the RMS value of the
input signal. The actual output differs from the ideal by
an average (or DC) error plus some amount of ripple.
The DC error term is a function of the value of CAV and
the input signal frequency. The output ripple is inverse-
True RMS-to-DC Converters
8 _______________________________________________________________________________________
100
0.1
0.22
0.65
1 100 1k
1
10
10
0.01
0.1
1
FREQUENCY (Hz)
EXTERNAL AVERAGING CAP, CAV (µF)
10 60
OUTPUT SETTLING TIME TO COMPLETE
99% OF STEP_ (seconds)
1% 0.1%
Figure 3. Lower Frequency for Stated % of Reading Error and
Settling Time for Circuit shown in Figure 2
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
-VS
R1 +VS
VOUT
CAV
R2
R3 R4
-VS
+VS
MX536A
500Ω
365Ω
750kΩ
50kΩ
MX636
200Ω
154Ω
470kΩ
500kΩ
R1
R2
R3
R4
MX536A
MX636
Figure 4. Optional External Gain and Output Offset Trims
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN +VS
RL
VOUT
CAV
R1
R2
MX536A
20kΩ
10kΩ
1
µ
F
MX636
20kΩ
39kΩ
3.3
µ
F
R1
R2
C2
10k TO 1k
0.1µF
0.1µF
C2
MX536A
MX636
Figure 5. Single-Supply Operation
ly proportional to the value of CAV. Waveforms with high
crest factors, such as a pulse train with low duty cycle,
should have an average time constant chosen to be at
least ten times the signal period.
Using a large value of CAV to remove the output ripple
increases the settling time for a step change in the input
signal level. Figure 3 shows the relationship between
CAV and settling time, where 115ms settling equals 1µF
of CAV. The settling time, or time for the RMS converter to
settle to within a given percent of the change in RMS
level, is set by the averaging time constant, which varies
approximately 2:1 between increasing and decreasing
input signals. For example, increasing input signals
require 2.3 time constants to settle to within 1%, and 4.6
time constants for decreasing signals levels.
In addition, the settling time also varies with input signal
levels, increasing as the input signal is reduced, and
decreasing as the input is increased as shown in
Figures 6a and 6b.
Using Post Filters
A post filter allows a smaller value of CAV, and reduces
ripple and improves the overall settling time. The value
of CAV should be just large enough to give the maxi-
mum DC error at the lowest frequency of interest. The
post filter is used to remove excess output ripple.
Figures 7, 8, and 9 give recommended filter connec-
tions and values for both the MX536A and MX636.
Table 1 lists the number of time constants required for
the RMS section to settle to within different percentages
of the final value for a step change in the input signal.
Decibel Output (dB)
The dB output of the MX536A/MX636 originates in the
squarer/divider section and works well over a 60dB
range. The connection for dB measurements is shown
in Figure 10. The dB output has a temperature drift of
0.03dB/°C, and in some applications may need to be
compensated. Figure 10 shows a compensation
scheme. The amplifier can be used to scale the output
for a particular application. The values used in Figure
10 give an output of +100mV/dB.
MX536A/MX636
True RMS-to-DC Converters
_______________________________________________________________________________________ 9
10
0
1
2.5
1m 100m 101
5
7.5
RMS INPUT LEVEL (V)
SETTLING TIME
RELATIVE TO 1VRMS INPUT SETTLING TIME
10m
MX536A
Figure 6a. MX536A Settling Time vs. Input Level
10
0
1
2.5
1m 100m 1
5
7.5
RMS INPUT LEVEL (V)
SETTLING TIME
RELATIVE TO 200mVRMS INPUT SETTLING TIME
10m
MX636
Figure 6b. MX636 Settling Time vs. Input Level
Settling Time
to Within
Stated % of
New RMS
Level
1%
0.1%
0.01%
4.6τ/4.6τ
6.9τ/6.9τ
9.2τ/9.2τ
Table 1. Number of RC Time Constants
(τ) Required for MX536A/MX636 RMS
Converters to Settle to Within Stated % of
Final Value
FOR
DECREASING
AMPLITUDES
Basic Formulas
PARAMETERS
Note:
(
τ
) Settling Times for Linear RC Filter
4.6τ/2.0τ
6.9τ/3.1τ
9.2τ/4.2τ
FOR
INCREASING
AMPLITUDES
∆V 1 - e-T/RC ∆V e-T/RC
MX536A/MX636
Frequency Response
The MX536A/MX636 utilizes a logarithmic circuit in per-
forming the RMS computation of the input signal. The
bandwidth of the RMS converters is proportional to sig-
nal level. Figures 11 and 12 represent the frequency
response of the converters from 10mV to 7VRMS for the
MX536A and 1mV to 1V for the MX636, respectively.
The dashed lines indicate the upper frequency limits for
1%, 10%, and ±3dB of reading additional error.
Caution must be used when designing RMS measuring
systems so that overload does not occur. The input
clipping level for the MX636 is ±12V, and for the
MX536A it is ±20V. A 7VRMS signal with a crest factor
of 3 has a peak input of 21V.
Application in a Low-Cost DVM
A low-cost digital voltmeter (DVM) using just two inte-
grated circuits plus supporting circuitry and LCD dis-
play is shown in Figure 13. The MAX130 is a 3 1/2 digit
integrating A/D converter with precision bandgap refer-
ence. The 10MΩinput attenuator is AC coupled to pin
6 of the MX636 buffer amplifier. The output from the
MX636 is connected to the MAX130 to give a direct
reading to the LCD display.
True RMS-to-DC Converters
10 ______________________________________________________________________________________
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
-VS
+VS
+VS
VRMS OUT
CAV
IOUT
RL
COMMON
N.C.
N.C.
N.C.
dB
N.C.
C2
MX536A
MX636
Figure 7. MX536A/MX636 with a One-Pole Output Filter
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
-VS
+VS
VRMS OUT
CAV
C3
C2 RX*
* MX536A = 25kΩ
MX636 = 10kΩ
MX536A
MX636
Figure 8. MX536A/MX636 with a Two-Pole Output Filter
0.1 10 1k 10k
1
10
FREQUENCY (Hz)
EDC ERROR OR RIPPLE (% OF READING)
100
PK-PK RIPPLE RX = 0
PK-PK RIPPLE
(ONE POLE)
C2 = 4.7µF
PK-PK RIPPLE
(TWO POLE)
C2 = C3 = 4.7µF
DC ERROR
(ALL FILTERS)
MX536A
2.2µF
1µF
2.2µF
2.2µF
1µF
MX636
4.7µF
1µF
4.7µF
4.7µF
1µF
ONE-POLE FILTER
C2
CAV
TWO-POLE FILTER
C2
C3
CAF
Figure 9. Performance Features of Various Filter Types for
MX536A/MX636
MX536A/MX636
True RMS-to-DC Converters
______________________________________________________________________________________ 11
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
+VS
-VS
+VS
4.5V TO 15V
+VSVIN
VOUT
2.5V
dB OUT
-3mV/dB
C1
C2 ZERO dB
0.1µF
LINEAR
RMS
OUTPUT
R1
R2
500Ω
GAIN
R3
1k*
R4
36k
R5
GROUND
COMPENSATED
dB OUT
+0.1V/dB
*SPECIAL TC COMP RESISTOR: +3500PPM, 1k, 1%
MX580J
MX536A
MX636
MAX400
Figure 10. dB Connection
1
1m
100µ1k 100k 10M1M
10m
30m
200m
100m
FREQUENCY (Hz)
VOUT (V)
10k
1VRMS INPUT
200mVRMS INPUT
100mVRMS INPUT
10mVRMS INPUT
30mVRMS INPUT
1VRMS INPUT
1% 10% ±3dB
Figure 12. MX636 High-Frequency Response
10
0.01
1k 100k 10M1M
0.1
1
FREQUENCY (Hz)
VOUT (V)
10k
7VRMS INPUT 1% 10%
±3dB
1VRMS INPUT
100mVRMS
INPUT
10mVRMS
INPUT
Figure 11. MX536A High-Frequency Response
*
Maxim reserves the right to ship ceramic packages in lieu of CERDIP packages.
**
Dice are specified at TA= +25°C.
MX536A/MX636
True RMS-to-DC Converters
PART
MX536ASH -55°C to +125°C
TEMP. RANGE PIN-PACKAGE
10 TO-100
MX536ASQ* -55°C to +125°C 14 CERDIP
MX636JC/D
MX636JCWE
MX636JD 0°C to +70°C
0°C to +70°C
0°C to +70°C Dice**
16 Wide SO
14 Ceramic
MX636JH 0°C to +70°C 10 TO-100
Pin Configurations (continued)
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
TOP VIEW
MX536A
MX636
SO
+VS
N.C.
N.C.
N.C.
COMMON
RL
IOUT
N.C.
VIN
N.C.
-VS
CAV
dB
BUF OUT
BUF IN
N.C.
14
13
12
11
10
9
8
1
2
3
4
5
6
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
R9
500k
0dB SET
R13
500Ω
LIN
SCALE dB
SCALE
R15
1M
R14
50k
R12
1k
R11
26k
R10
20k
MAX130
C4
2.2µF
C3
0.02µF
6.8µF
R6
1M
R5
47k
1W
10%
R7
20k
D2
IN4148
D1
IN4148
2V
20V
200V
R1
9M
R2
900k
R3
90k
R4
10k
COM
200mV
C7
6.8µF
LIN
LIN
dB
LIN
1N4148
dB
dB
D3
D4
D5
C6
0.01µF
+VDD
+VS
REF HI
COM
REF LO
IN HI
IN LO
31⁄2
DIGIT
LCD
DISPLAY
31⁄2 DIGIT
ADC
V-
V+
9V
BATTERY
MX636
10k
10k
Figure 13. Portable High-Z Input RMS DPM and dB Meter
Typical Operating
________________Circuits (continued)
8
9
10
1
2
3
4
56
7
ABSOLUTE
VALUE
SQUARER
DIVIDER
CURRENT
MIRROR
BUF
VIN
+VS
-VS
VOUT
CAV
___________________________________________Ordering Information (continued)
PART TEMP. RANGE PIN-PACKAGE
MX636JQ*
MX636KCWE 0°C to +70°C
0°C to +70°C 14 CERDIP
16 Wide SO
MX636KD
MX636KH 0°C to +70°C
0°C to +70°C 14 Ceramic
10 TO-100
MX636KN 0°C to +70°C 14 Plastic DIP
MX636KQ* 0°C to +70°C 14 CERDIP
MX636JN 0°C to +70°C 14 Plastic DIP
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
12
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