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 may 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.
19-0824; Rev 2; 3/96
True RMS-to-DC Converters
The MX536A and MX636 are true RMS-to-DC converters. 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 components. They can be operated from either a single supply or dual supplies. Both devices draw less than 1mA
of quiescent supply current, making them ideal for battery-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
Pin Configurations
TOP VIEW
RL 1
IOUT
10
8 BUF OUT
COMMON 2
MX536A
MX636B
+VS 3
VIN
9 BUF IN
4
5
-VS
7 dB
6 CAV
____________________________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)
Ordering Information
TEMP. RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
-55°C to +125°C
PART
MX536AJC/D
MX536AJCWE
MX536AJD
MX536AJH
MX536AJN
MX536AJQ*
MX536AKCWE
MX536AKD
MX536AKH
MX536AKN
MX536AKQ*
MX536ASD
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.
_________Typical Operating Circuits
CAV
TO-100
VIN 1
14 +VS
N.C. 2
13 N.C.
-VS 3
12 N.C.
CAV 4
dB 5
MX536A
MX636
1
ABSOLUTE
VALUE
2
-VS
SQUARER
DIVIDER
3
IOUT
VOUT
CURRENT
MIRROR
12
10
9
6
7
+VS
11
5
RL
14
13
4
10 COMMON
8
BUF IN 7
VIN
11 N.C.
9
BUF OUT 6
PIN-PACKAGE
Dice**
16 Wide SO
14 Ceramic
10 TO-100
14 Plastic DIP
14 CERDIP
16 Wide SO
14 Ceramic
10 TO-100
14 Plastic DIP
14 CERDIP
14 Ceramic
BUF
8
DIP
Pin Configurations continued at end of data sheet.
Typical Operating Circuits continued at end of data sheet.
________________________________________________________________ Maxim Integrated Products
1
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
General Description
MX536A/MX636
True RMS-to-DC Converters
ABSOLUTE MAXIMUM RATINGS
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
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.
ELECTRICAL CHARACTERISTICS—MX536A
(TA = +25°C, +VS = +15V, -VS = -15V, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
Averaging Time Constant
TYP
MAX
UNITS
VOUT = [avg. (VIN)2] 1/2
Transfer Equation
Figure 3
25
ms/µF CAV
CONVERSION ACCURACY
Total Error, Internal Trim (Note 1)
Total Error vs. Temperature
MX536AJ, AS
±5 ±0.5
MX536AK
±2 ±0.2
TMIN to +70°C
+70°C to +125°C
Total Error vs. Supply
Total Error vs. DC Reversal
Total Error, External Trim
(Note 1)
MX536AJ
±0.1 ±0.01
MX536AK
±0.05 ±0.005
MX536AS
±0.1 ±0.005
MX536AS
±0.03 ±0.005
±0.1 ±0.01
MX536AJ, AS
±0.2
MX536AK
±0.1
MX536AJ, AS
±3 ±0.3
MX536AK
±2 ±0.1
mV ±% of
Reading
mV ±% of
Reading/°C
mV ±% of
Reading/V
% of
Reading
mV ±% of
Reading
ERROR vs. CREST FACTOR (Note 2)
Crest Factor 1 to 2
Additional Error
Specified Accuracy
Crest Factor = 3
-0.1
Crest Factor = 7
-1.0
% of
Reading
FREQUENCY RESPONSE (Note 3)
Bandwidth for 1%
Additional Error (0.09dB)
±3dB Bandwidth
2
VIN = 10mV
5
VIN = 100mV
45
VIN = 1V
120
VIN = 10mV
90
VIN = 100mV
450
VIN = 1V
2.3
_______________________________________________________________________________________
kHz
kHz
MHz
True RMS-to-DC Converters
MX536A/MX636
ELECTRICAL CHARACTERISTICS—MX536A (continued)
(TA = +25°C, +VS = +15V, -VS = -15V, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
INPUT CHARACTERISTICS
±15V Supplies
Continuous RMS Peak Transient
0 to 7
±5V Supplies
Continuous RMS Peak Transient
0 to 2
Input Signal Range
Safe Input
VPK
VRMS
±7
VPK
±25
VPK
16.7
20.00
kΩ
MX536AJ, AS
0.8
±2
MX536AK
0.5
±1
All Supplies
Input Resistance
13.33
Input Offset Voltage
VRMS
±20
mV
OUTPUT CHARACTERISTICS
TA = +25°C
MX536AJ
±1
±2
MX536AK
±0.5
±1
MX536AS
Offset Voltage
TA = TMIN to TMAX
Supply Voltage
Output Voltage Swing
MX536AJ, AK
±0.1
MX536AS
±0.2
MX536AJ, AK
±0.1
MX536AS
±0.2
±15V Supplies
0 to 11
±5V Supplies
0 to 2
Source
Output Current
12.5
V
mA
-130
Short Circuit Current
mV/°C
mV/V
5
Sink
mV
±2
µA
20
Output Resistance
mA
0.5
Ω
dB OUTPUT
VIN = 7mV to 7VRMS,
0dB = 1VRMS
Error
MX536AJ
±0.4
±0.6
MX536AK
±0.2
±0.3
MX536AS
±0.5
±0.6
Scale Factor
Scale Factor TC (Uncompensated)
IREF
0dB = 1VRMS
IREF Range
5
dB
-3
mV/dB
0.33
% of
Reading/°C
20
1
80
µA
100
µA
IOUT TERMINAL
IOUT Scale Factor
40
IOUT Scale Factor Tolerance
Output Resistance
20
Voltage Compliance
µA/VRMS
±10
±20
%
25
30
kΩ
-VS to
(+VS - 2.5)
V
_______________________________________________________________________________________
3
MX536A/MX636
True RMS-to-DC Converters
ELECTRICAL CHARACTERISTICS—MX536A (continued)
(TA = +25°C, +VS = +15V, -VS = -15V, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
BUFFER AMPLIFIER
-VS to
(+VS - 2.5)
Input and Output Voltage Range
Input Offset Voltage
±0.5
±4
mV
Input Bias Current
20
300
nA
Input Resistance
108
Output Current
RS = 25kΩ
V
Source
Ω
+5
Sink
mA
-130
µA
Short-Circuit Current
20
mA
Small-Signal Bandwidth
1
MHz
Slew Rate (Note 4)
5
V/µs
ELECTRICAL CHARACTERISTICS—MX636
(TA = +25°C, +VS = +3V, -VS = -5V, unless otherwise noted.)
PARAMETER
CONDITIONS
MIN
Averaging Time Constant
TYP
MAX
UNITS
VOUT = [avg. (VIN)2]1/2
Transfer Equation
Figure 3
25
ms/µF CAV
CONVERSION ACCURACY
Total Error, Internal Trim
(Notes 5, 6)
MX636J
±0.5 ±1.0
MX636K
±0.2 ±0.5
Total Error vs. Temperature
(0°C to +70°C)
MX636J
±0.1 ±0.01
MX636K
±0.1 ±0.005
Total Error vs. Supply
±0.1 ±0.01
MX636J
±0.2
MX636K
±0.1
Total Error vs. DC Reversal
VIN = 200mV
Total Error, External Trim
(Note 5)
MX636J
±0.3 ±0.1
MX636K
±0.1 ±0.1
mV ±% of
Reading
mV ±% of
Reading/°C
mV ±% of
Reading/V
±% of
Reading
mV ±% of
Reading
ERROR vs. CREST FACTOR (Note 3)
Crest Factor 1 to 2
Additional Error
Specified Accuracy
Crest Factor = 3
-0.2
Crest Factor = 6
-0.5
±% of
Reading
FREQUENCY RESPONSE (Notes 6, 8)
Bandwidth for 1%
Additional Error (0.09dB)
±3dB Bandwidth
4
VIN = 10mV
14
VIN = 100mV
90
VIN = 200mV
130
VIN = 10mV
100
VIN = 100mV
900
VIN = 200mV
1.5
_______________________________________________________________________________________
kHz
kHz
MHz
True RMS-to-DC Converters
MX536A/MX636
ELECTRICAL CHARACTERISTICS—MX636 (continued)
(TA = +25°C, +VS = +3V, -VS = -5V, unless otherwise noted.)
PARAMETER
INPUT CHARACTERISTICS
CONDITIONS
MIN
Continuous RMS, All Supplies
+3V, -5V Supplies
Peak Transient
±2.5V Supplies
±5V Supplies
All Supplies
Input Signal Range
Safe Input
Input Resistance
TYP
0 to 200
6.7
MX636J
Input Offset Voltage
MX636K
OUTPUT CHARACTERISTICS (Note 5)
Offset Voltage
Output Voltage Swing
MX636J
MX636K
UNITS
mVRMS
±2.8
±2
±5
±12
5.33
TA = +25°C
MAX
8.00
±0.5
±0.2
±0.5
±0.2
±10
±0.1
VPK
VPK
kΩ
mV
mV
TA = TMIN to TMAX
With Supply Voltage
+3V, -5V Supplies
0 to 1
±5V to ±16.5V Supplies
0 to 1
1.4
8
10
12
kΩ
±0.5
±0.2
dB
2
±0.3
±0.1
-3
+0.33
-0.033
4
Output Resistance
µV/°C
mV/V
V
dB OUTPUT
7mV ≤ VIN ≤ 300mV
Error
MX636J
MX636K
Scale Factor
Scale Factor Tempco
IREF
0dB = 1VRMS
IREF Range
1
8
mV/dB
%/°C
dB/°C
µA
50
µA
IOUT TERMINAL
IOUT Scale Factor
100
IOUT Scale Factor Tolerance
Output Resistance
µA/VRMS
-20
±10
+20
%
8
10
12
kΩ
-VS to
(+VS - 2.0)
Voltage Compliance
V
BUFFER AMPLIFIER
-VS to
(+VS - 2)
Input and Output Voltage Range
Input Current
±0.8
±0.5
100
Input Resistance
108
Input Offset Voltage
RS = 10kΩ
MX636J
MX636K
V
±2
±1
300
mV
nA
Short-Circuit Current
20
Ω
mA
µA
mA
Small-Signal Bandwidth
Slew Rate (Note 9)
1
5
MHz
V/µs
Source
Sink
Output Current
+5
-130
_______________________________________________________________________________________
5
MX536A/MX636
True RMS-to-DC Converters
ELECTRICAL CHARACTERISTICS—MX636 (continued)
(TA = +25°C, +VS = +3V, -VS = -5V, unless otherwise noted.)
PARAMETER
POWER SUPPLY
Rated Performance
Dual Supplies
Single Supply
Quiescent Current (Note 10)
CONDITIONS
MIN
TYP
MAX
UNITS
±16.5
+24
1
V
V
V
mA
+3/-5
+2/-2.5
+5
0.8
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 2kΩ external 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 10kΩ external pull-down resistor from pin 6 (BUF OUT) to -VS.
Note 10: With BUF input tied to COMMON.
_______________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 computation 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 converter that produces a unipolar current I1 (Figure 1).
This current drives one input of a squarer/divider that
produces a current I4 that has a transfer function:
I 2
I4 = 1
I3
The current I4 drives the internal current mirror through
a lowpass filter formed by R1 and an external capacitor, CAV. As long as the time constant of this filter is
greater than the longest period of the input signal, I4 is
averaged. The current mirror returns a current, I3, to the
square/divider to complete the circuit. The current I4 is
then a function of the average of (I12/I4), which is equal
to I1RMS.
The current mirror also produces a 2 · I4 output current,
IOUT, that can be used directly or converted to a voltage 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
6
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 external component, C AV . In this configuration the
MX536A/MX636 measures the RMS of the AC and DC
levels present at the input, but shows an error for lowfrequency inputs as a function of the CAV filter capacitor. Figure 3 gives practical values of CAV for various
values of averaging error over frequency for the standard 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 circuit, or for other applications such as buffering highimpedance input signals. The MX536A/MX636 can be
used in current output mode by disconnecting the internal 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
MX536A/MX636
CURRENT MIRROR
+VS
14
COM
10
MX536A
0.2mA
F.S.
0.4mA
F.S.
R1
25k
I3
CAV 4
R2
9
25k
I4
ABSOLUTE VALUE/
VOLTAGE-CURRENT
CONVERTER
RL
IOUT 8
dB
OUT
IREF
A3
5
I1
VIN R-1
R4
50k
Q1
Q3
VIN
BUFF
IN
1
7
A1
12k
R3
A4
Q5
BUFF
OUT
6
A2
25k
ONE-QUADRANT
SQUARER/DIVIDER
12k
25k
Q4
Q2
BUFFER
-VS
3
Figure 1. MX536A Simplified Schematic
CAV
10
VIN
1
ABSOLUTE
VALUE
2
-VS
SQUARER
DIVIDER
3
9
1
12
BUF
MX536A
MX636
2
CURRENT
MIRROR
8
3
SQUARER
DIVIDER
7
11
CURRENT
MIRROR
5
10
9
6
7
+VS
13
4
VOUT
14
BUF
+VS
4
VIN
8
VOUT
ABSOLUTE
VALUE
6
5
CAV
-VS
Figure 2. MX536A/MX636 Standard RMS Connection
_______________________________________________________________________________________
7
100
10
10
1
0.1
1
0.65
1%
0.1%
0.22
CAV
OUTPUT SETTLING TIME TO COMPLETE
99% OF STEP_ (seconds)
EXTERNAL AVERAGING CAP, CAV (µF)
MX536A/MX636
True RMS-to-DC Converters
VIN
1
-VS
1
10
60 100
13
SQUARER
DIVIDER
3
12
4
11
CURRENT
MIRROR
5
VOUT
7
+VS
10
R2
9
6
1k
+VS
14
2
0.01
0.1
ABSOLUTE
VALUE
R1
BUF
8
Figure 3. Lower Frequency for Stated % of Reading Error and
Settling Time for Circuit shown in Figure 2
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 limitation 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 resistor is necessary to provide output sink current. The
input signal is coupled through C2 and the value chosen so that the desired low-frequency break point is
obtained with the input resistance of 16.7kΩ for the
MX536A and 6.7kΩ for 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 resistors 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,
8
R1
R2
R3
R4
MX536A
MX636
MX536A
500Ω
365Ω
750kΩ
50kΩ
R4
R3
FREQUENCY (Hz)
-VS
MX636
200Ω
154Ω
470kΩ
500kΩ
Figure 4. Optional External Gain and Output Offset Trims
CAV
C2
VIN
1
+VS
ABSOLUTE
VALUE
2
13
SQUARER
DIVIDER
3
4
CURRENT
MIRROR
7
R1
12
10
9
6
RL
0.1µF
11
5
VOUT
14
0.1µF
R2
BUF
8
10k TO 1k
MX536A
MX636
R1
R2
C2
MX536A
20kΩ
10kΩ
1µF
MX636
20kΩ
39kΩ
3.3µF
Figure 5. Single-Supply Operation
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
MX636
SETTLING TIME
RELATIVE TO 200mVRMS INPUT SETTLING TIME
SETTLING TIME
RELATIVE TO 1VRMS INPUT SETTLING TIME
MX536A/MX636
MX536A
10
7.5
5
2.5
1
10
7.5
5
2.5
0
1
0
1m
10m
100m
1
10
1m
RMS INPUT LEVEL (V)
10m
100m
1
RMS INPUT LEVEL (V)
Figure 6a. MX536A Settling Time vs. Input Level
Figure 6b. MX636 Settling Time vs. Input Level
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.
Table 1. Number of RC Time Constants
(τ) Required for MX536A/MX636 RMS
Converters to Settle to Within Stated % of
Final Value
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 maximum 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 connections 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.
PARAMETERS
Basic Formulas
Settling Time
to Within
Stated % of
New RMS
Level
FOR
INCREASING
AMPLITUDES
FOR
DECREASING
AMPLITUDES
∆V 1 - e -T/RC
∆V
e -T/RC
1%
4.6τ/2.0τ
4.6τ/4.6τ
0.1%
6.9τ/3.1τ
6.9τ/6.9τ
0.01%
9.2τ/4.2τ
9.2τ/9.2τ
Note: (τ) Settling Times for Linear RC Filter
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.
_______________________________________________________________________________________
9
MX536A/MX636
True RMS-to-DC Converters
VIN
1
N.C. 2
CAV
-VS
SQUARER
DIVIDER
dB 5
10
9
6
7
-VS
BUF
8
COMMON
SQUARER
DIVIDER
3
CAV
CURRENT
MIRROR
7
C2
C2
+VS
MX536A
MX636
10
9
6
IOUT
12
11
5
RL
14
13
4
11 N.C.
CURRENT
MIRROR
ABSOLUTE
VALUE
2
12 N.C.
4
VRMS OUT
1
13 N.C.
3
+VS
VIN
14 +VS
ABSOLUTE
VALUE
BUF
8
C3
RX*
VRMS OUT
MX536A
MX636
* MX536A = 25kΩ
MX636 = 10kΩ
Figure 7. MX536A/MX636 with a One-Pole Output Filter
Figure 8. MX536A/MX636 with a Two-Pole Output Filter
EDC ERROR OR RIPPLE (% OF READING)
Frequency Response
PK-PK RIPPLE
10
The MX536A/MX636 utilizes a logarithmic circuit in performing the RMS computation of the input signal. The
bandwidth of the RMS converters is proportional to signal 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.
RX = 0
PK-PK RIPPLE
(ONE POLE)
C2 = 4.7µF
1
DC ERROR
(ALL FILTERS)
PK-PK RIPPLE
(TWO POLE)
C2 = C3 = 4.7µF
0.1
10
1k
100
10k
FREQUENCY (Hz)
MX536A
ONE-POLE FILTER
C2
2.2µF
CAV
1µF
TWO-POLE FILTER
2.2µF
C2
2.2µF
C3
1µF
CAF
MX636
4.7µF
1µF
4.7µF
4.7µF
1µF
Application in a Low-Cost DVM
A low-cost digital voltmeter (DVM) using just two integrated circuits plus supporting circuitry and LCD display is shown in Figure 13. The MAX130 is a 3 1/2 digit
integrating A/D converter with precision bandgap reference. The 10MΩ input 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.
Figure 9. Performance Features of Various Filter Types for
MX536A/MX636
10
______________________________________________________________________________________
True RMS-to-DC Converters
MX536A/MX636
MX536A
MX636
VIN
1
14
ABSOLUTE
VALUE
2
-VS
+VS
0.1µF
CURRENT
MIRROR
5
7
R4
36k
MX580J
11
10
6
dB OUT
-3mV/dB
VOUT
2.5V
12
4
C2
+VS
4.5V TO 15V
13
SQUARER
DIVIDER
3
C1
VIN
+VS
ZERO dB
COMPENSATED
dB OUT
+0.1V/dB
MAX400
R1
GROUND
R3
1k*
9
BUF
8
LINEAR
RMS
OUTPUT
R2
500Ω
GAIN
R5
*SPECIAL TC COMP RESISTOR: +3500PPM, 1k, 1%
Figure 10. dB Connection
10
1
1
0.1
0.01
1%
10%
200m
±3dB
1VRMS INPUT
VOUT (V)
VOUT (V)
7VRMS INPUT
100m
30m
10m
100mVRMS
INPUT
1m
10mVRMS
INPUT
1VRMS INPUT
1%
200mVRMS INPUT
100mVRMS INPUT
10%
±3dB
30mVRMS INPUT
10mVRMS INPUT
1VRMS INPUT
100µ
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 11. MX536A High-Frequency Response
10M
1k
10k
100k
1M
10M
FREQUENCY (Hz)
Figure 12. MX636 High-Frequency Response
______________________________________________________________________________________
11
MX536A/MX636
True RMS-to-DC Converters
200mV
VIN
D1
IN4148
R1
9M
C4
2.2µF
C3
0.02µF
2V
R6
1M
R2
900k
+VS
1
ABSOLUTE
VALUE
2
13
SQUARER
DIVIDER
3
R5
47k
1W
10%
20V
R3
90k
4
6.8µF
5
200V
10k
BUF
7
D2
IN4148
12
R9
500k
0dB SET
9
C7
6.8µF
ADC
MAX130
R14
50k
9V
BATTERY
VREF LO
COM
dB
SCALE
IN HI
R15
1M
LIN
COM
V+
31⁄2 DIGIT
REF HI
LIN
SCALE
LIN
+VDD
dB
R13
500Ω
dB
MX636
1N4148
LIN
R12
1k
R10
20k
8
10k
R7
20k
10
D3
D4
D5
R11
26k
11
CURRENT
MIRROR
6
R4
10k
14
C6
0.01µF
IN LO
dB
31⁄2
DIGIT
LCD
DISPLAY
Figure 13. Portable High-Z Input RMS DPM and dB Meter
Typical Operating
________________Circuits (continued)
Pin Configurations (continued)
TOP VIEW
10
9
1
2
CURRENT
MIRROR
3
SQUARER
DIVIDER
VOUT
BUF
8
VIN 1
16 +VS
N.C. 2
15 N.C.
-VS 3
14 N.C.
CAV 4
dB 5
+VS
4
VIN
CAV
MX536A
MX636
BUF OUT 6
7
ABSOLUTE
VALUE
13 N.C.
12 COMMON
11 RL
BUF IN 7
10 IOUT
N.C. 8
9 N.C.
6
5
SO
-VS
___________________________________________Ordering Information (continued)
PART
MX536ASH
MX536ASQ*
MX636JC/D
MX636JCWE
MX636JD
MX636JH
MX636JN
TEMP. RANGE
-55°C to +125°C
-55°C to +125°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
PIN-PACKAGE
10 TO-100
14 CERDIP
Dice**
16 Wide SO
14 Ceramic
10 TO-100
14 Plastic DIP
PART
MX636JQ*
MX636KCWE
MX636KD
MX636KH
MX636KN
MX636KQ*
TEMP. RANGE
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
0°C to +70°C
PIN-PACKAGE
14 CERDIP
16 Wide SO
14 Ceramic
10 TO-100
14 Plastic DIP
14 CERDIP
* Maxim reserves the right to ship ceramic packages in lieu of CERDIP packages.
** Dice are specified at TA = +25°C.
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 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 1998 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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