Datasheet MX536AJC-D, MX536AJCWE, MX536AJD, MX536AJH, MX536AJN Datasheet (Maxim)

General Description

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 7V

RMS

for the MX536A

and 0 to 200mV

RMS

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

____________________________Features

♦ True RMS-to-DC Conversion
♦ Computes RMS of AC and DC Signals
♦ Wide Response:

2MHz Bandwidth for V

RMS

> 1V (MX536A)

1MHz Bandwidth for V

RMS

> 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

+V

S

N.C.
N.C.
N.C.
COMMON
R

L

I

OUT

V

IN

N.C.

-V

S

C

AV

dB

BUF OUT

BUF IN

MX536A

MX636

DIP

TOP VIEW

MX536A
MX636B

TO-100

8

9

10

1

2

3

4

5

6

7

BUF IN

BUF OUT

dB

C

AV

-V

S

+V

S

I

OUT

R

L

COMMON

V

IN

Pin Configurations

14
13
12
11
10

9
8

1
2
3
4
5
6
7

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

V

IN

-V

S

+V

S

V

OUT

C

AV

_________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 [phone].
For small orders, phone [phone] 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

T

MIN

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

V

OUT

= [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 C

AV

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

V

RMS

0 to 7

±15V Supplies
Continuous RMS Peak Transient

±20 V

PK

0 to 2 V

RMS

Input Signal Range

±5V Supplies
Continuous RMS Peak Transient

±7 V

PK

Safe Input All Supplies ±25 V

PK

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= T

MIN

to T

MAX

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 7V

RMS

,

0dB = 1V

RMS

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.)

I

REF

0dB = 1V

RMS

5 20 80 µA

I

REF

Range 1 100 µA

I

OUT

Scale Factor 40 µA/V

RMS

I

OUT

Scale Factor Tolerance ±10 ±20 %

Output Resistance 20 25 30 kΩ
Voltage Compliance

-VSto

(+V

S

- 2.5)

V

INPUT CHARACTERISTICS

OUTPUT CHARACTERISTICS

dB OUTPUT

I

OUT

TERMINAL

MX536A/MX636

True RMS-to-DC Converters

4 _______________________________________________________________________________________

Input Bias Current 20 300 nA
Input Resistance 10

8

Ω

Output Current

Source +5 mA

Sink -130 µA
Short-Circuit Current 20 mA

UNITS

MIN TYP MAX

CONDITIONSPARAMETER

Input and Output Voltage Range

-VSto

(+V

S

- 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

V

OUT

= [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 C

AV

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.)

I

REF

±2.8

2 4 8

±2

Input Signal Range

µA

Peak Transient

±5

V

PK

Safe Input

I

REF

Range

±12 V

PK

Input Resistance 5.33 6.7 8.00 kΩ

1 50

MX636J ±0.5

mVInput Offset Voltage

MX636K ±0.2

µA

MX636J ±0.5

TA= +25°C

MX636K ±0.2

mV

I

OUT

Scale Factor

TA= T

MIN

to T

MAX

±10

With Supply Voltage ±0.1

µV/°C

100

Offset Voltage

µA/V

RMS

I

OUT

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

8 10 12 kΩ

Voltage Compliance

-VSto

(+V

S

- 2.0)

V

Continuous RMS, All Supplies

All Supplies

±5V Supplies

±2.5V Supplies

+3V, -5V Supplies

mV

RMS

Input and Output Voltage Range

-VSto

(+V

S

- 2)

V

RS= 10kΩ

±0.8 ±2

mVInput Offset Voltage

±0.5 ±1
Input Current 100 300 nA
Input Resistance 10

8

Ω

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 = 1V

RMS

MX636J
MX636K

-0.033 dB/°C

mV/V

dB

INPUT CHARACTERISTICS

OUTPUT CHARACTERISTICS (Note 5)

dB OUTPUT

I

OUT

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 compu­tation of the RMS. The actual computation performed
by the MX536A/MX636 follows the equation:

V

RMS

= Avg. [V

IN

2

/V

RMS

]

The input voltage, V

IN

, 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 I4that has a transfer function:

I

4

=

I

1

2

I

3

The current I4drives 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, 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 (I

1

2

/I4), which is equal

to I

1

RMS

.

The current mirror also produces a 2 · I

4

output current,

I

OUT

, 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:

V

OUT

= 2 · R2 · I

RMS

= V

IN

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, 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 low­frequency inputs as a function of the CAVfilter capaci­tor. Figure 3 gives practical values of CAVfor 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, R

L

, from ground. The current output is
available at pin 8 (pin 10 on the “H” package) with a
nominal scale of 40µA/V

RMS

input for the MX536A and

100µA/V

RMS

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 7V

RMS

, 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 1V

RMS

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 (I

OUT

), with pin 9 tied to COMMON.

Note 7: Error vs. crest factor is specified as an additional error for 200mV

RMS

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 -V

S

.

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

V

IN

R

-1

I1

I3

I4

I

REF

Q1

Q2

Q3

A3

Q4

Q5

14

10

9

8

1

3

4

5

6

7

V

IN

-V

S

I

OUT

0.2mA
F.S.

0.4mA
F.S.

+V

S

COM

dB

OUT

BUFF
OUT

BUFF

IN

R

L

C

AV

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

8

V

OUT

9

10

1

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

V

IN

V

IN

-V

S

+V

S

+V

S

-V

S

V

OUT

C

AV

C

AV

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 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,

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 C

AV

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, C

AV

(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

V

IN

-V

S

R1

+V

S

V

OUT

C

AV

R2

R3

R4

-V

S

+V

S

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

V

IN

+V

S

R

L

V

OUT

C

AV

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 CAVto remove the output ripple
increases the settling time for a step change in the input
signal level. Figure 3 shows the relationship between
CAVand 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 CAVshould 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.

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 1V

RMS

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 200mV

RMS

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 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 7V

RMS

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 7V

RMS

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 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.

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

V

IN

-V

S

+V

S

+V

S

V

RMS

OUT

C

AV

I

OUT

R

L

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

V

IN

-V

S

+V

S

V

RMS

OUT

C

AV

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
C

AV

TWO-POLE FILTER
C2
C3
C

AF

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

V

IN

+V

S

-V

S

+V

S

4.5V TO 15V

+V

S

V

IN

V

OUT

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

100µ

1k 100k 10M1M

10m

30m

200m
100m

FREQUENCY (Hz)

V

OUT

(V)

10k

1V

RMS

INPUT

200mV

RMS

INPUT

100mV

RMS

INPUT

10mV

RMS

INPUT

30mV

RMS

INPUT

1V

RMS

INPUT

1%

10%

±3dB

Figure 12. MX636 High-Frequency Response

10

0.01

1k 100k 10M1M

0.1

1

FREQUENCY (Hz)

V

OUT

(V)

10k

7V

RMS

INPUT

1%

10%

±3dB

1V

RMS

INPUT

100mV

RMS

INPUT
10mV

RMS

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

+V

S

N.C.
N.C.
N.C.
COMMON
R

L

I

OUT

N.C.

V

IN

N.C.

-V

S

C

AV

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

V

IN

R9
500k
0dB SET

R13
500Ω

LIN

SCALEdBSCALE

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

+V

DD

+V

S

REF HI

COM

REF LO

IN HI

IN LO

31⁄

2

DIGIT

LCD

DISPLAY

3

1

⁄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

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

V

IN

+V

S

-V

S

V

OUT

C

AV

___________________________________________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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