Analog Devices AD737 f Datasheet

Low Cost, Low Power,

FEATURES

Computes:

True rms value
Average rectified value
Absolute value

Provides:

200 mV full-scale input range

(larger inputs with input attenuator)
Direct interfacing with 3 1/2 digit CMOS ADCs
High input impedance: 10
Low input bias current: 25 pA max
High accuracy: ±0.2 mV ± 0.3% of reading
RMS conversion with signal crest factors up to 5
Wide power supply range: ±2.5 V to ±16.5 V
Low power: 160 µA max supply current
No external trims needed for specified accuracy
AD736—a general-purpose, buffered voltage output

version also available

GENERAL DESCRIPTION

The AD7371 is a low power, precision, monolithic true rms-to­dc converter. It is laser trimmed to provide a maximum error of
±0.2 mV ± 0.3% of reading with sine wave inputs. Furthermore,
it maintains high accuracy while measuring a wide range of
input waveforms, including variable duty-cycle pulses and triac
(phase) controlled sine waves. The low cost and small physical
size of this converter make it suitable for upgrading the
performance of non-rms precision rectifiers in many applica­tions. Compared to these circuits, the AD737 offers higher
accuracy at equal or lower cost.

The AD737 can compute the rms value of both ac and dc input
voltages. It can also be operated ac-coupled by adding one
external capacitor. In this mode, the AD737 can resolve input
signal levels of 100 µV rms or less, despite variations in tem­perature or supply voltage. High accuracy is also maintained for
input waveforms with crest factors of 1 to 3. In addition, crest
factors as high as 5 can be measured (while introducing only

2.5% additional error) at the 200 mV full-scale input level.

The AD737 has no output buffer amplifier, thereby significantly
reducing dc offset errors occurring at the output, which makes
the device highly compatible with high input impedance ADCs.

______________________________________

1

Protected under U.S. Patent Number 5,495,245.

12

Ω

True RMS-to-DC Converter

AD737

FUNCTIONAL BLOCK DIAGRAM

8kΩ

1

C

C

V

2

POWER

DOWN

–V

IN

3

4

S

INPUT

AMPLIFIER

BIAS

SECTION

Figure 1.

Requiring only 160 µA of power supply current, the AD737 is
optimized for use in portable multimeters and other battery­powered applications. This converter also provides a power­down feature that reduces the power-supply standby current to
less than 30 µA.

Two signal input terminals are provided in the AD737. A high

12

impedance (10

Ω) FET input interfaces directly with high R
input attenuators, and a low impedance (8 kΩ) input accepts
rms voltages to 0.9 V while operating from the minimum power
supply voltage of ±2.5 V. The two inputs can be used either
single-ended or differentially.

The AD737 achieves 1% of reading error bandwidth, exceeding
10 kHz for input amplitudes from 20 mV rms to 200 mV rms
while consuming only 0.72 mW.

The AD737 is available in four performance grades. The
AD737J and AD737K grades are rated over the commercial
temperature range of 0°C to 70°C. The AD737JR-5 is tested with
supply voltages of ±2.5 V dc. The AD737A and AD737B grades
are rated over the industrial temperature range of −40°C to
+85°C. The AD737 is available in three low cost, 8-lead
packages: PDIP, SOIC, and CERDIP.

PRODUCT HIGHLIGHTS

1. The AD737 is capable of computing the average rectified

value, absolute value, or true rms value of various input
signals.

2. Only one external component, an averaging capacitor, is

required for the AD737 to perform true rms measurement.

3. The low power consumption of 0.72 mW makes the

AD737 suitable for battery-powered applications.

AD737

FULL

WAVE

RECTIFIER

RMS CORE

8kΩ

8

COM

+V

7

6

OUTPUT

C

5

S

AV

00828-001

Rev. F

Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]
Fax: [phone] © 2005 Analog Devices, Inc. All rights reserved.

www.analog.com

AD737

TABLE OF CONTENTS

Specifications..................................................................................... 3

Absolute Maximum Ratings............................................................ 5

ESD Caution.................................................................................. 5

Pin Configurations and Function Descriptions ........................... 6

Typical Performance Characteristics ............................................. 7

Calculating Settling Time.......................................................... 10

Types of AC Measurement ........................................................ 10

Theory of Operation ...................................................................... 12

Rapid Settling Times via the Average Responding
Connection

DC Error, Output Ripple, and Averaging Error ..................... 13

Ac Measurement Accuracy and Crest Factor ......................... 13

Selecting Practical Values for Capacitors................................ 13

Application Circuits....................................................................... 15

Outline Dimensions....................................................................... 17

Ordering Guide .......................................................................... 18

................................................................................. 12

RMS Measurement—Choosing Optimum Value For C

AV

.... 12

REVISION HISTORY

1/05—Rev. E to Rev. F

Updated Format..................................................................Universal

Added Functional Block Diagram.................................................. 1

Changes to General Description Section ...................................... 1

Changes to Pin Configurations and Function

Descriptions Section .................................................................... 6

Changes to Typical Performance Characteristics Section........... 7

Changes to Table 4.......................................................................... 11

Change to Figure 24 ....................................................................... 12

Change to Figure 27 ....................................................................... 15

Changes to Ordering Guide.......................................................... 18

6/03—Rev. D to Rev. E

Added AD737JR-5..............................................................Universal

Changes to FEATURES ................................................................... 1

Changes to GENERAL DESCRIPTION ....................................... 1

Changes to SPECIFICATIONS....................................................... 2

Changes to ABSOLUTE MAXIMUM RATINGS ........................ 4

Changes to ORDERING GUIDE ................................................... 4

Added TPCs 16 through 19............................................................. 6

Changes to Figures 1 and 2.............................................................. 8

Changes to Figure 8........................................................................ 11

Updated OUTLINE DIMENSIONS ............................................ 12

12/02—Rev. C to Rev. D

Changes to FUNCTIONAL BLOCK DIAGRAM ........................ 1

Changes to PIN CONFIGURATION .............................................4

Figure 1 replaced ...............................................................................8

Changes to Figure 2...........................................................................8

Figure 5 replaced ............................................................................ 10

Changes to Application Circuits Figures 4, 6–8 ......................... 10

OUTLINE DIMENSIONS updated............................................. 12

12/99—Rev. B to Rev. C

Rev. F | Page 2 of 20

AD737

(

SPECIFICATIONS

T = 25°C, ±5 V supplies, except AD737J-5, ±2.5 V, CAV = 33 µF, CC = 10 µF, f = 1 kHz, sine wave input applied to Pin 2, unless otherwise
specified. Specifications shown in boldface are tested on all production units at final electrical test. Results from these tests are used to
calculate outgoing quality levels.

Table 1.

AD737J/AD737A AD737K/AD737B AD737J-5

Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit

TRANSFER FUNCTION

OUT

2

VAvgV =

IN

)

CONVERSION ACCURACY

Total Error EIN = 0 to 200 mV rms 0.2/0.3

0.4/0.5

±VS = ±2.5 V 0.2/0.3
±VS = ±2.5 V, Input to

0.2/0.3

0.2/0.2

0.2/0.3

±mV/±POR

0.4/0.5

0.4/0.5

Pin 1

E

T

to T

MIN

MAX

A and B Grades EIN = 200 mV rms
J and K Grades EIN = 200 mV rms,

= 200 mV − 1 V rms −1.2

IN

±2.0

−1.2

±2.0

POR

0.5/0.7

0.3/0.5

±mV/POR

0.007 0.007 0.02 ±POR/°C

±V

= ±2.5 V

S

vs. Supply Voltage EIN = 200 mV rms
±VS = ±2.5 V to ±5 V
±VS = ±5 V to ±16.5 V

DC Reversal Error,

VIN = 600 mV dc

0
0

−0.18

0.06

−0.3 0

0.1 0

−0.18

0.06

−0.3 0

0.1 0

−0.18

0.06

−0.3

0.1

1.3 2.5 1.3 2.5 POR

DC-Coupled

V

Nonlinearity

2

E
Input to Pin 1,

AC-Coupled

3

Total Error, External

= 200 mV dc,

IN

±V

= ±2.5 V

S

EIN = 0 to 200 mV rms

= 100 mV rms

IN

EIN = 100 mV rms, after
correction, ±V

= ±2.5 V

S

0

0.02 0.1 POR

1.7 2.5 POR

0.25

0.35

0 0.25

0.35

POR

EIN = 0 to 200 mV rms 0.1/0.2 0.1/0.2 0.1/0.2 ±mV/±POR

Trim

ADDITIONAL CREST FACTOR ERROR

4

Crest Factor = 1 to 3 CAV = CF = 100 µF 0.7 0.7 %

C

= 22 µF, CF =

AV

100 µF, ±V

= ±2.5 V,

S

1.7 %

input to Pin 1

Crest Factor = 5 CAV = CF = 100 µF 2.5 2.5 %

INPUT CHARACTERISTICS

High Z Input (Pin 2)

Signal Range

Continuous RMS

±VS = +2.5 V

200

Level

±VS = +2.8 V, −3.2 V

Peak Transient Input ±VS = +2.5 V, input to

±VS = ±5 V to ±16.5 V

200
1

200
1

mV rms
V rms

V

±0.6

Pin 1

±VS = +2.8 V, −3.2 V

±0.9

±0.9

V
±VS = ±5 V ±2.7 ±2.7 V
±VS = ±16.5 V

±4.0

Input Resistance 10

12

10

±4.0

12

V

10

12

Ω

Input Bias Current ±VS = ±5 V 1 25 1 25 1 25 pA
Low Z Input (Pin 1)
Signal Range

Continuous RMS

±VS = +2.5 V 300 mV rms

Level
±VS = +2.8 V, −3.2 V 300 300 mV rms
±VS = ±5 V to ±16.5 V 1 1 V rms

1

POR
POR

%/V
%/V

mV rms

Rev. F | Page 3 of 20

AD737

AD737J/AD737A AD737K/AD737B AD737J-5

Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit

Peak Transient Input ±VS = +2.5 V ±1.7 V
±VS = +2.8 V, −3.2 V ±1.7 ±1.7 V
±VS = ±5 V ±3.8 ±3.8 V
±VS = ±16.5 V ±11 ±11 V

Input Resistance 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 kΩ
Maximum Continuous
Nondestructive Input,

All Supply Voltages

Input Offset Voltage

vs. Temperature 8 30 8 30 8 30 µV/°C

vs. Supply VS = ±2.5 V to ±5 V 80 80 80 µV/V
V
OUTPUT CHARACTERISTICS—NO LOAD

Output Voltage Swing ±VS = +2.8 V, −3.2 V −1.6 −1.7 −1.6 −1.7 V
±VS = ±5 V −3.3 −3.4 −3.3 −3.4 V
±VS = ±16.5 V
±VS = ±2.5 V (Pin 1) −1.1 –0.9 V
Output Resistance dc 6.4 8 9.6 6.4 8 9.6 6.4 8 9.6 kΩ

FREQUENCY RESPONSE

High Impedance Input

(Pin 2)

1% Additional Error VIN = 1 mV rms 1 1 1 kHz
V
V
V

3 dB Bandwidth VIN = 1 mV rms 5 5 5 kHz
V
V
V
Low Z Input (Pin 1)

1% Additional Error VIN = 1 mV rms 1 1 1 kHz
V
V
V
V

3 dB Bandwidth VIN = 1 mV rms 5 5 5 kHz
V
V
V
POWER-DOWN

Disable Voltage 0 0 V
Input Current, PD

Enabled
POWER SUPPLY

Operating Voltage

Range

Current No input 120
Rated iInput 170 210 170 210 170 210 µA
Power-down 25 40 25 40 25 40 µA
TEMPERATURE RANGE See the Ordering Guide

1

POR = % of reading.

2

Nonlinearity is defined as the maximum deviation (in percent error) from a straight line connecting the readings at 0 V and 200 mV rms.

3

After fourth-order error correction using the equation y = − 0.31009x 4 − 0.21692x3 − 0.06939x2 + 0.99756x + 11.1 × 10−6, where y is the corrected result and x is the

device output between 0.01 V and 0.3 V.

4

Crest factor error is specified as the additional error resulting from the specific crest factor, using a 200 mV rms signal as a reference. The crest factor is defined as

/V rms.

V

PEAK

5

DC offset does not limit ac resolution.

±12 ±12 ±12 V p-p

5

AC-coupled

= ±5 V to ±16.5 V 50 150 50 150 µV/V

S

−4 −5

±3

−4 −5

±3

±3

mV

V

= 10 mV rms 6 6 6 kHz

IN

= 100 mV rms 37 37 37 kHz

IN

= 200 mV rms 33 33 33 kHz

IN

= 10 mV rms 55 55 55 kHz

IN

= 100 mV rms 170 170 170 kHz

IN

= 200 mV rms 190 190 190 kHz

IN

= 10 mV rms 6 6 6 kHz

IN

= 40 mV rms 25 kHz

IN

= 100 mV rms 90 90 90 kHz

IN

= 200 mV rms 90 90 90 kHz

IN

= 10 mV rms 55 55 55 kHz

IN

= 100 mV rms 350 350 350 kHz

IN

= 200 mV rms 460 460 460 kHz

IN

VPD = V

S

+2.8/

11 11 µA

−3.2

±5 ±16.5 +2.8/

−3.2

160

120

±5 ±16.5 ±2.5 ±5 ±16.5 V

160

120

160

µA

Rev. F | Page 4 of 20

AD737

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Supply Voltage ±16.5 V
Internal Power Dissipation
Input Voltage ±V

1

200 mW

S

Output Short-Circuit Duration Indefinite
Differential Input Voltage +VS and −V

S

Storage Temperature Range (Q) −65°C to +150°C
Storage Temperature Range (N, R) −65°C to +125°C
Lead Temperature Range (Soldering 60 sec) 300°C
ESD Rating 500 V

1

8-Lead PDIP package: θJA = 165°C/W.

8-Lead CERDIP package: θ
8-Lead SOIC: θ

= 155°C/W.

JA

= 110°C/W.

JA

ESD CAUTION

ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on
the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.

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 indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

Rev. F | Page 5 of 20

AD737

PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

1

C

C

AD737

2

V

IN

–V

3
4

S

TOP VIEW

(Not to Scale)

POWER DOWN

Figure 2. R-8 (SOIC) Pin Configuration

Table 3. Pin Function Descriptions

Pin No. Mnemonic Description

1 C
2 V

C

IN

3 POWER DOWN Disables the AD737. Low is enabled; high is powered-down.
4

5 C

−V

S

AV

6 OUTPUT Output.
7 +V

S

8 COM Common.

8

COM

7

+V

6

OUTPUT

5

C

S

AV

00828-029

POWER DOWN

C

V

–V

1

C

2

IN

3
4

S

AD737

TOP VIEW

(Not to Scale)

Figure 3. Q-8 (CERDIP) Pin Configuration

Coupling Capacitor for Indirect DC Coupling.
RMS Input.

Negative Power Supply.

Averaging Capacitor.

Positive Power Supply.

8

COM
+V

7

S

6

OUTPUT
C

5

AV

00828-030

1

C

C

AD737

2

V

IN

TOP VIEW

POWER DOWN

–V

3
4

S

(Not to Scale)

Figure 4. N-8 (PDIP) Pin Configuration

8

COM
+V

7
6

OUTPUT
C

5

S

AV

00828-031

Rev. F | Page 6 of 20

AD737

TYPICAL PERFORMANCE CHARACTERISTICS

T = 25°C, ±5 V supplies, except AD737J-5, ±2.5 V, CAV = 33 µF, CC = 10 µF, f = 1 kHz, sine wave input applied to Pin 2, unless
otherwise specified.

0.7
VIN = 200mV rms
C

0.5

C

= 100µF

AV

= 22µF

F

10V

CAV = 22µF, CF = 4.7µF, CC = 22µF

1V

0.3

0.1

0

–0.1

–0.3

ADDITIONAL ERROR (% OF READING)

–0.5

04286121410 16

SUPPLY VOLTAGE (±V)

Figure 5. Additional Error vs. Supply Voltage

16

DC-COUPLED

14

12

10

8

6

4

PEAK INPUT BEFORE CLIPPING (V)

2

0

04286121410 16

PIN 1

PIN 2

SUPPLY VOLTAGE (±V)

Figure 6. Maximum Input Level vs. Supply Voltage

25

20

00828-015

00828-016

100mV

10mV

INPUT LEVEL (rms)

1mV

100µV

0.1 1 10010 1000

1% ERROR

–3dB

10% ERROR

FREQUENCY (kHz)

Figure 8. Frequency Response Driving Pin 1

10V

CAV = 22µF, CF = 4.7µF, CC = 22µF

1V

100mV

10mV

INPUT LEVEL (rms)

1mV

100µV

0.1 1 10010 1000

1% ERROR

10% ERROR

–3dB

FREQUENCY (kHz)

Figure 9. Frequency Response Driving Pin 2

6

3ms BURST OF 1kHz =
3 CYCLES
200mV rms SIGNAL

5

= 22µF

C

C

C

= 100µF

F

4

CAV = 10µF

CAV = 33µF

00828-017

00828-018

15

SUPPLY CURRENT (µA)

10

5

024681012141618

DUAL SUPPLY VOLTAGE (±V)

Figure 7. Power-Down Current vs. Supply Voltage

00828-F-002

Rev. F | Page 7 of 20

3

2

1

ADDITIONAL ERROR (% OF READING)

0

1234

CREST FACTOR (V

CAV = 250µF

/V rms)

PEAK

Figure 10. Additional Error vs. Crest Factor vs. C

CAV = 100µF

AV

00828-019

5

AD737

0.8

0.6

0.4

0.2

0

–0.2

–0.4

ADDITIONAL ERROR (% OF READING)

–0.6

–0.8

–60 –20–40 200 60 80 100 12040 140

500

400

300

200

VIN = 200mV rms
CAV = 100µF
C

= 22µF

F

TEMPERATURE (°C)

Figure 11. Additional Error vs. Temperature

00828-020

1.0

0.5

0

–0.5

–1.0

–1.5

ERROR (% OF READING)

–2.0

CAV = 22µF, CC = 47µF,

= 4.7µF

C

F

–2.5

10mV 100mV 1V 2V

INPUT LEVEL (rms)

Figure 14. Error vs. RMS Input Voltage Using Circuit of Figure 29

100

(µF)

10

AV

C

–0.5%

VIN = 200mV rms
C

= 47µF

C

C

= 47µF

F

00828-022

DC SUPPLY CURRENT (µA)

100

5

0 0.2 0.4 0.6 0.8 1.0

RMS INPUT LEVEL (V)

Figure 12. DC Supply Current vs. RMS Input Level

10mV

AC-COUPLED

1mV

INPUT LEVEL (rms)

100µV

10µV

100 1k 10k 100k

–3dB FREQUENCY (Hz)

Figure 13. RMS Input Level vs. –3 dB Frequency

00828-003

00828-021

–1%

1

10 100 1k

Figure 15. C

AV

FREQUENCY (Hz)

vs. Frequency for Specified Averaging Error

1V

–1%

100mV

10mV

INPUT LEVEL (rms)

1mV

1 10 100 1k

FREQUENCY (Hz)

–0.5%

AC-COUPLED
CAV = 10µF, CC = 47µF,

= 47µF

C

F

Figure 16. RMS Input Level vs. Frequency for Specified Averaging Error

00828-023

00828-024

Rev. F | Page 8 of 20

AD737

4.0

3.5

10

VS=±2.5V,

C

= 22µF, CF = 4.7µF, CC = 22µF

AV

1

3.0

2.5

2.0

INPUT BIAS CURRENT (pA)

1.5

1.0

02468 121410 16

SUPPLY VOLTAGE (±V)

Figure 17. Input Bias Current vs. Supply Voltage

1V

100mV

CAV = 10µF

10mV

INPUT LEVEL (rms)

1mV

100µV

1ms 10ms 100ms 1s 10s 100s

CAV = 33µF

SETTLING TIME

CC = 22µF
C

= 0µF

F

CAV = 100µF

Figure 18. Settling Time vs. RMS Input Level for Various Values of C

10nA

1nA

100pA

10pA

INPUT BIAS CURRENT

1pA

100fA

–55 –35 –15 5 25 65 85 10545 125

TEMPERATURE (°C)

Figure 19. Input Bias Current vs. Temperature

00828-025

00828-026

AV

00828-027

100m

10m

INPUT LEVEL (V rms)

1m

100µ

0.1 1 10 100 1000
FREQUENCY (kHz)

00828-004

Figure 20. Frequency Response Driving Pin 1

10

VS=±2.5V,

C

= 22µF, CF = 4.7µF, CC = 22µF

AV

1

100m

10m

INPUT LEVEL (V rms)

1m

100µ

0.1 1 10 100 1000

0.5%

1%

FREQUENCY (kHz)

10%

3dB

00828-005

Figure 21. Error Contours Driving Pin 1

5

3 CYCLES OF 1kHz
200mV rms

= ±2.5V

V

S

4

= 22µF

C

C

= 100µF

C

F

3

2

1

ADDITIONAL ERROR (% OF READING)

0

1234

CREST FACTOR

Figure 22. Additional Crest Factor Error for Various Values of C

22µF

CAV =
10µF

100µF

33µF

220µF

00828-006

5

AV

Rev. F | Page 9 of 20

AD737

(

1.0

0.5

0

–0.5

–1.0

–1.5

ERROR (% OF READING)

–2.0

CAV = 22µF, VS = ±2.5V

= 47µF, CF = 4.7µF

C

C

–2.5

10m 100m 1 2

Figure 23. Error vs. Input Voltage Driving Pin 1

INPUT LEVEL (V rms)

CALCULATING SETTLING TIME

Figure 18 can be used to closely approximate the time required
for the AD737 to settle when its input level is reduced in
amplitude. The net time required for the rms converter to settle
is the difference between two times extracted from the graph:
the initial time minus the final settling time. As an example,
consider the following conditions: a 33 µF averaging capacitor,
an initial rms input level of 100 mV, and a final (reduced) input
level of 1 mV. From Figure 18, the initial settling time (where
the 100 mV line intersects the 33 µF line) is approximately
80 ms. The settling time corresponding to the new or final input
level of 1 mV is approximately 8 seconds. Therefore, the net
time for the circuit to settle to its new value is 8 seconds minus
80 ms, which is 7.92 seconds.

Note that because of the smooth decay characteristic inherent
with a capacitor/diode combination, this is the total settling
time to the final value (not the settling time to 1%, 0.1%, and so
on, of t he final value). Also, t his graph provides the worst-case
settling time, because the AD737 settles very quickly with
increasing input levels.

00828-007

TYPES OF AC MEASUREMENT

The AD737 is capable of measuring ac signals by operating as
either an average responding or a true rms-to-dc converter. As
its name implies, an average responding converter computes the
average absolute value of an ac (or ac and dc) voltage or current
by full wave rectifying and low-pass filtering the input signal;
this approximates the average. The resulting output, a dc
average level, is then scaled by adding (or reducing) gain; this
scale factor converts the dc average reading to an rms equiva­lent value for the waveform being measured. For example, the
average absolute value of a sine wave voltage is 0.636 that of

; the corresponding rms value is 0.707 times V

V

PEAK

Therefore, for sine wave voltages, the required scale factor is

1.11 (0.707 divided by 0.636).

In contrast to measuring the average value, true rms measure­ment is a universal language among waveforms, allowing the
magnitudes of all types of voltage (or current) waveforms to be
compared to one another and to dc. RMS is a direct measure of
the power or heating value of an ac voltage compared to that of
a dc voltage; an ac signal of 1 V rms produces the same amount
of heat in a resistor as a 1 V dc signal.

Mathematically, the rms value of a voltage is defined (using a
simplified equation) as

2

)

rms VAvgV =

This involves squaring the signal, taking the average, and then
obtaining the square root. True rms converters are smart
rectifiers; they provide an accurate rms reading regardless of the
type of waveform being measured. However, average respond­ing converters can exhibit very high errors when their input
signals deviate from their precalibrated waveform; the magni­tude of the error depends on the type of waveform being
measured. As an example, if an average responding converter is
calibrated to measure the rms value of sine wave voltages and
then is used to measure either symmetrical square waves or dc
voltages, the converter has a computational error 11% (of
reading) higher than the true rms value (see Table 4).

PEAK

.

Rev. F | Page 10 of 20

AD737

Table 4. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms

Type of Waveform
1 V Peak Amplitude

Undistorted Sine Wave 1.414 0.707 0.707 0
Symmetrical Square Wave 1.00 1.00 1.11 11.0
Undistorted Triangle Wave 1.73 0.577 0.555 −3.8
Gaussian Noise
(98% of Peaks <1 V) 3 0.333 0.295 −11.4
Rectangular 2 0.5 0.278 −44
Pulse Train 10 0.1 0.011 −89
SCR Waveforms
50% Duty Cycle 2 0.495 0.354 −28
25% Duty Cycle 4.7 0.212 0.150 −30

Crest Factor

/V rms)

(V

PEAK

True RMS
Value (V)

Reading of an Average Responding Circuit
Calibrated to an RMS Sine Wave Value (V)

Error (%)

Rev. F | Page 11 of 20

AD737

THEORY OF OPERATION

As shown in Figure 24, the AD737 has four functional
subsections: input amplifier, full-wave rectifier, rms core, and
bias section. The FET input amplifier allows both a high
impedance, buffered input (Pin 2) or a low impedance, wide
dynamic range input (Pin 1). The high impedance input, with
its low input bias current, is well suited for use with high
impedance input attenuators. The input signal can be either
dc-coupled or ac-coupled to the input amplifier. Unlike other
rms converters, the AD737 permits both direct and indirect
ac coupling of the inputs. AC coupling is provided by placing
a series capacitor between the input signal and Pin 2 (or Pin 1)
for direct coupling and between Pin 1 and ground (while
driving Pin 2) for indirect coupling.

AC

10µF

C

C =

+

DC

OPTIONAL RETURN PATH

CURRENT
MODE
ABSOLUTE
VALUE

C

1

C

8kΩ

V

IN

2

V

IN

POWER

DOWN

–V

S

FET

OP AMP

1

<10pA

B

BIAS

3

SECTION

RMS

TRANSLINEAR

CORE

4

POSITIVE SUPPLY

NEGATIVE SUPPLY

COMMON

C

33µF

A

+

0.1µF

0.1

8kΩ

µ

F

Figure 24. AD737 True RMS Circuit ( Test Circuit)

The output of the input amplifier drives a full-wave precision
rectifier, which, in turn, drives the rms core. It is in the core that
the essential rms operations of squaring, averaging, and square
rooting are performed, using an external averaging capacitor,

. Without CAV, the rectified input signal travels through the

C

AV

core unprocessed, as is done with the average responding
connection (Figure 25).

8

COM

C

F

+

10µF
(OPTIONAL

+V

7

S

LPF)

6

OUTPUT

C

5

AV

+V

S

–V

S

In the average responding connection, all the averaging is
carried out by an RC post filter consisting of an 8 kΩ internal
scale-factor resistor connected between Pin 6 and Pin 8, and an
external averaging capacitor, C

. In the rms circuit, this addi-

F

tional filtering stage helps reduce any output ripple that was not
removed by the averaging capacitor, C

.

AV

A final subsection, the bias section, permits a power-down
function. This reduces the idle current of the AD737 from
160 µA down to a mere 30 µA. This feature is selected by tying
Pin 3 to the +V

terminal.

S

RMS MEASUREMENT—CHOOSING OPTIMUM
VALUE FOR C

Because the external averaging capacitor, CAV, holds the rectified
input signal during rms computation, its value directly affects
the accuracy of the rms measurement, especially at low frequen­cies. Furthermore, because the averaging capacitor appears
across a diode in the rms core, the averaging time constant
increases exponentially as the input signal is reduced. This
means that as the input level decreases, errors due to nonideal
averaging decrease while the time it takes for the circuit to settle
to the new rms level increases. Therefore, lower input levels
allow the circuit to perform better (due to increased averaging)
but increase the waiting time between measurements. When
selecting C
settling time is required.

AV

, a trade-off between computational accuracy and

AV

RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION

Because the average responding connection shown in Figure 25
does not use an averaging capacitor, its settling time does not
vary with input signal level; it is determined solely by the RC
time constant of C
resistor.

C

1

C

00828-008

V

IN

POWER

DOWN

–V

2

3

4

S

NEGATIVE SUPPLY

and the internal 8 kΩ output scaling

F

8kΩ

INPUT

AMPLIFIER

BIAS

SECTION

POSITIVE SUPPLY

COMMON

FULL

WAVE

RECTIFIER

RMS

CORE

AD737

0.1µF

0.1µF

8kΩ

8

COM

+V

7

6

OUTPUT

C

5

+V

S

–V

S

Figure 25. AD737 Average Responding Circuit

+

C

F

S

33µF

V

OUT

AV

00828-009

Rev. F | Page 12 of 20

AD737

DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR

Figure 26 shows the typical output waveform of the AD737 with
a sine wave input voltage applied. As with all real-world devices,
the ideal output of V
the output contains both a dc and an ac error component.

E

O

Figure 26. Output Waveform for Sine Wave Input Voltage

As shown, the dc error is the difference between the average of
the output signal (when all the ripple in the output has been
removed by external filtering) and the ideal dc output. The dc
error component is, therefore, set solely by the value of the
averaging capacitor used—no amount of post filtering (using a
very large C

) allows the output voltage to equal its ideal value.

F

The ac error component, an output ripple, can be easily
removed using a large enough postfiltering capacitor, C

In most cases, the combined magnitudes of the dc and ac error
components must be considered when selecting appropriate
values for capacitors C
senting the maximum uncertainty of the measurement, is
termed the averaging error and is equal to the peak value of the
output ripple plus the dc error. As the input frequency increases,
both error components decrease rapidly; if the input frequency
doubles, the dc error and ripple reduce to one-quarter and one­half of their original values, respectively, and rapidly become
insignificant.

= VIN is never exactly achieved; instead,

OUT

IDEAL

E

O

DC ERROR = EO– EO (IDEAL)

DOUBLE-FREQUENCY

RIPPLE

and CF. This combined error, repre-

AV

AVERAGE E

= E

O

O

TIME

00828-028

.

F

AC MEASUREMENT ACCURACY AND CREST FACTOR

The crest factor of the input waveform is often overlooked
when determining the accuracy of an ac measurement. Crest
factor is defined as the ratio of the peak signal amplitude to the
rms amplitude (Crest Factor = V

/V rms). Many common

PEAK

waveforms, such as sine and triangle waves, have relatively low
crest factors (≥2). Other waveforms, such as low duty-cycle
pulse trains and SCR waveforms, have high crest factors. These
types of waveforms require a long averaging time constant to
average out the long time periods between pulses. Figure 10
shows the additional error vs. the crest factor of the AD737 for
various values of C

.

AV

SELECTING PRACTICAL VALUES FOR CAPACITORS

Table 5 provides practical values of CAV and CF for several
common applications.

The input coupling capacitor, C
internal input scaling resistor, determines the −3 dB low fre­quency roll-off. This frequency, F

=

F

L

()

()

000,82

π

Note that at F

, the amplitude error is approximately −30%

L

(−3 dB) of reading. To reduce this error to 0.5% of reading,
choose a value of C

that sets FL at one-tenth of the lowest

C

frequency to be measured.

In addition, if the input voltage has more than 100 mV of dc
offset, then the ac coupling network at Pin 2 should be used in
addition to capacitor C

.

C

, in conjunction with the 8 kΩ

C

, is equal to

L

1

FaradsinCofValueThe

C

Rev. F | Page 13 of 20

AD737

Table 5. AD737 Capacitor Selection

RMS Input

Application

Level

General-Purpose RMS Computation 0−1 V 20 Hz 5 150 µF 10 µF 360 ms
200 Hz 5 15 µF 1 µF 36 ms
0−200 mV 20 Hz 5 33 µF 10 µF 360 ms
200 Hz 5 3.3 µF 1 µF 36 ms
General-Purpose Average Responding 0−1 V 20 Hz None 33 µF 1.2 sec
200 Hz None 3.3 µF 120 ms
0−200 mV 20 Hz None 33 µF 1.2 sec
200 Hz None 3.3 µF 120 ms
SCR Waveform Measurement 0−200 mV 50 Hz 5 100 µF 33 µF 1.2 sec
60 Hz 5 82 µF 27 µF 1.0 sec
0−100 mV 50 Hz 5 50 µF 33 µF 1.2 sec
60 Hz 5 47 µF 27 µF 1.0 sec
Audio Applications

Speech 0−200 mV 300 Hz 3 1.5 µF 0.5 µF 18 ms
Music 0−100 mV 20 Hz 10 100 µF 68 µF 2.4 sec

1

Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times are greater for decreasing amplitude input signals.

Low Frequency
Cutoff (−3 dB)

Maximum
Crest Factor

CAV CF

Settling Time
to 1%

1

Rev. F | Page 14 of 20

AD737

V

APPLICATION CIRCUITS

SWITCH CLOSED

ACTIVATES

POWER-DOWN

MODE. AD737 DRAWS

JUST 40µA IN THIS MODE

1PRV

0.01µF

IN

9MΩ

900kΩ

90kΩ

10kΩ

200mV

2V

20V

200V

1N4148

47kΩ

1N4148

1µF

1W

+

POWER

DOWN

–V

10µF

C

V

C

C

+

C

8kΩ

1

IN

2

3

S
4

INPUT

AMPLIFIER

BIAS

SECTION

Figure 27. 3 1/2 Digit DVM Circuit

INPUT SCALE FACTOR ADJ

R3

69.8kΩ
1%

R2

5kΩ

0.01µF

C

1

C

V

2

3

4

IN

AD737JR

POWER
DOWN

–V

S

C2

INPUT

C1

0.47µF

RECTIFIER

COM

+V

OUTPUT

C

+

FULL

WAVE

RMS

CORE

S

AV

1µF

8

7

6

5

AD737

5V

COM

8

+V

7

8kΩ

OUTPUT

6

C

5

R4

5kΩ

R3

78.7kΩ

OUTPUT ZERO

ADJUST

0.01µF

20kΩ

+V

S

AD589

1.23V

S

+V

AV

+

33µF

200kΩ

50kΩ

1MΩ

0.1µF

C

F

0.47µF

R5

80.6kΩ

1

2

7

AD8541AR

3

C3

5

4

3 1/2 DIGIT ICL7136
TYPE CONVERTER

REF HIGH

REF LOW

COMMON

LOW

ANALOG

HIGH

5V

0.01µF

6

–V

S

OUTPUT

+

9V

00828-010

2.5V

5V

00828-011

C

33µF

+

AV

2.2µF

1µF

C4

+

C5

100kΩ

R8

100kΩ

R7

Figure 28. Battery-Powered Operation for 200 mV Maximum RMS Full-Scale Input

Rev. F | Page 15 of 20

AD737

C

C

C

C

V

IN

POWER

DOWN

10µF
+

8kΩ

1

FULL

WAVE

2

AMPLIFIER

BIAS

3

SECTION

–V

S

4

INPUT

RECTIFIER

RMS

CORE

100Ω

AD737

8kΩ

SCALE FACTOR

ADJUST

COM

8

200Ω

C

+V

7

S

10µF

OUTPUT

6

C

AV

5

+

F

V

OUT

+

C

AV

33µF

00828-012

Figure 29. External Scale Factor Trim

14

13

C

C

10µF

+

V

IN

POWER

DOWN

Q1

12

*

2

3

8kΩ

COM

NOT

8

CONNECTED

+V

7

S

OUTPUT

6

C

AV

5

C

C

8kΩ

1

2

3

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

FULL

WAVE

RECTIFIER

RMS

CORE

AD737

SCALE

FACTOR

AD711

1kΩ

3500

PPM/°C

60.4Ω

TRIM

PRECISION
RESISTOR

TYPE PT/ST

6

CORP

2kΩ

31.6kΩ

dB OUTPUT
100mV/dB

*

11

10

Q2

9

*Q1, Q2 PART OF

RCA CA3046 OR
SIMILAR NPN
TRANSISTOR ARRAY

00828-013

** R

** R1 + R

CAL

C

IN Ω = 10,000 ×

CAL

+

AV

** R1

I

REF

4.3V

0dB INPUT LEVEL IN V

Figure 30. dB Output Connection

OFFSET ADJUST

V

IN

POWER

DOWN

S

500kΩ

INPUT

AMPLIFIER

+V

C

C

8kΩ

1

2

3

Figure 31. DC-Coupled V

1MΩ

FULL

WAVE

RECTIFIER

–V

S

1kΩ

COM

AD737

and Scale Factor Trims

OS

8

7

6

499Ω

1kΩ
SCALE

FACTOR

+V

ADJUST

S

V

OUT

00828-014

Rev. F | Page 16 of 20

AD737

OUTLINE DIMENSIONS

5.00 (0.1968)

4.80 (0.1890)

4.00 (0.1574)

3.80 (0.1497)

0.25 (0.0098)

0.10 (0.0040)

COPLANARITY

0.10

CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

85

1.27 (0.0500)

SEATING

PLANE

COMPLIANT TO JEDEC STANDARDS MS-012AA

BSC

6.20 (0.2440)

5.80 (0.2284)

41

1.75 (0.0688)

1.35 (0.0532)

0.51 (0.0201)

0.31 (0.0122)

0.25 (0.0098)

0.17 (0.0067)

0.50 (0.0196)

0.25 (0.0099)

8°

1.27 (0.0500)

0°

0.40 (0.0157)

Figure 32. 8-Lead Standard Small Outline Package [SOIC]

Narrow Body (R-8)

Dimensions shown in millimeters and (inches)

0.400 (10.16)

0.365 (9.27)

0.355 (9.02)

8

1

PIN 1

0.100 (2.54)

0.210
(5.33)

MAX

0.150 (3.81)

0.130 (3.30)

0.115 (2.92)

0.022 (0.56)

0.018 (0.46)

0.014 (0.36)

0.070 (1.78)

0.060 (1.52)

0.045 (1.14)

CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN.
CORNER LEADS MAY BE CONFIGURED AS WHOLE OR HALF LEADS.

COMPLIANT TO JEDEC STANDARDS MS-001-BA

Figure 34. 8-Lead Plastic Dual-In-Line Package [PDIP]

Dimensions shown in inches and (millimeters)

BSC

× 45°

5

4

0.280 (7.11)

0.250 (6.35)

0.240 (6.10)

0.015
(0.38)
MIN

SEATING
PLANE

0.005 (0.13)
MIN

(N-8)

0.060 (1.52)
MAX

0.015 (0.38)
GAUGE

PLANE

0.005 (0.13)

PIN 1

0.200 (5.08)
MAX

0.200 (5.08)

0.125 (3.18)

0.023 (0.58)

0.014 (0.36)

CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN

0.055 (1.40)

MIN

0.100 (2.54) BSC

0.405 (10.29) MAX

MAX

85

0.310 (7.87)

4

0.070 (1.78)

0.030 (0.76)

0.220 (5.59)

0.060 (1.52)

0.015 (0.38)

1

0.150 (3.81)
MIN

SEATING
PLANE

0.320 (8.13)

0.290 (7.37)

15°

0°

0.015 (0.38)

0.008 (0.20)

Figure 33. 8-Lead Ceramic Dual In-Line Package [CERDIP]

(Q-8)

Dimensions shown in inches and (millimeters)

0.325 (8.26)

0.310 (7.87)

0.300 (7.62)

0.430 (10.92)
MAX

0.195 (4.95)

0.130 (3.30)

0.115 (2.92)

0.014 (0.36)

0.010 (0.25)

0.008 (0.20)

Rev. F | Page 17 of 20

AD737

ORDERING GUIDE

Model Temperature Range Package Description Package Option

AD737AQ −40°C to +85°C 8-Lead Ceramic Dual In-Line Package (CERDIP) Q-8
AD737BQ −40°C to +85°C 8-Lead Ceramic Dual In-Line Package (CERDIP) Q-8
AD737JN 0°C to 70°C 8-Lead Plastic Dual-In-Line Package (PDIP) N-8
AD737JNZ
AD737JR 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JR-5 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JR-5-REEL 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JR-5-REEL7 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JR-REEL 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JR-REEL7 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JRZ1 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JRZ-51 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JRZ-5-R71 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JRZ-5-RL1 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JRZ-R71 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737JRZ-RL1 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737KN 0°C to 70°C 8-Lead Plastic Dual-In-Line Package (PDIP) N-8
AD737KNZ1 0°C to 70°C 8-Lead Plastic Dual-In-Line Package (PDIP) N-8
AD737KR 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737KR-REEL 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737KR-REEL7 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737KRZ1 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737KRZ-R71 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8
AD737KRZ-RL1 0°C to 70°C 8-Lead Standard Small Outline Package (SOIC) Narrow Body R-8

1

Z = Pb-free part.

1

0°C to 70°C 8-Lead Plastic Dual-In-Line Package (PDIP) N-8

Rev. F | Page 18 of 20

AD737

NOTES

Rev. F | Page 19 of 20

AD737

NOTES

© 2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.

C00828–0–1/05(F)

Rev. F | Page 20 of 20