Datasheet AD736 Datasheet (Analog Devices)

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)
High input impedance of 1012 V
Low input bias current: 25 pA max
High accuracy: ±0.3 mV ±0.3% of reading
RMS conversion with signal crest factors up to 5
Wide power supply range: +2.8 V, –3.2 V to ±16.5 V
Low power: 200 mA max supply current
Buffered voltage output
No external trims needed for specified accuracy
AD737—an unbuffered voltage output version with chip

power-down also available

GENERAL DESCRIPTION

The AD736 is a low power, precision, monolithic true rms-to-dc
converter. It is laser trimmed to provide a maximum error of
±0.3 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 size of
this converter make it suitable for upgrading the performance
of non-rms precision rectifiers in many applications. Compared
to these circuits, the AD736 offers higher accuracy at an equal
or lower cost.

The AD736 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 AD736 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 AD736 has its own output buffer amplifier, thereby pro­viding a great deal of design flexibility. Requiring only 200 µA
of power supply current, the AD736 is optimized for use in
portable multimeters and other battery-powered applications.

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.

True RMS-to-DC Converter

FUNCTIONAL BLOCK DIAGRAM

8kΩ

1

C

C

FULL

WAVE

V

2

IN

AMPLIFIER

C

3

F

–V

S

4

BIAS

SECTION

INPUT

RECTIFIER

RMS CORE

Figure 1.

AD736

8kΩ

OUTPUT

AMPLIFIER

The AD736 allows the choice of two signal input terminals: a
high impedance FET input (10

12

Ω) that directly interfaces with
high Z input attenuators and a low impedance input (8 kΩ) that
allows the measurement of 300 mV input levels while operating
from the minimum power supply voltage of +2.8 V, −3.2 V. The
two inputs may be used either single-ended or differentially.

The AD736 has a 1% reading error bandwidth that exceeds
10 kHz for the input amplitudes from 20 mV rms to 200 mV
rms while consuming only 1 mW.

The AD736 is available in four performance grades. The
AD736J and AD736K grades are rated over the 0°C to +70°C
and −20°C to +85°C commercial temperature ranges. The
AD736A and AD736B grades are rated over the −40°C to +85°C
industrial temperature range. The AD736 is available in three
low c ost, 8-lead packages: PDIP, SOIC, and CERDIP.

PRODUCT HIGHLIGHTS

The AD736 is capable of computing the average rectified value,
absolute value, or true rms value of various input signals.

Only one external component, an averaging capacitor, is
required for the AD736 to perform true rms measurement.

The low power consumption of 1 mW makes the AD736
suitable for many battery-powered applications.

12

A high input impedance of 10
external buffer when interfacing with input attenuators.

A low impedance input is available for those applications that
require an input signal up to 300 mV rms operating from low
power supply voltages.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]
Fax: [phone] © 2004 Analog Devices, Inc. All rights reserved.

www.analog.com

Ω eliminates the need for an

8

7

6

5

AD736

COM

+V

S

OUTPUT

C

AV

00834-F-001

AD736

TABLE OF CONTENTS

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

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

Pin Configuration ........................................................................ 5

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

Typical Performance Characteristics ............................................. 6

Rapid Settling Times via the Average Responding

Connection ............................................................................. 11

DC Error, Output Ripple, and Averaging Error..................... 11

AC Measurement Accuracy and Crest Factor........................ 11

Selecting Practical Values for Input Coupling (CC), Averaging

), and Filtering (CF) Capacitors..................................... 11

(C

AV

Calculating Settling Time Using Figure 16............................... 8

Types of AC Measureme nt .......................................................... 9

Theory of Operation ...................................................................... 10

RMS Measurement—Choosing the Optimum

Val u e f o r C

........................................................................... 10

AV

REVISION HISTORY

5/04—Data Sheet Changed from Rev. E to Rev. F.

Changes to Specifications............................................................ 2

Replaced Figure 18 ..................................................................... 10

Updated Outline Dimensions................................................... 16

Changes to Ordering Guide...................................................... 16

4/03—Data Sheet Changed from Rev. D to Rev. E.

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

Changes to SPECIFICATIONS................................................... 3

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

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

11/02—Data Sheet Changed from Rev. C to Rev. D.

Application Circuits....................................................................... 13

Outline Dimensions....................................................................... 15

Ordering Guide .......................................................................... 16

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

Changes to PIN CONFIGURATION ........................................ 3

Figure 1 Replaced ......................................................................... 6

Changes to Figure 2...................................................................... 6

Changes to Application Circuits Figures 4 to 8 ........................ 8

OUTLINE DIMENSIONS updated........................................... 8

Rev. F | Page 2 of 16

AD736

SPECIFICATIONS

Table 1. @25°C ±5 V supplies, ac-coupled with 1 kHz sine wave input applied, unless otherwise noted. Specifications in bold are
tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels.

AD736J/AD736A AD736K/AD736B
Parameter Conditions Min Typ Max Min Typ Max Unit

TRANSFER FUNCTION

= √

V

OUT

CONVERSION ACCURACY 1 kHz sine wave

Tot al E rror, Inter nal Trim

All Grades 0 mV rms–200 mV rms 0.3/0.3

200 mV to 1 V rms −1.2

T

to T

MIN

MAX

A and B Grades @ 200 mV rms
J and K Grades @ 200 mV rms 0.007

vs. Supply Voltage

@ 200 mV rms Input VS = ±5 V to ±16.5 V

V

DC Reversal Error, DC-Coupled @ 600 mV dc
Nonlinearity2, 0 mV–200 mV @ 100 mV rms

1

Using C

C

0.5/0.5
±2.0

0.7/0.7

+0.06

−0.18

+0.1 0

−0.3 0

1.3 2.5

0.25

0.35 0

= ±5 V to ±3 V

S

0
0

0

Total Error, External Trim 0 mV rms–200 mV rms 0.1/0.5 0.1/0.3 ±mV/±% of Reading

ERROR VS. CREST FACTOR

3

Crest Factor = 1 to 3 CAV, CF = 100 µF 0.7 0.7 % Additional Error
Crest Factor = 5 CAV, CF = 100 µF 2.5 2.5 % Additional Error

INPUT CHARACTERISTICS

High Impedance Input (Pin 2)

Signal Range

Continuous rms Level VS = +2.8 V, −3.2 V

V

Peak Transient Input VS = +2.8 V, −3.2 V
V
V

= ±5 V to ±16.5 V

S

±0.9

= ±5 V

S

= ±16.5 V

S

±4.0

±2.7

Input Resistance 10

12

200
1

10

Input Bias Current VS = ±3 V to ±16.5 V 1 25 1 25 pA

Low Impedance Input (Pin 1)
Signal Range

Continuous rms Level VS = +2.8 V, –3.2 V 300 300 mV rms
V

= ±5 V to ±16.5 V 1 1 V rms

S

Peak Transient Input VS = +2.8 V, −3.2 V ±1.7 ±1.7 V
V
V

= ±5 V ±3.8 ±3.8 V

S

= ±16.5 V ±11 ±11 V

S

Input Resistance 6.4 8 9.6 6.4 8 9.6 kΩ

Maximum Continuous

All supply voltages ±12 ±12 V p-p

Nondestructive Input
Input Offset Voltage

J and K Grades

A and B Grades

4

±3
±3

vs. Temperature 8 30 8 30 µV/°C

vs. Supply VS = ±5 V to ±16.5 V 50
V

= ±5 V to ±3 V 80 80 µV/V

S

150

1

Accuracy is specified with the AD736 connected as shown in Figure 18 with capacitor CC.

2

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

adjusted to zero.

3

Error versus crest factor is specified as additional error for a 200 mV rms signal. Crest factor = V

4

DC offset does not limit ac resolution.

PEAK

/V rms.

2

Avg(V

)

IN

0.2/0.2

−1.2

0.007

+0.06

−0.18

1.3 2.5 % of Reading

0.25

±0.9

±4.0

V
±2.7 V
V

12

50

0.3/0.3
±2.0

0.5/0.5

+0.1

−0.3

0.35

200
1

±mV/±% of Reading
% of Reading

±mV/±% of Reading
±% of Reading/°C

%/V
%/V

% of Reading

mV rms
V rms

Ω

±3
±3

150

mV
mV

µV/V

Rev. F | Page 3 of 16

AD736

AD736J/AD736A AD736K/AD736B
Parameter Conditions Min Typ Max Min Typ Max Unit

OUTPUT CHARACTERISTICS

Output Offset Voltage

J and K Grades ±0.1
A and B Grades

±0.5
±0.5

vs.Temperature 1 20 1 20 µV/°C
vs. Supply VS = ±5 V to ±16.5 V 50

V

= ±5 V to ±3 V 50 50 µV/V

S

130

Output Voltage Swing

2 kΩ Load VS = +2.8 V, −3.2 V 0–1.6 1.7 0–1.6 1.7 V

V
V

No Load VS = ±16.5 V

= ±5 V 0–3.6 3.8 0–3.6 3.8 V

S

= ±16.5 V

S

0–4
0–4

5

12
Output Current 2 2 mA
Short-Circuit Current 3 3 mA
Output Resistance @ dc 0.2 0.2 Ω

FREQUENCY RESPONSE

High Impedance Input (Pin 2)

Sine wave input

for 1% Additional Error
VIN = 1 mV rms 1 1 kHz
VIN = 10 mV rms 6 6 kHz
VIN = 100 mV rms 37 37 kHz
VIN = 200 mV rms 33 33 kHz

±3 dB Bandwidth Sine wave input

VIN = 1 mV rms 5 5 kHz
VIN = 10 mV rms 55 55 kHz
VIN = 100 mV rms 170 170 kHz
VIN = 200 mV rms 190 190 kHz

Low Impedance Input (Pin 1)

Sine wave input

for 1% Additional Error
VIN = 1 mV rms 1 1 kHz
VIN = 10 mV rms 6 6 kHz
VIN = 100 mV rms 90 90 kHz
VIN = 200 mV rms 90 90 kHz

±3 dB Bandwidth Sine wave input

VIN = 1 mV rms 5 5 kHz
VIN = 10 mV rms 55 55 kHz
VIN = 100 mV rms 350 350 kHz
VIN = 200 mV rms 460 460 kHz

POWER SUPPLY

Operating Voltage Range +2.8, −3.2 ±5 ±16.5 +2.8, −3.2 ±5 ±16.5 V
Quiescent Current Zero signal 160

200

200 mV rms, No Load Sine wave input 230 270 230 270 µA

TEMPERATURE RANGE

Operating, Rated Performance

Commercial 0°C to 70°C AD736JN, AD736JR AD736KN, AD736KR
Industrial −40°C to +85°C AD736AQ, AD736AR AD736BQ, AD736BR

±0.1

50

0–4
0–4

5 V
12 V

160

±0.3
±0.3

130

200

mV
mV

µV/V

µA

Rev. F | Page 4 of 16

AD736

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

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

5

200 mW

S

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

S

Storage Temperature Range (Q) –65°C to +150°C

1

C

C

V

2

IN

C

3

F

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

–V

4

S

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress

Figure 2. Pin Configuration for 8-Lead PDIP (N-8), 8-Lead SOIC (RN-8),

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.

PIN CONFIGURATION

8kΩ

FULL

WAVE

RECTIFIER

INPUT

AMPLIFIER

BIAS

SECTION

RMS CORE

and 8-Lead CERDIP (Q-8) Packages

AD736

8kΩ

OUTPUT

AMPLIFIER

8

COM

+V

7

6

OUTPUT

C

5

S

AV

00834-F-001

5

8-Lead PDIP Package: θJA = 165°C/W

8-Lead CERDIP Package: θ
8-Lead SOIC Package: θ

= 110°C/W

JA

= 155°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.

Rev. F | Page 5 of 16

AD736

TYPICAL PERFORMANCE CHARACTERISTICS

0.7
VIN = 200mV rms
1kHz SINE WAVE

C

0.5

0.3

0.1

–0.1

–0.3

ADDITIONAL ERROR (% OF READING)

= 100µF

AV

C

= 22µF

F

0

10V

SINEWAVE INPUT,VS=±5V,

= 22µF, CF = 4.7µF, CC = 22µF

C

AV

1V

100mV

10mV

INPUT LEVEL (rms)

1mV

1% ERROR

–3dB

10% ERROR

–0.5

04286121410 16

SUPPLY VOLTAGE (±V)

Figure 3. 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 4. Maximum Input Level vs. Supply Voltage

16

1kHz SINE WAVE INPUT

14

12

10

00834-F-002

100µV

0.1 1 10010 1000
–3dB FREQUENCY (kHz)

00834-F-005

Figure 6. Frequency Response Driving Pin 1

10V

SINEWAVE INPUT,VS=±5V,

= 22µF, CF = 4.7µF, CC = 22µF

C

AV

1V

100mV

10mV

INPUT LEVEL (rms)

1mV

00834-F-003

100µV

0.1 1 10010 1000

1% ERROR

–3dB FREQUENCY (kHz)

10% ERROR

–3dB

00834-F-006

Figure 7. Frequency Response Driving Pin 2

6

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

5

= ±5V

V

S

C

= 22µF

C

C

= 100µF

F

4

CAV = 10µF

CAV = 33µF

8

6

4

PEAK BUFFER OUTPUT (V)

2

0

0246 1081214

SUPPLY VOLTAGE (±V)

Figure 5. Peak Buffer Output vs. Supply Voltage

00834-F-004

16

Rev. F | Page 6 of 16

3

2

1

ADDITIONAL ERROR (% OF READING)

0

1234

CREST FACTOR (V

CAV = 250µF

/V rms)

PEAK

Figure 8. Additional Error vs. Crest Factor vs. C

CAV = 100µF

AV

00834-F-007

5

AD736

0.8
VIN = 200mV rms
1kHz SINE WAVE

0.6
C

= 100µF

AV

C

= 22µF

F

0.4

V

= ±5V

S

0.2

0

–0.2

–0.4

ADDITIONAL ERROR (% OF READING)

–0.6

–0.8

–60 –20–40 200 60 80 100 12040 140

TEMPERATURE (°C)

Figure 9. Additional Error vs. Temperature

00834-F-008

1.0

0.5

0

–0.5

–1.0

–1.5

ERROR (% OF READING)

VIN = SINE WAVE @ 1kHz

–2.0

C

= 22µF, CC = 47µF,

AV

= 4.7µF, VS = ±5V

C

F

–2.5

10mV 100mV 1V 2V

INPUT LEVEL (rms)

Figure 12. Error vs. RMS Input Voltage (Pin 2),

Output Buffer Offset Is Adjusted to Zero

00834-F-011

600

VIN = 200mV rms
1kHz SINE WAVE

= 100µF

C

AV

500

= 22µF

C

F

V

= ±5V

S

400

300

DC SUPPLY CURRENT (µA)

200

100

0 0.2 0.4 0.6 0.8 1.0

rms INPUT LEVEL (V)

Figure 10. DC Supply Current vs. RMS Input Level

10mV

VIN = 1kHz
SINE WAVE INPUT

AC-COUPLED

= ±5V

V

S

1mV

100

(µF)

10

AV

C

00834-F-009

1

10 100 1k

Figure 13. C

AV

FREQUENCY (Hz)

vs. Frequency for Specified Averaging Error

–0.5%

–1%

= 200mV rms

V

IN

C

= 47µF

C

C

= 47µF

F

VS = ±5V

00834-F-012

1V

–1%

100mV

–0.5%

INPUT LEVEL (rms)

100µV

10µV

100 1k 10k 100k

–3dB FREQUENCY (Hz)

Figure 11. –3 dB Frequency vs. RMS Input Level (Pin 2)

00834-F-010

Rev. F | Page 7 of 16

10mV

INPUT LEVEL (rms)

VIN SINE WAVE
AC-COUPLED

= 10µF, CC = 47µF,

C

AV

= 47µF, VS = ±5V

C

1mV

1 10 100 1k

FREQUENCY (Hz)

F

Figure 14. RMS Input Level vs. Frequency for Specified Averaging Error

00834-F-013

AD736

4.0

3.5

3.0

2.5

2.0

INPUT BIAS CURRENT (pA)

1.5

1.0

1V

100mV

02468 121410 16

SUPPLY VOLTAGE (±V)

Figure 15. Pin 2 Input Bias Current vs. Supply Voltage

VS = 5V
C

= 22µF

C

C

= 0µF

F

CALCULATING SETTLING TIME USING FIGURE 16

Figure 16 may be used to closely approximate the time required
for the AD736 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, a
100 mV initial rms input level, and a final (reduced) 1 mV input
level. From Figure 16, the initial settling time (where the
100 mV line intersects the 33 µF line) is approximately 80 ms.

00834-F-014

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 (i.e., not the settling
time to 1%, 0.1%, and so on, of the final value). Als o, this g raph
provides the worst-case settling time since the AD736 settles
very quickly with increasing input levels.

CAV = 10µF

10mV

INPUT LEVEL (rms)

1mV

100µV

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

CAV = 33µF

SETTLING TIME

CAV = 100µF

Figure 16. Settling Time vs. RMS Input Level for Various Values of C

10nA

1nA

100pA

10pA

INPUT BIAS CURRENT

1pA

00834-F-015

AV

100fA

–55 –35 –15 5 25 65 85 10545 125

TEMPERATURE (°C)

00834-F-016

Figure 17. Pin 2 Input Bias Current vs. Temperature

Rev. F | Page 8 of 16

AD736

TYPES OF AC MEASUREMENT

The AD736 is capable of measuring ac signals by operating as
either an average responding converter 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 times that of V
times V

. Therefore, for sine wave voltages, the required scale

PEAK

; the corresponding rms value is 0.707

PEAK

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

)(2VAvgrmsV =

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. For example, if an average responding converter is
calibrated to measure the rms value of sine wave voltages and is
then used to measure either symmetrical square waves or dc
voltages, the converter will have a computational error 11% (of
reading) higher than the true rms value (see Table 3).

Table 3. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms

Average Responding Circuit
Waveform Type 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)

Calibrated to Read RMS Value of

Sine Waves Will Read (V)

% of Reading Error Using
Average Responding Circuit

Rev. F | Page 9 of 16

AD736

THEORY OF OPERATION

V

IN

OPTIONAL RETURN PATH

FWR

CURRENT

MODE

ABSOLUTE

C

IN

1

2

AMPLIFIER

I

VALUE

8kΩ

INPUT

<10pA

B

C

V

C

DC

C =

+

AC

10µF

AD736

OUTPUT

AMPLIFIER

8kΩ

COM

8

7

0.1µF

+V

S

BIAS

3

SECTION

RMS

TRANSLINEAR

CORE

–V

S

4

0.1µF

TO
COM
PIN

Figure 18. AD736 True RMS Circuit

As shown by Figure 18, the AD736 has five functional subsec­tions: the input amplifier, full-wave rectifier (FWR), rms core,
output amplifier, and bias section. The FET input amplifier
allows both a high impedance, buffered input (Pin 2) and 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 output of the input amplifier drives a full-wave precision
rectifier, which in turn drives the rms core. The essential rms
operations of squaring, averaging, and square rooting are
performed in the core, 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 19).

A final subsection, an output amplifier, buffers the output from
the core and allows optional low-pass filtering to be performed
via the external capacitor, C

, which is connected across the

F

feedback path of the amplifier. In the average responding con­nection, this is where all of the averaging is carried out. In the
rms circuit, this additional filtering stage helps reduce any out­put ripple that was not removed by the averaging capacitor, C

AV

C

33µF

C

10µF

.

A

+

F

+

6

C

5

(OPTIONAL)

RMS
OUTPUT

AV

00834-F-017

RMS MEASUREMENT—CHOOSING THE OPTIMUM
VALUE FOR C

Since 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 required 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. Obviously, when selecting C
computational accuracy and settling time is required.

AV

, a trade-off between

AV

Rev. F | Page 10 of 16

AD736

RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION

Because the average responding connection shown in Figure 19
does not use the C

averaging capacitor, its settling time does

AV

not vary with input signal level. It is determined solely by the
RC time constant of C

and the internal 8 kΩ resistor in the

F

output amplifier’s feedback path.

C

C

10µF

+

(OPTIONAL)

8kΩ

C

1

C

V

IN

2

V

IN

C

F

3

–V

S

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

POSITIVE SUPPLY

COMMON

NEGATIVE SUPPLY

FULL

WAVE

RECTIFIER

rms

CORE

C

33µF

AD736

8kΩ

OUTPUT

AMPLIFIER

+

F

0.1µF

0.1µF

Figure 19. AD736 Average Responding Circuit

8

COM

+V

S

7

+V

S

OUTPUT

6

5

+V

S

–V

S

V

OUT

C

AV

00834-F-018

DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR

Figure 20 shows the typical output waveform of the AD736 with
a sine wave input applied. As with all real-world devices, the
ideal output of V
output contains both a dc and an ac error component.

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 (i.e., using a
very large C
value. The ac error component, an output ripple, may be easily
removed by using a large enough post-filtering capacitor, C

In most cases, the combined magnitudes of both the dc and ac
error components need to be considered when selecting appro­priate values for capacitors C
representing 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.

= VIN is never achieved exactly. Instead, the

OUT

) will allow the output voltage to equal its ideal

F

and CF. This combined error,

AV

.

F

E

O

IDEAL

E

O

DC ERROR = EO– EO (IDEAL)

DOUBLE-FREQUENCY

RIPPLE

AVERAGE E

= E

O

O

00834-F-019

TIME

Figure 20. Output Waveform for Sine Wave Input Voltage

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 one half of their original
values, respectively, and rapidly become insignificant.

AC MEASUREMENT ACCURACY AND CREST FAC TO R

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 8
shows the additional error versus the crest factor of the AD736
for various values of C

.

AV

SELECTING PRACTICAL VALUES FOR INPUT
COUPLING (C
(C

) CAPACITORS

F

Table 4 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

Note that at F
(–3 dB) of reading. To reduce this error to 0.5% of reading,
choose a value of C
frequency to be measured.

In addition, if the input voltage has more than 100 mV of dc
offset, then the ac-coupling network shown in Figure 23 should
be used in addition to capacitor C

), AVERAGING (CAV), AND FILTERING

C

, in conjunction with the 8 kΩ

C

, is equal to

L

=

F

L

π

, the amplitude error is approximately −30%

L

that sets FL at one-tenth of the lowest

C

1

FaradsinCofValueThe

C

.

C

))(000,8(2

Rev. F | Page 11 of 16

AD736

Table 4. Capacitor Selection Chart

Low Frequency

Application RMS Input Level

Cutoff (−3 dB)

General-Purpose rms Computation 0 V to 1 V 20 Hz 5 150 10 360 ms
200 Hz 5 15 1 36 ms
0 mV to 200 mV 20 Hz 5 33 10 360 ms
200 Hz 5 3.3 1 36 ms
General-Purpose 0 V to 1 V 20 Hz None 33 1.2 sec
Average 200 Hz None 3.3 120 ms
Responding 0 mV to 200 mV 20 Hz None 33 1.2 sec
200 Hz None 3.3 120 ms
SCR Waveform Measurement 0 mV to 200 mV 50 Hz 5 100 33 1.2 sec
60 Hz 5 82 27 1.0 sec
0 mV to 100 mV 50 Hz 5 50 33 1.2 sec
60 Hz 5 47 µF 27 µF 1.0 sec
Audio Applications
Speech 0 mV to 200 mV 300 Hz 3 1.5 µF 0.5 µF 18 ms
Music 0 mV to 100 mV 20 Hz 10 100 µF 68 µF 2.4 sec

6

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.

Max Crest
Factor

C

AV

(µF)

CF
(µF)

Settling Time

6

to 1%

Rev. F | Page 12 of 16

AD736

APPLICATION CIRCUITS

OPTIONAL

AC COUPLING

CAPACITOR
V

IN

0.01µF
1kV

200mV

9M

Ω

2V

900k

Ω

20V

90k

Ω

200V

10k

Ω

3

–IN

2

+IN

INPUT IMPEDANCE: 10
INPUT IMPEDANCE: 10pF

8kΩ

C

1

C

V

IN

2

C

F

3

–V

S

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

47kΩ

1W

+V

–V

S

1N4148

1N4148

S

1µF

Figure 21. AD736 with a High Impedance Input Attenuator

C

AD711

6

10µF

C

C

C

8kΩ

+

1

V

IN

2

12

Ω

C

F

3

–V

S

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

WAVE

RECTIFIER

rms

CORE

FULL

WAVE

RECTIFIER

rms

CORE

C

10µF

+

FULL

C

AV

33µF

C

10µF

C

+

+

F

(OPTIONAL)

AD736

AMPLIFIER

(OPTIONAL)

AD736

8kΩ

OUTPUT

AMPLIFIER

8kΩ

OUTPUT

8

+V

S

7

OUTPUT

6

C

AV

5

8

+V

S

7

OUTPUT

6

C

AV

5

COM

1µF

COM

1µF

+V

S

OUTPUT

+V

S

OUTPUT

00834-F-020

+

C

1µF

AV

33µF

C

10µF

+

F

(OPTIONAL)

00834-F-021

Figure 22. Differential Input Connection

Rev. F | Page 13 of 16

AD736

V

DC-COUPLED

V

IN

AC-COUPLED

1MΩ

0.1µF

IN

0.1µF

+V

–V

1MΩ

S

S

1MΩ

39MΩ

OUTPUT
V

OS

ADJUST

C

V

–V

V

IN

V

C

C

10µF
+

8kΩ

C

1

C

V

IN

2

C

F

3

–V

S

–V

S

4

INPUT

AMPLIFIER

BIAS

SECTION

FULL

WAVE

RECTIFIER

rms

CORE

+

C

1µF

Figure 23. External Output V

33µF

C

10µF

AV

+

F

OS

C

C

10µF

+

C

8kΩ

1

IN

2

C

F

3

S

4

INPUT

AMPLIFIER

BIAS

SECTION

FULL

WAVE

RECTIFIER

rms

CORE

AD736

8kΩ

OUTPUT

AMPLIFIER

+

C

F

10µF

(OPTIONAL)

Figure 24. Battery-Powered Option

CCC

C

8kΩ

+

1

2

IN

3

INPUT

AMPLIFIER

FULL

WAVE

RECTIFIER

AD736

Figure 25. Low Z, AC-Coupled Input Connection

(OPTIONAL)

AD736

8kΩ

OUTPUT

AMPLIFIER

(OPTIONAL)

Adjustment

8

7

6

C

5

8

7

6

COM

V

2

+V

S

OUTPUT

AV

+

33µF

COM

+V

S

OUTPUT

S

8

COM

+V

S

7

OUTPUT

6

C

AV

5

100kΩ

100kΩ

+V

S

1µF

OUTPUT

00834-F-022

4.7µF
9V

4.7µF

00834-F-023

00834-F-024

Rev. F | Page 14 of 16

AD736

C

R

N

OUTLINE DIMENSIONS

0.375 (9.53)

0.365 (9.27)

0.355 (9.02)

8

1

0.100 (2.54)

0.180

(4.57)

MAX

0.150 (3.81)

0.130 (3.30)

0.110 (2.79)

0.022 (0.56)

0.018 (0.46)

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

COMPLIANT TO JEDEC STANDARDS MO-095AA

Figure 26. 8-Lead Plastic Dual In-Line Package [PDIP]

Dimensions shown in inches and (millimeters)

0.005 (0.13)
MIN

PIN 1

0.405 (10.29) MAX

0.200 (5.08)
MAX

0.200 (5.08)

0.125 (3.18)

0.023 (0.58)

0.014 (0.36)

ONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETERS DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FO
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIG

Figure 27. 8-Lead Ceramic Dual In-Line Package [CERDIP]

Dimensions shown in inches and (millimeters)

5

0.295 (7.49)

0.285 (7.24)

0.275 (6.98)

4

BSC

0.015
(0.38)
MIN

SEATING
PLANE

0.060 (1.52)

0.050 (1.27)

0.045 (1.14)

(N-8)

0.055 (1.40)
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)

SEATING
PLANE

1

0.100 (2.54) BSC

(Q-8)

0.150 (3.81)
MIN

0.325 (8.26)

0.310 (7.87)

0.300 (7.62)

0.320 (8.13)

0.290 (7.37)

15
0

0.150 (3.81)

0.135 (3.43)

0.120 (3.05)

0.015 (0.38)

0.010 (0.25)

0.008 (0.20)

0.015 (0.38)

0.008 (0.20)

Rev. F | Page 15 of 16

AD736

Y

4.00 (0.1574)

3.80 (0.1497)

0.25 (0.0098)

0.10 (0.0040)

COPLANARIT

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

5.00 (0.1968)

4.80 (0.1890)

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)

8°
0°

0.50 (0.0196)

0.25 (0.0099)

1.27 (0.0500)

0.40 (0.0157)

× 45°

Figure 28. 8-Lead Standard Small Outline Package [SOIC] Narrow Body

(R-8)

Dimensions shown in millimeters and (inches)

ORDERING GUIDE

Model Temperature Range Package Description Package Option

AD736JN 0°C to +70°C PDIP N-8
AD736KN 0°C to +70°C PDIP N-8
AD736AQ –40°C to +85°C CERDIP Q-8
AD736BQ –40°C to +85°C CERDIP Q-8
AD736AR –40°C to +85°C SOIC R-8
AD736AR-Reel –40°C to +85°C SOIC R-8
AD736AR-Reel-7 –40°C to +85°C SOIC R-8
AD736BR –40°C to +85°C SOIC R-8
AD736BR-Reel –40°C to +85°C SOIC R-8
AD736BR-Reel-7 –40°C to +85°C SOIC R-8
AD736JR 0°C to +70°C SOIC R-8
AD736JR-Reel 0°C to +70°C SOIC R-8
AD736JR-Reel-7 0°C to +70°C SOIC R-8
AD736JRZ1 0°C to +70°C SOIC R-8
AD736JRZ-RL1 0°C to +70°C SOIC R-8
AD736JRZ-R71 0°C to +70°C SOIC R-8
AD736KR 0°C to +70°C SOIC R-8
AD736KR-Reel 0°C to +70°C SOIC R-8
AD736KR-Reel-7 0°C to +70°C SOIC R-8
AD736KRZ1 0°C to +70°C SOIC R-8
AD736KRZ-RL1 0°C to +70°C SOIC R-8
AD736KRZ-R71 0°C to +70°C SOIC R-8

1

Z = Pb-Free Part.

© 2004 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.

C00834–0–5/04(F)

Rev. F | Page 16 of 16