AD736AR-Reel-7 - Analog Devices - Datasheet.Directory

Low Cost, Low Power,

True RMS-to-DC Converter

AD736

Rev. F

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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 exter-

nal 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 pro-

viding 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.

FUNCTIONAL BLOCK DIAGRAM

COM

OUTPUT

AD736

FULL

WAVE

RECTIFIER

BIAS

SECTION

RMS CORE

INPUT

AMPLIFIER

OUTPUT

AMPLIFIER

8kΩ

–V

00834-F-001

Figure 1.

The AD736 allows the choice of two signal input terminals: a

high impedance FET input (1012 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 cost, 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.

A high input impedance of 1012 Ω eliminates the need for an

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.

AD736

Rev. F | Page 2 of 16

TABLE OF CONTENTS

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

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

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

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

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

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

Types of AC Measurement .......................................................... 9

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

RMS Measurement—Choosing the Optimum

Valu e for CAV ........................................................................... 10

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

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

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

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

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

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.

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

AD736

Rev. F | Page 3 of 16

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 VOUT = √Avg(VIN2)

CONVERSION ACCURACY 1 kHz sine wave

Total Error, Internal Trim1Using CC

All Grades 0 mV rms–200 mV rms 0.3/0.3 0.5/0.5 0.2/0.2

0.3/0.3 ±mV/±% of Reading

200 mV to 1 V rms −1.2 ±2.0 −1.2

±2.0 % of Reading

TMIN to TMAX

A and B Grades @ 200 mV rms 0.7/0.7 0.5/0.5 ±mV/±% of Reading

J and K Grades @ 200 mV rms 0.007 0.007

±% of Reading/°C

vs. Supply Voltage

@ 200 mV rms Input VS = ±5 V to ±16.5 V 0 +0.06 +0.1 0 +0.06 +0.1 %/V

S = ±5 V to ±3 V 0 −0.18 −0.3 0 −0.18 −0.3 %/V

DC Reversal Error, DC-Coupled @ 600 mV dc 1.3 2.5 1.3 2.5 % of Reading

Nonlinearity2, 0 mV–200 mV @ 100 mV rms 0 0.25 0.35 0 0.25 0.35 % of Reading

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

ERROR VS. CREST FACTOR3

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 200 200 mV rms

S = ±5 V to ±16.5 V 1 1 V rms

Peak Transient Input VS = +2.8 V, −3.2 V ±0.9 ±0.9 V

S = ±5 V ±2.7 ±2.7 V

S = ±16.5 V ±4.0 ±4.0 V

Input Resistance 1012 1012 Ω

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

S = ±5 V to ±16.5 V 1 1 V rms

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

S = ±5 V ±3.8 ±3.8 V

S = ±16.5 V ±11 ±11 V

Input Resistance 6.4 8 9.6 6.4 8 9.6 kΩ

Maximum Continuous

Nondestructive Input

All supply voltages ±12 ±12 V p-p

Input Offset Voltage4

J and K Grades ±3 ±3 mV

A and B Grades ±3 ±3 mV

vs. Temperature 8 30 8 30 µV/°C

vs. Supply VS = ±5 V to ±16.5 V 50 150 50

150 µV/V

S = ±5 V to ±3 V 80 80 µV/V

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

2Nonlinearity 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.

3Error versus crest factor is specified as additional error for a 200 mV rms signal. Crest factor = VPEAK/V rms.

4 DC offset does not limit ac resolution.

AD736

Rev. F | Page 4 of 16

AD736J/AD736A AD736K/AD736B

Parameter Conditions Min Typ Max Min Typ Max Unit

OUTPUT CHARACTERISTICS

Output Offset Voltage

J and K Grades ±0.1 ±0.5 ±0.1

±0.3 mV

A and B Grades ±0.5 ±0.3 mV

vs.Temperature 1 20 1 20 µV/°C

vs. Supply VS = ±5 V to ±16.5 V 50 130 50

130 µV/V

S = ±5 V to ±3 V 50 50 µV/V

Output Voltage Swing

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

S = ±5 V 0–3.6 3.8 0–3.6 3.8 V

S = ±16.5 V 0–4 5 0–4 5 V

No Load VS = ±16.5 V 0–4 12 0–4 12 V

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)

for 1% Additional Error

Sine wave input

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)

for 1% Additional Error

Sine wave input

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 160 200 160

200 µA

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

AD736

Rev. F | Page 5 of 16

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Supply Voltage ±16.5 V

Internal Power Dissipation5200 mW

Input Voltage ±VS

Output Short-Circuit Duration Indefinite

Differential Input Voltage +VS and –VS

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

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.

5 8-Lead PDIP Package: θJA = 165°C/W

8-Lead CERDIP Package: θJA = 110°C/W

8-Lead SOIC Package: θJA = 155°C/W

PIN CONFIGURATION

COM

OUTPUT

AD736

FULL

WAVE

RECTIFIER

BIAS

SECTION

RMS CORE

INPUT

AMPLIFIER

OUTPUT

AMPLIFIER

8kΩ

–V

00834-F-001

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

and 8-Lead CERDIP (Q-8) Packages

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.

AD736

Rev. F | Page 6 of 16

TYPICAL PERFORMANCE CHARACTERISTICS

= 200mV rms

1kHz SINE WAVE

= 100µF

= 22µF

–0.5

–0.3

–0.1

0.3

0.1

0.5

0.7

04286121410 16

SUPPLY VOLTAGE (±V)

ADDITIONAL ERROR (% OF READING)

00834-F-002

Figure 3. Additional Error vs. Supply Voltage

04286121410 16

SUPPLY VOLTAGE (±V)

PEAK INPUT BEFORE CLIPPING (V)

00834-F-003

PIN 1

PIN 2

DC-COUPLED

Figure 4. Maximum Input Level vs. Supply Voltage

0246 1081214

SUPPLY VOLTAGE (±V)

PEAK BUFFER OUTPUT (V)

00834-F-004

1kHz SINE WAVE INPUT

Figure 5. Peak Buffer Output vs. Supply Voltage

100µV

1mV

10mV

100mV

10V

0.1 1 10010 1000

–3dB FREQUENCY (kHz)

INPUT LEVEL (rms)

00834-F-005

SINEWAVEINPUT,V

=±5V,

= 22µF, C

= 4.7µF, C

= 22µF

1% ERROR

–3dB

10% ERROR

Figure 6. Frequency Response Driving Pin 1

100µV

1mV

10mV

100mV

10V

0.1 1 10010 1000

–3dB FREQUENCY (kHz)

INPUT LEVEL (rms)

00834-F-006

SINEWAVEINPUT,V

=±5V,

= 22µF, C

= 4.7µF, C

= 22µF

1% ERROR

10% ERROR

–3dB

Figure 7. Frequency Response Driving Pin 2

= 100µF

= 250µF

1234

CREST FACTOR (V

PEAK

/V rms)

ADDITIONAL ERROR (% OF READING)

00834-F-007

= 10µF

= 33µF

3ms BURST OF 1kHz =

3 CYCLES

200mV rms SIGNAL

= ±5V

= 22µF

= 100µF

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

AD736

Rev. F | Page 7 of 16

VIN = 200mV rms

1kHz SINE WAVE

CAV = 100µF

CF = 22µF

VS = ±5V

–0.8

–0.6

–0.2

–0.4

0.4

0.2

0.6

0.8

–60 –20–40 200 60 80 100 12040 140

TEMPERATURE (°C)

ADDITIONAL ERROR (% OF READING)

00834-F-008

Figure 9. Additional Error vs. Temperature

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1.0

rms INPUT LEVEL (V)

DC SUPPLY CURRENT (µA)

00834-F-009

VIN = 200mV rms

1kHz SINE WAVE

CAV = 100µF

CF = 22µF

VS = ±5V

Figure 10. DC Supply Current vs. RMS Input Level

10µV

100µV

1mV

10mV

100 1k 10k 100k

–3dB FREQUENCY (Hz)

INPUT LEVEL (rms)

00834-F-010

VIN = 1kHz

SINE WAVE INPUT

AC-COUPLED

VS = ±5V

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

–2.5

–2.0

–1.5

–1.0

–0.5

0.5

1.0

10mV 100mV 1V 2V

INPUT LEVEL (rms)

ERROR (% OF READING)

00834-F-011

VIN = SINE WAVE @ 1kHz

CAV = 22µF, CC = 47µF,

CF = 4.7µF, VS = ±5V

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

Output Buffer Offset Is Adjusted to Zero

100

10 100 1k

FREQUENCY (Hz)

CAV (µF)

00834-F-012

–1%

–0.5%

VS = ±5V

VIN = 200mV rms

CC = 47µF

CF = 47µF

Figure 13. CAV vs. Frequency for Specified Averaging Error

1mV

10mV

100mV

1 10 100 1k

FREQUENCY (Hz)

INPUT LEVEL (rms)

00834-F-013

VIN SINE WAVE

AC-COUPLED

CAV = 10µF, CC = 47µF,

CF = 47µF, VS = ±5V

–0.5%

–1%

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

AD736

Rev. F | Page 8 of 16

CALCULATING SETTLING TIME USING FIGURE 16

1.0

1.5

2.0

2.5

3.0

4.0

3.5

02468 121410 16

SUPPLY VOLTAGE (±V)

INPUT BIAS CURRENT (pA)

00834-F-014

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

100µV

1mV

10mV

100mV

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

SETTLING TIME

INPUT LEVEL (rms)

00834-F-015

= 5V

= 22µF

= 0µF

= 10µF

= 33µF

= 100µF

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

100fA

10nA

1nA

100pA

10pA

1pA

–55 –35 –15 5 25 65 85 10545 125

TEMPERATURE (°C)

INPUT BIAS CURRENT

00834-F-016

Figure 17. Pin 2 Input Bias Current vs. Temperature

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.

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). Also, this graph

provides the worst-case settling time since the AD736 settles

very quickly with increasing input levels.

AD736

Rev. F | Page 9 of 16

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 VPEAK; the corresponding rms value is 0.707

times VPEAK. 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 measure-

ment 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

VAvgrmsV =

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

Waveform Type 1 V Peak

Amplitude

Crest Factor

(VPEAK/V rms)

True rms

Value (V)

Average Responding Circuit

Calibrated to Read RMS Value of

Sine Waves Will Read (V)

% of Reading Error Using

Average Responding Circuit

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

AD736

Rev. F | Page 10 of 16

THEORY OF OPERATION

RMS

TRANSLINEAR

CORE

COM

AD736

FWR

CURRENT

MODE

ABSOLUTE

VALUE

33µF

C =

10µF

0.1µF

–V

OPTIONAL RETURN PATH

8kΩ

INPUT

AMPLIFIER

<10pA

OUTPUT

AMPLIFIER

8kΩ

00834-F-017

RMS

OUTPUT

COM

10µF(OPTIONAL)

BIAS

SECTION

Figure 18. AD736 True RMS Circuit

As shown by Figure 18, the AD736 has five functional subsec-

tions: 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,

CAV. Without CAV , the rectified input signal travels through the

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, CF, which is connected across the

feedback path of the amplifier. In the average responding con-

nection, this is where all of the averaging is carried out. In the

rms circuit, this additional filtering stage helps reduce any out-

put ripple that was not removed by the averaging capacitor, CAV .

RMS MEASUREMENT—CHOOSING THE OPTIMUM

VALUE FOR CAV

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 measure-

ments. Obviously, when selecting CAV, a trade-off between

computational accuracy and settling time is required.

AD736

Rev. F | Page 11 of 16

RAPID SETTLING TIMES VIA THE AVERAGE

RESPONDING CONNECTION

Because the average responding connection shown in Figure 19

does not use the CAV averaging capacitor, its settling time does

not vary with input signal level. It is determined solely by the

RC time constant of CF and the internal 8 kΩ resistor in the

output amplifier’s feedback path.

33µF

10µF

COM

OUTPUT

(OPTIONAL)

POSITIVE SUPPLY +V

0.1µF

–V

0.1µF

COMMON

NEGATIVE SUPPLY

OUT

AD736

rms

CORE

FULL

WAVE

RECTIFIER

–V

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

OUTPUT

AMPLIFIER

8kΩ

00834-F-018

Figure 19. AD736 Average Responding Circuit

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 VOUT = VIN is never achieved exactly. Instead, the

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 aver-

aging capacitor used—no amount of post filtering (i.e., using a

very large CF) will allow the output voltage to equal its ideal

value. The ac error component, an output ripple, may be easily

removed by using a large enough post-filtering capacitor, CF.

In most cases, the combined magnitudes of both the dc and ac

error components need to be considered when selecting appro-

priate values for capacitors CAV and CF. This combined error,

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.

DC ERROR = E

– E

(IDEAL)

AVERAGE E

= E

IDEAL

DOUBLE-FREQUENCY

RIPPLE

TIME

00834-F-019

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

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 = VPEAK/V rms). Many common

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 CAV.

SELECTING PRACTICAL VALUES FOR INPUT

COUPLING (CC), AVERAGING (CAV), AND FILTERING

(CF) CAPACITORS

Table 4 provides practical values of CAV and CF for several

common applications.

The input coupling capacitor, CC, in conjunction with the 8 kΩ

internal input scaling resistor, determines the −3 dB low

frequency roll-off. This frequency, FL, is equal to

))(000,8(2

FaradsinCofValueThe

Lπ

Note that at FL, the amplitude error is approximately −30%

(–3 dB) of reading. To reduce this error to 0.5% of reading,

choose a value of CC that sets FL at one-tenth of the lowest

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 CC.

AD736

Rev. F | Page 12 of 16

Table 4. Capacitor Selection Chart

Application RMS Input Level

Low Frequency

Cutoff (−3 dB)

Max Crest

Factor

CAV

(µF)

(µF) Settling Time6 to 1%

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.

AD736

Rev. F | Page 13 of 16

APPLICATION CIRCUITS

+VS

CAV

33µF

47kΩ

10µF

COM

OUTPUT

(OPTIONAL)

OUTPUT

AD736

rms

CORE

VIN FULL

WAVE

RECTIFIER

–VS

+VS

–VS

CAV

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

OUTPUT

AMPLIFIER

8kΩ

10µF

1µF

(OPTIONAL)

00834-F-020

OPTIONAL

AC COUPLING

CAPACITOR

0.01µF

1kV

20V

200V

Ω

900k

Ω

90k

Ω

10k

Ω

200mV

1N4148

Figure 21. AD736 with a High Impedance Input Attenuator

33µF

COM

OUTPUT

AD736

rms

CORE

FULL

WAVE

RECTIFIER

–V

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

OUTPUT

AMPLIFIER

8kΩ

10µF

1µF

(OPTIONAL)

00834-F-021

+IN

INPUT IMPEDANCE: 10

Ω

INPUT IMPEDANCE: 10pF

–IN AD711

Figure 22. Differential Input Connection

AD736

Rev. F | Page 14 of 16

33µF

10µF

COM

OUTPUT

(OPTIONAL)

OUTPUT

AD736

rms

CORE

FULL

WAVE

RECTIFIER

–V

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

OUTPUT

AMPLIFIER

8kΩ

10µF

1µF

(OPTIONAL)

00834-F-022

OUTPUT

ADJUST

1MΩ39MΩ

AC-COUPLED

DC-COUPLED

1MΩ

0.1µF

Figure 23. External Output VOS Adjustment

10µF

COM

OUTPUT

AD736

rms

CORE

FULL

WAVE

RECTIFIER

–V

BIAS

SECTION

INPUT

AMPLIFIER

8kΩ

OUTPUT

AMPLIFIER

8kΩ

10µF(OPTIONAL)

00834-F-023

1MΩ

0.1µF

33µF

100kΩ

4.7µF

Figure 24. Battery-Powered Option

COM

OUTPUT

AD736

FULL

WAVE

RECTIFIER

INPUT

AMPLIFIER

8kΩ

00834-F-024

Figure 25. Low Z, AC-Coupled Input Connection

AD736

Rev. F | Page 15 of 16

OUTLINE DIMENSIONS

SEATING

PLANE

0.015

(0.38)

MIN

0.180

(4.57)

MAX

0.150 (3.81)

0.130 (3.30)

0.110 (2.79) 0.060 (1.52)

0.050 (1.27)

0.045 (1.14)

0.295 (7.49)

0.285 (7.24)

0.275 (6.98)

0.100 (2.54)

BSC

0.375 (9.53)

0.365 (9.27)

0.355 (9.02)

0.150 (3.81)

0.135 (3.43)

0.120 (3.05)

0.015 (0.38)

0.010 (0.25)

0.008 (0.20)

0.325 (8.26)

0.310 (7.87)

0.300 (7.62)

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]

(N-8)

Dimensions shown in inches and (millimeters)

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

0.310 (7.87)

0.220 (5.59)

PIN 1

0.005 (0.13)

MIN 0.055 (1.40)

MAX

0.100 (2.54) BSC

0.320 (8.13)

0.290 (7.37)

0.015 (0.38)

0.008 (0.20)

SEATING

PLANE

0.200 (5.08)

MAX

0.405 (10.29) MAX

0.150 (3.81)

MIN

0.200 (5.08)

0.125 (3.18)

0.023 (0.58)

0.014 (0.36) 0.070 (1.78)

0.030 (0.76)

0.060 (1.52)

0.015 (0.38)

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

(Q-8)

Dimensions shown in inches and (millimeters)

AD736

Rev. F | Page 16 of 16

0.25 (0.0098)

0.17 (0.0067)

1.27 (0.0500)

0.40 (0.0157)

0.50 (0.0196)

0.25 (0.0099)× 45°

8°

0°

1.75 (0.0688)

1.35 (0.0532)

SEATING

PLANE

0.25 (0.0098)

0.10 (0.0040)

5.00 (0.1968)

4.80 (0.1890)

4.00 (0.1574)

3.80 (0.1497)

1.27 (0.0500)

BSC

6.20 (0.2440)

5.80 (0.2284)

0.51 (0.0201)

0.31 (0.0122)

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

COMPLIANT TO JEDEC STANDARDS MS-012AA

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.

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C00834–0–5/04(F)