AD736.PDF

Low Cost, Low Power,

True RMS-to-DC Converter

AD736

FEATURES

COMPUTES

True RMS Value

Average Rectified Value

Absolute Value

FUNCTIONAL BLOCK DIAGRAM

PROVIDES

200 mV Full-Scale Input Range

(Larger Inputs with Input Attenuator)

High Input Impedance of 10 12

0.3% of Reading

RMS Conversion with Signal Crest Factors Up to 5

Wide Power Supply Range: +2.8 V, –3.2 V to

0.3 mV

16.5 V

A max Supply Current

Buffered Voltage Output

No External Trims Needed for Specified Accuracy

AD737—An Unbuffered Voltage Output Version with

Chip Power Down Is Also Available

which 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 singly or differentially.

The AD736 achieves a 1% of reading error bandwidth exceeding

10 kHz for 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 commercial tem-

perature range of 0°C to +70°C. The AD736A and AD736B

grades are rated over the industrial temperature range of –40°C

to +85°C.

The AD736 is available in three low-cost, 8-pin packages: plastic

mini-DIP, plastic SO and hermetic cerdip.

PRODUCT 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. Fur-

thermore, 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 applica-

tions. Compared to these circuits, the AD736 offers higher ac-

curacy at 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 ex-

ternal capacitor. In this mode, the AD736 can resolve input sig-

nal levels of 100 mV 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 provid-

ing a great deal of design flexibility. Requiring only 200 mA of

power supply current, the AD736 is optimized for use in por-

table multimeters and other battery powered applications.

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

high impedance (10 12 W) FET input which will directly interface

with high Z input attenuators and a low impedance (8 kW) input

PRODUCT HIGHLIGHTS

1. The AD736 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 AD736 to perform true rms measurement.

3. The low power consumption of 1 mW makes the AD736

suitable for many battery powered applications.

4. A high input impedance of 10 12 W eliminates the need for an

external buffer when interfacing with input attenuators.

5. A low impedance input is available for those applications

requiring up to 300 mV rms input signal operating from low

power supply voltages.

REV. C

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

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 617/329-4700

Low Input Bias Current: 25 pA max

High Accuracy:

Low Power: 200

Fax: 617/326-8703

AD736–SPECIFICATIONS

5 V supplies, ac coupled with 1 kHz sine-wave input applied unless

otherwise noted.)

AD736J/A

AD736K/B

Model

Conditions

Min

Typ

Max

Min

Typ

Max

Units

TRANSFER FUNCTION

V OUT

Avg .( V IN 2 )

V OUT

Avg .( V IN 2 )

CONVERSION ACCURACY

1 kHz Sine Wave

Total Error, Internal Trim 1

ac Coupled Using C C

0.3/0.3 0.5/0.5

0.2/0.2 0.3/0.3

mV/

% of Reading

200 mV–1 V rms

–1.2

6 2.0

–1.2

6 2.0

% of Reading

T MIN –T MAX

A&B Grades

@ 200 mV rms

0.7/0.7

0.5/0.5

± mV/± % of Reading

J&K Grades

@ 200 mV rms

0.007

% of Reading/

vs. Supply Voltage

@ 200 mV rms Input

V S =

5 V to

16.5 V 0

+0.06

+0.1

+0.06

+0.1

%/V

@ 200 mV rms Input

V S = ± 5 V to ± 3 V

–0.18

–0.3

–0.18

–0.3

%/V

dc Reversal Error, dc Coupled @ 600 mV dc

1.3

2.5

1.3

2.5

% of Reading

Nonlinearity 2 , 0 mV–200 mV @ 100 mV rms

+0.25

+0.35

+0.25

+0.35

% of Reading

Total Error, External Trim

0–200 mV rms

0.1/0.5

0.1/0.3

mV/

% of Reading

ERROR vs. CREST FACTOR 3

Crest Factor 1 to 3

C AV , C F = 100

0.7

% Additional Error

Crest Factor = 5

C AV , C F = 100 mF

2.5

% Additional Error

INPUT CHARACTERISTICS

High Impedance Input (Pin 2)

Signal Range

Continuous rms Level

V S = +2.8 V, –3.2 V

200

mV rms

Continuous rms Level

V S =

5 V to

16.5 V

V rms

Peak Transient Input

V S = +2.8 V, –3.2 V

6 0.9

Peak Transient Input

V S =

5 V

2.7

Peak Transient Input

V S = ± 16.5 V

6 4.0

Input Resistance

10 12

Input Bias Current

V S = ± 3 V to ± 16.5 V

Low Impedance Input (Pin 1)

Signal Range

Continuous rms Level

V S = +2.8 V, –3.2 V

300

mV rms

Continuous rms Level

V S =

5 V to

16.5 V

V rms

Peak Transient Input

V S = +2.8 V, –3.2 V

± 1.7

Peak Transient Input

V S =

5 V

3.8

Peak Transient Input

V S = ± 16.5 V

± 11

Input Resistance

6.4

9.6

6.4

9.6

Maximum Continuous

Nondestructive Input

All Supply Voltages

± 12

V p-p

Input Offset Voltage 4

ac Coupled

J&K Grades

6 3

A&B Grades

vs. Temperature

mV/°C

vs. Supply

V S =

5 V to

16.5 V

150

V/V

vs. Supply

V S = ± 5 V to ± 3 V

mV/V

OUTPUT CHARACTERISTICS

Output Offset Voltage

J&K Grades

± 0.1

6 0.5

± 0.1

6 0.3

A&B Grades

6 0.5

6 0.3

vs.Temperature

vs. Supply

V S =

5 V to

16.5 V

130

V/V

V S = ± 5 V to ± 3 V

mV/V

Output Voltage Swing

2 kW Load

V S = +2.8 V, –3.2 V

0 to +1.6 +1.7

2 k

Load

V S = ± 5 V

0 to +3.6 +3.8

2 k

Load

V S =

16.5 V

0 to +4 +5

0 to +4

No Load

V S =

16.5 V

0 to +4 +12

0 to +4

+12

Output Current

Short-Circuit Current

Output Resistance

@ dc

0.2

FREQUENCY RESPONSE

High Impedance Input (Pin 2)

For 1% Additional Error

Sine-Wave Input

V IN = 1 mV rms

kHz

V IN = 10 mV rms

kHz

V IN = 100 mV rms

kHz

V IN = 200 mV rms

kHz

–2–

REV. C

(@ +25

All Grades

0–200 mV rms

AD736

AD736J/A

AD736K/B

Model

Conditions

Min

Typ

Max

Min

Typ

Max

Units

3 dB Bandwidth

Sine-Wave Input

V IN = 1 mV rms

kHz

V IN = 10 mV rms

kHz

V IN = 100 mV rms

170

kHz

V IN = 200 mV rms

190

kHz

FREQUENCY RESPONSE

Low Impedance Input (Pin 1)

For 1% Additional Error

V IN = 1 mV rms

Sine-Wave Input

kHz

V IN = 10 mV rms

kHz

V IN = 100 mV rms

kHz

V IN = 200 mV rms

kHz

3 dB Bandwidth

Sine-Wave Input

V IN = l mV rms

kHz

V IN = 10 mV rms

kHz

V IN = 100 mV rms

350

kHz

V IN = 200 mV rms

460

kHz

POWER SUPPLY

OperatingVoltageRange

+2.8, –3.2

16.5

+2.8, –3.2

16.5

Volts

Quiescent Current

Zero Signal

160

200

160

200

200 mV rms, No Load

Sine-Wave Input

230

270

230

270

TEMPERATURE RANGE

Operating, Rated Performance

Commercial (0

C to +70

AD736J

AD736K

Industrial (–40°C to +85°C)

AD736A

AD736B

NOTES

l Accuracy is specified with the AD736 connected as shown in Figure 16 with capacitor C C .

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

3 Error vs. Crest Factor is specified as additional error for a 200 mV rms signal. C.F. = V PEAK /V rms.

4 DC offset does not limit ac resolution.

Specifications are subject to change without notice.

Specifications shown in boldface are tested on all production units at final electrical test.

Results from those tests are used to calculate outgoing quality levels.

ABSOLUTE MAXIMUM RATINGS 1

Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 16.5 V

Internal Power Dissipation 2 . . . . . . . . . . . . . . . . . . . . . 200 mW

Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ± V S

Output Short-Circuit Duration . . . . . . . . . . . . . . . . . Indefinite

Differential Input Voltage . . . . . . . . . . . . . . . . . . +V S and –V S

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

Storage Temperature Range (N, R) . . . . . –65°C to +125°C

Operating Temperature Range

AD736J/K . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C

AD736A/B . . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C

ESD Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 V

NOTES

1 Stresses above those listed under “Absolute Maximum Ratings” may cause

permanent damage to the device. This is a stress rating only and 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 .

2 8-Pin Plastic Package: q JA = 165°C/W

8-Pin Cerdip Package: q JA = 110°C/W

8-Pin Small Outline Package: q JA = 155°C/W

ORDERING GUIDE

PIN CONFIGURATION

8-Pin Mini-DIP (N-8), 8-Pin SOIC (R-8),

8-Pin Cerdip (Q-8)

Temperature Package

Package

Model

Range

Description

Option

AD736JN

0°C to +70°C Plastic Mini-DIP N-8

AD736KN

0°C to +70°C Plastic Mini-DIP N-8

AD736JR

0°C to +70°C Plastic SOIC

SO-8

AD736KR

0°C to +70°C Plastic SOIC

SO-8

AD736AQ

–40°C to +85°C Cerdip

Q-8

AD736BQ

–40°C to +85°C Cerdip

Q-8

AD736JR-REEL

0°C to +70°C Plastic SOIC

SO-8

AD736JR-REEL-7 0°C to +70°C Plastic SOIC

SO-8

AD736KR-REEL

0°C to +70°C Plastic SOIC

SO-8

AD736KR-REEL-7 0°C to +70°C Plastic SOIC

SO-8

REV. C

–3–

Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300

AD736

–Typical Characteristics

Figure 1. Additional Error vs.

Supply Voltage

Figure 2. Maximum Input Level

vs. SupplyVoltage

Figure 3. Peak Buffer Output vs.

Supply Voltage

Figure 4. Frequency Response

Driving Pin 1

Figure 5. Frequency Response

Driving Pin 2

Figure 6. Additional Error vs.

Crest Factor vs. C AV

Figure 7. Additional Error vs.

Temperature

Figure 8. DC Supply Current vs.

RMS lnput Level

Figure 9. –3 dB Frequency vs.

RMS Input Level (Pin2)

–4–

REV. C

Typical Characteristics–

AD736

Figure 10. Error vs. RMS Input

Voltage (Pin 2), Output Buffer Off-

set Is Adjusted To Zero

Figure 11. C AV vs. Frequency for

Specified Averaging Error

Figure 12. RMS Input Level vs.

Frequency for Specified Averag-

ing Error

Figure 13. Pin 2 Input Bias Current

vs. Supply Voltage

Figure 14. Settling Time vs. RMS

Input Level for Various

Values of C AV

Figure 15. Pin 2 Input Bias Cur-

rent vs. Temperature

CALCULATING SETTLING TIME USING FIGURE 14

The graph of Figure 14 may be used to closely approximate the

time required for the AD736 to settle when its input level is re-

duced in amplitude. The net time required for the rms converter

to settle will be 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 mF averaging

capacitor, an initial rms input level of 100 mV and a final (re-

duced) input level of 1 mV. From Figure 14, the initial settling

time (where the 100 mV line intersects the 33 mF line) is around

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 will be 8 seconds minus

80 ms which is 7.92 seconds. Note that, because of the smooth

decay characteristic inherent with a capacitor/diode combina-

tion, this is the total settling time to the final value (i.e., not the

settling time to 1%, 0.1%, etc., of final value). Also, this graph

provides the worst case settling time, since the AD736 will settle

very quickly with increasing input levels.

REV. C

–5–

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