FUNCTIONAL BLOCK DIAGRAMS

REV. E

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.

a

High Precision,

Wide-Band RMS-to-DC Converter

AD637

The AD637 is available in two accuracy grades (J, K) for com-

mercial (0

°C to +70°C) temperature range applications; two

accuracy grades (A, B) for industrial (–40

°C to +85°C) applica-

tions; and one (S) rated over the –55

°C to +125°C temperature

range. All versions are available in hermetically-sealed, 14-lead

side-brazed ceramic DIPs as well as low cost cerdip packages. A

16-lead SOIC package is also available.

PRODUCT HIGHLIGHTS

1. The AD637 computes the true root-mean-square, mean

square, or absolute value of any complex ac (or ac plus dc)

input waveform and gives an equivalent dc output voltage.

The true rms value of a waveform is more useful than an

average rectified signal since it relates directly to the power of

the signal. The rms value of a statistical signal is also related

to the standard deviation of the signal.

2. The AD637 is laser wafer trimmed to achieve rated perfor-

mance without external trimming. The only external compo-

nent required is a capacitor which sets the averaging time

period. The value of this capacitor also determines low fre-

quency accuracy, ripple level and settling time.

3. The chip select feature of the AD637 permits the user to

power down the device down during periods of nonuse,

thereby, decreasing battery drain in remote or hand-held

applications.

4. The on-chip buffer amplifier can be used as either an input

buffer or in an active filter configuration. The filter can be

used to reduce the amount of ac ripple, thereby, increasing

the accuracy of the measurement.

PRODUCT DESCRIPTION

The AD637 is a complete high accuracy monolithic rms-to-dc

converter that computes the true rms value of any complex

waveform. It offers performance that is unprecedented in inte-

grated circuit rms-to-dc converters and comparable to discrete

and modular techniques in accuracy, bandwidth and dynamic

range. A crest factor compensation scheme in the AD637 per-

mits measurements of signals with crest factors of up to 10 with

less than 1% additional error. The circuit’s wide bandwidth per-

mits the measurement of signals up to 600 kHz with inputs of

200 mV rms and up to 8 MHz when the input levels are above

1 V rms.

As with previous monolithic rms converters from Analog Devices,

the AD637 has an auxiliary dB output available to the user. The

logarithm of the rms output signal is brought out to a separate

pin allowing direct dB measurement with a useful range of

60 dB. An externally programmed reference current allows the

user to select the 0 dB reference voltage to correspond to any

level between 0.1 V and 2.0 V rms.

A chip select connection on the AD637 permits the user to

decrease the supply current from 2.2 mA to 350

µA during

periods when the rms function is not in use. This feature facili-

tates the addition of precision rms measurement to remote or

hand-held applications where minimum power consumption is

critical. In addition when the AD637 is powered down the out-

put goes to a high impedance state. This allows several AD637s

to be tied together to form a wide-band true rms multiplexer.

The input circuitry of the AD637 is protected from overload

voltages that are in excess of the supply levels. The inputs will

not be damaged by input signals if the supply voltages are lost.

FEATURES

High Accuracy

0.02% Max Nonlinearity, 0 V to 2 V RMS Input

0.10% Additional Error to Crest Factor of 3

Wide Bandwidth

8 MHz at 2 V RMS Input

600 kHz at 100 mV RMS

Computes:

True RMS

Square

Mean Square

Absolute Value

dB Output (60 dB Range)

Chip Select-Power Down Feature Allows:

Analog “3-State” Operation

Quiescent Current Reduction from 2.2 mA to 350

A

Side-Brazed DIP, Low Cost Cerdip and SOIC

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

Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1999

BUFFER

AD637

ABSOLUTE

VALUE

SQUARER/DIVIDER

BIAS

SECTION

FILTER

25k

25k

1

2

3

4

5

6

7

14

13

12

11

10

9

8

16

15

SOIC (R) Package

BUFFER

AD637

ABSOLUTE

VALUE

SQUARER/DIVIDER

BIAS

SECTION

FILTER

25k

25k

1

2

3

4

5

6

7

14

13

12

11

10

9

8

Ceramic DIP (D) and

Cerdip (Q) Packages