High Precision, Wideband

RMS-to-DC Converter

AD637

Rev. K

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FEATURES

High accuracy

0.02% maximum 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 three-state operation

Quiescent current reduction from 2.2 mA to 350 μA

14-lead SBDIP, 14-lead low cost CERDIP, and 16-lead SOIC_W

FUNCTIONAL BLOCK DIAGRAM

ABSOLUTE

VALUE

DEN INPUT

RMS OUT

dB OUTPUT

BUFF IN

BUFF

OUT

25kΩ

25kΩ

COMMON

CS

OUTPUT

OFFSET

SQUARER/

DIVIDER

BIAS

AD637

Figure 1.

GENERAL 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

integrated 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 permits measurements of signals with crest factors of

up to 10 with less than 1% additional error. The wide band-

width of the AD637 permits 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, Inc., the AD637 has an auxiliary dB output available to

users. 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 facilitates the

addition of precision rms measurement to remote or handheld

applications where minimum power consumption is critical. In

addition, when the AD637 is powered down, the output goes to a

high impedance state. This allows several AD637s to be tied

together to form a wideband true rms multiplexer.

The input circuitry of the AD637 is protected from overload

voltages in excess of the supply levels. The inputs are not

damaged by input signals if the supply voltages are lost.

The AD637 is available in accuracy Grade J and Grade K for

commercial temperature range (0°C to 70°C) applications, accuracy

Grade A and Grade B for industrial range (−40°C to +85°C) appli-

cations, and accuracy Grade S rated over the −55°C to +125°C

temperature range. All versions are available in hermetically sealed,

14-lead SBDIP, 14-lead CERDIP, and 16-lead SOIC_W packages.

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 because 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.

The AD637 is laser wafer trimmed to achieve rated performance

without external trimming. The only external component

required is a capacitor that sets the averaging time period. The

value of this capacitor also determines low frequency accuracy,

ripple level, and settling time.

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

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

to reduce the amount of ac ripple, thereby increasing accuracy.