High Precision, Wideband

RMS-to-DC Converter

Data Sheet

AD637

FEATURES

High accuracy

0.02% maximum nonlinearity, 0 V to 2 V rms input

0.1% 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

FUNCTIONAL BLOCK DIAGRAM

VIN

25k

Ω

25k

Ω

ABSOLUTE

VALUE

DEN

INPUT

CAV

RMS OUT

BUFF IN

BUFF OUT

SQUARER/

DIVIDER

BIAS

COMMON

CS

OUTPUT

OFFSET

dB OUTPUT

–

+

+

–

–

+

Figure 1.

GENERAL DESCRIPTION

The AD637 is a complete, high accuracy, 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. 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 relates to the standard deviation of

the signal.

Superior crest factor compensation permits measurements

of signals with crest factors of up to 10 with less than 1%

additional error. The wide bandwidth 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.

Direct dB value of the rms output with a range of 60 dB is

available on a separate pin. 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 laser wafer trimmed to achieve rated performance

without external trimming. The only external component

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

of this capacitor also determines low frequency accuracy, ripple

level, and settling time.

The on-chip buffer amplifier is 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.

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.

Rev. L

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