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Low Level, True RMS-to-DC Converter

Data Sheet

AD636

Rev. E

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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 that may result from its use. Specifications subject to change without notice. No

license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

Trademarks and registered trademarks are the property of their respective owners.

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

Technical Support

www.analog.com

FEATURES

True rms-to-dc conversion

200 mV full scale

Laser-trimmed to high accuracy

0.5% maximum error (AD636K)

1.0% maximum error (AD636J)

Wide response capability

Computes rms of ac and dc signals

1 MHz, 3 dB bandwidth: V rms > 100 mV

Signal crest factor of 6 for 0.5% error

dB output with 50 dB range

Low power: 800 糀 quiescent current

Single or dual supply operation

Monolithic integrated circuit

Low cost

GENERAL DESCRIPTION

The AD636 is a low power monolithic IC that performs true

rms-to-dc conversion on low level signals. It offers performance

that is comparable or superior to that of hybrid and modular

converters costing much more. The AD636 is specified for a

signal range of 0 mV to 200 mV rms. Crest factors up to 6 can

be accommodated with less than 0.5% additional error, allowing

accurate measurement of complex input waveforms.

The low power supply current requirement of the AD636,

typically 800 糀, is ideal for battery-powered portable

instruments. It operates from a wide range of dual and single

power supplies, from ?.5 V to ?6.5 V or from +5 V to +24 V.

The input and output terminals are fully protected; the input

signal can exceed the power supply with no damage to the device

(allowing the presence of input signals in the absence of supply

voltage), and the output buffer amplifier is short-circuit protected.

The AD636 includes an auxiliary dB output derived from an

internal circuit point that represents the logarithm of the rms

output. The 0 dB reference level is set by an externally supplied

current and can be selected to correspond to any input level from

0 dBm (774.6 mV) to 20 dBm (77.46 mV). Frequency response

ranges from 1.2 MHz at 0 dBm to greater than 10 kHz at 50 dBm.

The AD636 is easy to use. The device is factory-trimmed at the

wafer level for input and output offset, positive and negative

waveform symmetry (dc reversal error), and full-scale accuracy

at 200 mV rms. Therefore, no external trims are required to

achieve full-rated accuracy.

FUNCTIONAL BLOCK DIAGRAM

BUFFER IN

BUFFER OUT

OUT

10k&

40k&

COM

CURRENT

MIRROR

SQUARER

DIVIDER

ABSOLUTE

VALUE

AD636

BUF

Figure 1.

The AD636 is available in two accuracy grades. The total error of the

J-version is typically less than ?.5 mV ?1.0% of reading, while

the total error of the AD636K is less than ?.2 mV to ?.5% of

reading. Both versions are temperature rated for operation

between 0癈 and 70癈 and available in 14-lead SBDIP and 10-lead

TO-100 metal can.

The AD636 computes the true root-mean-square of a complex ac

(or ac plus dc) input signal and gives an equivalent dc output level.

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

the average rectified value because it is a measure of the power

in the signal. The rms value of an ac-coupled signal is also its

standard deviation.

The 200 mV full-scale range of the AD636 is compatible with

many popular display-oriented ADCs. The low power supply

current requirement permits use in battery-powered hand-held

instruments. An averaging capacitor is the only external

component required to perform measurements to the fully

specified accuracy is. Its value optimizes the trade-off between

low frequency accuracy, ripple, and settling time.

An optional on-chip amplifier acts as a buffer for the input or the

output signals. Used in the input, it provides accurate

performance from standard 10 M?input attenuators. As an

output buffer, it sources up to 5 mA.

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