Integrated Circuit

True RMS-to-DC Converter

Data Sheet

AD536A

Rev. G

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FEATURES

True rms-to-dc conversion

Laser trimmed to high accuracy

±0.2% maximum error (AD536AK)

±0.5% maximum error (AD536AJ)

Wide response capability

Computes rms of ac and dc signals

450 kHz bandwidth: V rms > 100 mV

2 MHz bandwidth: V rms > 1 V

Signal crest factor of 7 for 1% error

dB output with 60 dB range

Low power: 1.2 mA quiescent current

Single- or dual-supply operation

Monolithic integrated circuit

−55°C to +125°C operation (AD536AS)

GENERAL DESCRIPTION

The AD536A is a complete monolithic integrated circuit that

performs true rms-to-dc conversion. It offers performance

comparable or superior to that of hybrid or modular units costing

much more. The AD536A directly computes the true rms value of

any complex input waveform containing ac and dc components.

A crest factor compensation scheme allows measurements with 1%

error at crest factors up to 7. The wide bandwidth of the device

extends the measurement capability to 300 kHz with less than 3 dB

errors for signal levels greater than 100 mV.

An important feature of the AD536A, not previously available

in rms converters, is an auxiliary dB output pin. The logarithm

of the rms output signal is brought out to a separate pin to allow

the dB conversion, with a useful dynamic range of 60 dB. Using

an externally supplied reference current, the 0 dB level can be

conveniently set to correspond to any input level from 0.1 V to

2 V rms.

The AD536A is laser trimmed to minimize input and output offset

voltage, to optimize positive and negative waveform symmetry

(dc reversal error), and to provide full-scale accuracy at 7 V rms.

As a result, no external trims are required to achieve the rated

unit accuracy.

The input and output pins are fully protected. The input circuitry

can take overload voltages well beyond the supply levels. Loss of

supply voltage with the input connected to external circuitry does

not cause the device to fail. The output is short-circuit protected.

FUNCTIONAL BLOCK DIAGRAM

dB

BUFFER IN

VIN

25kΩ

80kΩ

25kΩ

CAV

+VS

–VS

+VS

COM

AD536A

RL

IOUT

BUFFER

OUT

CURRENT

MIRROR

SQUARER/

DIVIDER

ABSOLUTE

VALUE

+

BUF

Figure 1.

The AD536A is available in two accuracy grades (J and K) for

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

one grade (S) rated for the −55°C to +125°C extended range.

The AD536AK offers a maximum total error of ±2 mV ± 0.2%

of reading, while the AD536AJ and AD536AS have maximum

errors of ±5 mV ± 0.5% of reading. All three versions are available

in a hermetically sealed 14-lead DIP or a 10-pin TO-100 metal

header package. The AD536AS is also available in a 20-terminal

leadless hermetically sealed ceramic chip carrier.

The AD536A computes the true root-mean-square level of a

complex ac (or ac plus dc) input signal and provides an equiva-

lent dc output level. The true rms value of a waveform is a more

useful quantity than the average rectified value because it relates

directly to the power of the signal. The rms value of a statistical

signal also relates to its standard deviation.

An external capacitor is required to perform measurements to the

fully specified accuracy. The value of this capacitor determines the

low frequency ac accuracy, ripple amplitude, and settling time.

The AD536A operates equally well from split supplies or a

single supply with total supply levels from 5 V to 36 V. With

1 mA quiescent supply current, the device is well suited for a

wide variety of remote controllers and battery-powered

instruments.