Low Cost, Low Power,

True RMS-to-DC Converter

Data Sheet

AD737

Rev. J

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FEATURES

Computes

True rms value

Average rectified value

Absolute value

Provides

200 mV full-scale input range (larger inputs with input

scaling)

Direct interfacing with 3½ digit CMOS analog-to-digital

converters (ADCs)

High input impedance: 1012 Ω

Low input bias current: 25 pA maximum

High accuracy: ±0.2 mV ± 0.3% of reading

RMS conversion with signal crest factors up to 5

Wide power supply range: ±2.5 V to ±16.5 V

Low power: 25 μA (typical) standby current

No external trims needed for specified accuracy

The AD737 output is negative going; the AD736 is a positive

output-going version of the same basic device

FUNCTIONAL BLOCK DIAGRAM

COM

OUTPUT

CAV

–VS

8kΩ

BIAS

SECTION

ABSOLUTE

VALUE

CIRCUIT

SQUARER

DIVIDER

CC

VIN

+VS

POWER

DOWN

CF

CAV

8kΩ

Figure 1.

GENERAL DESCRIPTION

The AD737 is a low power, precision, monolithic, true rms-to-dc

converter. It is laser trimmed to provide a maximum error of

±0.2 mV ± 0.3% of reading with sine wave inputs. Furthermore,

it maintains high accuracy while measuring a wide range of

input waveforms, including variable duty cycle pulses and

triac (phase) controlled sine waves. The low cost and small

physical size of the AD737 make it suitable for upgrading the

performance of non-rms precision rectifiers in many applications.

Compared to these circuits, the AD737 offers higher accuracy at

equal or lower cost.

The AD737 computes the rms value of both ac and dc input

voltages, and is ac-coupled by adding an input capacitor. In this

mode, the AD737 resolves input signal levels of 100 μV rms or

less, despite variations in temperature or supply voltage. High

accuracy is maintained for input waveforms with crest factors of

1 to 3 and crest factors at 2.5% or less with respect to full-scale

input level.

The AD737 has no output buffer amplifier, thereby significantly

reducing dc offset errors occurring at the output and making

the device highly compatible with high input impedance ADCs.

Requiring only 160 μA of power supply current, the AD737 is

optimized for use in portable multimeters and other battery-

powered applications. In power-down mode, the standby supply

current in is typically 25 μA.

The AD737 has both high (1012 Ω) and low impedance input

options. The high-Z FET input connects high source impedance

input attenuators, and a low impedance (8 kΩ) input accepts

rms voltages of up to 0.9 V while operating from the minimum

power supply voltage of ±2.5 V. The two inputs can be used

either single-ended or differentially.

The AD737 achieves 1% of reading error bandwidth, exceeding

10 kHz for input amplitudes from 20 mV rms to 200 mV rms,

while consuming only 0.72 mW.

The AD737 is available in two performance grades. The AD737J

and AD737K grades operate over the commercial temperature

range of 0°C to 70°C. The AD737JR-5 is tested with supply

voltages of ±2.5 V dc. The AD737A grade operates over the

industrial temperature range of −40°C to +85°C. The AD737 is

available in two low cost, 8­lead packages: PDIP and SOIC_N.

PRODUCT HIGHLIGHTS

1.

Computes the average rectified, absolute, or true rms value

of a signal regardless of waveform.

2.

Only one external component, an averaging capacitor, is

required for the AD737 to perform true rms measurement.

3.

The standby power consumption of 125 μW makes the

AD737 suitable for battery-powered applications.