Low Cost Low Power Integrated Circuit Chip True RMS-to-DC Converter AD737JRZ

Low Cost, Low Power, True RMS-to-DC Converter AD737

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 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

GENERAL DESCRIPTION

The AD737 is a low power, precision, monolithic, true rms-todc 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 this converter 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 can compute the rms value of both ac and dc input voltages. It can also be operated ac-coupled by adding one external capacitor. In this mode, the AD737 can resolve input signal levels of 100 µV rms or less, despite variations in temperature or supply voltage. High accuracy is also maintained for input waveforms with crest factors of 1 to 3. In addition, crest factors as high as 5 can be measured (while introducing only 2.5% additional error) at the 200 mV full-scale input level.

The AD737 has no output buffer amplifier, thereby significantly reducing dc offset errors occurring at the output, which makes 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 batterypowered 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 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 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.

ABSOLUTE MAXIMUM RATINGS

Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.