AD737 RMS to DC Convertor

The AD737 is a low power, precision, monolithic, true RMS to DC Convertor.

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.

Features of AD737 RMS to DC Converter

• 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