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 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 mV 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 occuring at the output. This allows the
device to be highly compatible with high input impedance A/D
converters.