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AD536AJQ to convert an phase-controlled signal to DC, RMS to DC

Question asked by HTWdd on Oct 28, 2015
Latest reply on Dec 9, 2015 by HTWdd

I am trying to use the AD536AJQ to measure trueRMS Voltage exactly like
described in Figure 3 of the manual with +5V Vs (single supply). The AD536AJQ operates after a
row of other Parts. These other Parts are a rectifier, an optocoupled DC/DC chip (Avago) at 100kHz gain 1  for
galvanic separation and last a differential amplifier which output is the input
for the AD536AJQ. So Input Signal is a phase-angle-controled sinusidal waveform (rectified to 100Hz pulsating DC)
0 to 1V. I controlled the correct input values and waveform
with an oscilloscope, the input of the RMS to DC chip is fine. However when Input signels phase angle goes below 90 to 100 degree and from that even lower to 0 degree the output DC signal of the AD536 is sinking
while input rms should be rising. I also testet an other trueRMS IC that shows
the same output effect.  Please, can you help and tell me what the reason for
this effect is and maybe how to avoid it? Maybe there is a flaw in the concept and you know an other way how it should be done.
Is it a cause of functioning of the AD536AJQ.

With kind regards Bernd Meischner