I am using the AD8436 for RMS measurement of a bipolar AC voltage source according to the attached schematic.
With two equal prototypes I observe different behavior. While one behaves as expected the other one exhibits a significant offset error at the output.
Here my measurements with a DMM:
Prototype 1 Prototype 2
VCC pin 5105 mVDC 5032 mVDC
TP276 (OBUFOUT pin) 243 mVDC 564 mVDC
TP275 (OUT pin) 131 mVDC 288 mVDC
TP280 (RMS pin) 2556 mVDC 2515 mVDC
278 mVAC 278 mVAC
Obviously, the RMS core of both prototypes has the same input signal but not the same output. In further measurements I was able to characterize the error to be an offset error. Prototype 2 behaves as expected, Prototype 1 exhibits an offset of about -150 mV .
Is this an influence of the supply voltage? Is the RMS core not connected correctly? I was not able to find the cause of this error, neither from simulation nor from the datasheet.
Any help is highly appreciated! Thank you.