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AD8436 RMS core DC Offset

Question asked by chris1234 on Aug 22, 2016
Latest reply on Aug 24, 2016 by chris1234



I am using the AD8436 for RMS measurement of a bipolar AC voltage source according to the attached schematic.

RMS conversion 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.