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Inconsistent VRMS readings

Question asked by loveny on Apr 4, 2018
Latest reply on Apr 18, 2018 by Gary.Kang.AnalogDevices

Further to this thread https://ez.analog.com/message/334215-ade9000-vrms-value-question

 

I have a board with eight ADE9000 devices. The single phase line voltage is attenuated, buffered, and then drives all eight devices in parallel (on all 3 phases). So all devices receive the identical voltage signal.

 

I now capture the VRMS, VRMSONE and VRMS1012 registers and get results as follows:-

RMS voltage registers

 

The largest difference between devices is 2.024V which is nearly 1%, much higher than what I was expecting.

 

I then powered up the board again and repeated, with the following results:-

RMS voltage registers

 

The largest difference is 2.536V (more than 1%), but of at least as much concern is the fact that the relative difference between channels are completely different! Previously, channel 3 to channel 1 was around +0.5V, now it's around -2V. If I repeat the exercise again then the channel differences will change again, seemingly at random. 

 

How would I calibrate this, when I have no idea which channel is measuring the correct voltage, and whichever channel is correct might then be incorrect the next time I power up the board?

 

Can anyone explain why there is so much difference, and why the difference changes (randomly) between power cycles?

 

Many thanks,

Jon

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