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ADE7758 | VRMS samples

Question asked by mths. on Apr 18, 2015
Latest reply on Jun 8, 2015 by mths.

Hi everybody,

 

I'm currently facing a problem with the VRMS samples and I would really appreciate if someone could help me to solve it.

 

Situation is as follows:

- The attenuation network is designed to measure +/- 500 V (353,55 Vrms), Gain of PGA2 is x1

- xVRMSGAIN registers are at default value (0x00) so far

- Sampling of VRMS registers is synchronized to zero-crossing

- 50 Samples are averaged to get the value of the VRMS register

 

I supplied known voltages Vmin (Vfullscale/20 = 353,55 Vrms/20 = 17,6 Vrms) and Vnom (230 Vrms) to the ADE7758 through the attenuation network, my results are as follows:

 

- AVRMS value at Vmin: 84308d / AVRMS value at Vfullscale: 1092176d

- BVRMS value at Vmin: 85961d / BVRMS value at Vfullscale: 1094660d

- CVRMS value at Vmin: 93908d / CVRMS value at Vfullscale: 1094360d

 

I calculated the offsets and wrote them to the corresponding xVRMSOS registers, those are:

 

- AVRMSOS: -12

- BVRMSOS: -39

- CVRMSOS: - 172

 

With this settings Phase A and Phase B readings are alright. For Phase C I get accurate readings on high voltage levels and quite big positive errors at low voltage levels (around Vmin) when converting to real world values. I found that there is a significant difference in the constant for turning register readings to real world values even after offset correction. At Vmin it is 0,00018 and for Vfullscale it is 0,00021. As I used the 0,00021 constant I think this explains the positive errors at low voltage levels. I don't think that using an averaged constant is the right way to solve the problem, so what can I do about this problem? Could calibration of VRMSGAIN be the solution?

 

Best regards

Mathias

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