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

Hi Mathias,

The CT definitely sounds like it is the issue, let me know how the results looks once you get a CT with better linearity under 1 A.

As for the questions:

Best Regards,

Dlath