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Reading Vrms ADE7753

Question asked by wwseb on Dec 15, 2015
Latest reply on Jan 16, 2016 by wwseb

Hi there,

 

I made my own design based on a ADE7753 + MCU. I respected closely the reference design and have a proper implementation (as far as I know) of the ADE on the board. I have a perfect behavior of the Zx pin, which makes me think that the overall hardware is pretty functional. However, I am not sure on the way to read the Vrms.

 

The voltage is supposed to be 235Vrms (measured with voltmeter). I have a voltage divider of /1000 through a 1M and 1k resistors, which is supposed to result in a +/-0,32V. The capa for filtering ar 33nF and the resistor on the negative input is 1k. I have an external reference that generate a proper 2,44V (measure with the scope). The power supply is a clean 5V, regulated.

 

I acquire the Vrms value, sync with the Zx crossing. Here is what I get on a 100 samples acquisition:

vrms.png

They are properly spaced of 10ms (Zx is working fine), however I can't manage to understand what I am supposed to measure here. Maximum value is 568981 and minimum is 95495, the period is 60ms...

 

The Vrms is supposed to be stable isn't it? I should have a flat line with, at worst, a little noise.

Plus, how can I convert this value into a value in "volt"?

 

To be complete I made an acquisition of the input. I measure directly on the V2p and V2n of the ADE:

IMG_20151214_200957.jpg

Amplitude and frequency are as expected, however the shape is a little "sharp". Do you think it can be an explanation? What would to that? Maybe the filtering components value?

 

Any help would be greatly appreciated.

 

Thank you in advance

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