ADE7854 Apparent Power calculation (AVA)

Document created by analog-archivist Employee on Feb 23, 2016
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a customer has a problem with the power values read back from an ADE7854.

He initializes the ADE and reads this data:


Voltage:  AVRMS = 1539660 => 150.3 V

Current:   AIRMS = 44  => 4.4 A

Power:     AVA = 8  ?????


What is the right way to get the power?

 

The rms value of the voltage they applied to the meter is V.
The full scale rms value of the voltage channel is VFS.
When they apply V, VRMS register has VRMS=1539660
When they apply VFS, VRMS register has VRMS_FS=4191910.
So we may write that V/VFS=VRMS/VRMS_FS.

The same approach may be done for the current:
I/IFS=IRMS/IRMS_FS.

PMAX is the instantaneous power computed by ADE7878 when the current and
voltage are at full scale:
IRMS_FS*VRMS_FS=4191910^2=17572109448100.00
But PMAX=33516139. The difference comes from the fact that the ADE7878 DSP
divides IRMS_FS*VRMS_FS by 2^19 (is practically a shift left to reduce the
number of bits of the result:
log2(IRMS_FS*VRMS_FS/PMAX)=log(4191910^2/33516139,2)=19.

The expression 40 gives AVA computed by ADE7878 as V/VFS*I/IFS*PMAX/16.
We can write it as:
VRMS/VRMS_FS*IRMS/IRMS_FS*IRMS_FS*VRMS_FS/2^19/16=VRMS*IRMS/2^23=1539660*44/2^23
=8.075.

So ADE7878 gives AVA=8.

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