Please help me understand the two formulas.

sqrt(3767^2 + 128)

( ------------------------ -1 ) * 100

3767

which is on page 41, ADE7880 manual. How is it concluded?

By equation (15) on that page, then I can calculate xIRMSOS value as

Irms^2 - Irms0^2

xIRMSOS = ------------------------------

128

Where:

xIRMSOS: AIRMSOS or BIRMSOS or CIRMSOS register offset value.

Irms : current input from accurate source

Irms0: xIRMS register reading without calibration.

How equation 15 is concluded?

are what I understand correct?

Thanks!

Hi ooben

Your understanding of equation 15 is almost correct. To explain it better, I would say:

1. The RMS register value read when say a Full-scale/1000 signal is applied to the chip is noted down as Irms0.

2. The RMS register value expected at FS/1000, based on comparison with nominal current register reading, is noted down as Irms.

For further explanation on that, refer to page 11 of the AN-1171 App note. It has an example worked out to help understand the calculations.

For the other equation above equation 15 in the datasheet:

sqrt(3767^2 + 128)

( ------------------------ -1 ) * 100

3767

This expression is equal to 0.00045%.

The LSBs corresponding to maximum RMS value is 3766572.

So, the LSBs obtained at 1000:1 ( 60 dB lower) signal will be 3766.572 or 3767.

So, in order to understand the effect of 1 LSB in RMS offset register on the RMS register value at 1000:1, this expression is provided. The take away for you would be to understand the actual LSB weight of 0.00045% w.r.t FS/1000 RMS register value.

Regards

hmani