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Doubt about CFxDEN calculations

Question asked by CieNTi on Jul 10, 2013
Latest reply on Jul 12, 2013 by CieNTi

Hi everyone!

 

I was following AN-1171 (Rev.A) to calibrate ADE7880 until I stop at:

CFxDEN1.png

(Page 8 of 16)

 

If you do the maths exactly as it appear the result is 3507.270118 (Rounded to 3507 = 0xDB3)

 

Now I take ADE7880 datasheet (Rev.A) to see if I'm missing something and I stop again at:

CFxDEN2.png

(Page 66 of 104)


Different way to calculate same register ... which one is the correct way? Why so different formulas?

 

I insist trying and use first equation result to calculate "n" with the second one and I get -4,050119. As datasheet says "(...)where n is a positive or negative integer(...)":

  1. If I round to -4, CFxDEN is 3125 (3507 - 3125 = 382)
  2. If I round to -5, CFxDEN is 31250 (3507 - 31250 = -27743)

 

Its a big difference no matter which one I choose (i know I can adjust CF later but .. I want to establish a nice accuracy from base first)

 

Resuming, please throw me light over CFxDEN calculations!

 

Thanks in advance and best regards,

 

CieNTi

 

CieNTi: Added documents revisions and formulas pages for better reference

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