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AD 7794 Ratiometric measurement vs direct ?

Question asked by dmp on Dec 19, 2011
Latest reply on Jan 11, 2012 by dmp

hi,

 

I don't understand why the mesurement of the ratio of two resistors is the same +-10ppm when i divide the individual measurements using internal reference and when i measure one using the other as reference (ratiometric). the two resistors are 1k AIN1and 4k AIN2, 1ppm/°C, iexc=200µA , g=1,buf on,Ain2-=.880V.

It seems to me that the current consumption of VREF2 should induce a difference of approx 350nA/200µA=.17%, the ref being unbuffered.

a zero and full scale internal calibration is done at each measurement.

Can someone explain this ?

 

Thanks

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