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AD7793 internal calibration leads to seemingly random readings

Question asked by AndreasKl on Feb 28, 2015
Latest reply on Apr 5, 2015 by jtee

Hi,

 

I'm using an AD7793 for resistance measurements in a simple ratio-metric configuration (see schematic) with one excitation current and a reference resistor. I try to measure a 0.1% tolerance resistor for testing purposes and I use a 0.01% 5k reference resistor.

 

I noticed that I get slightly different readings (+/-10 Ohm) for the resistor if I connect the circuit to different AD7793s and I figured that calibration might help me solve that problem and lead to more equal readings across devices.

 

However, when I carry out internal zero scale and full scale calibrations, things get much worse. While without calibration I might just be off by a few Ohm, after every calibration my readings seem to get distorted completely. I get widely differing results for the 2.2k resistance, ranging anywhere from 50-5000 Ohm. This is the case even if I calibrate again right after a calibration.

 

Can you help me figure out what could be the problem here? It seems like something is interfering with the calibration but I can't figure out what.

 

Thanks a lot!

 

AD7793_calibrationproblem.jpg

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