Question asked by PeterVers on Dec 26, 2014
Latest reply on Feb 10, 2015 by jtee

Hi,

Currently I am working with the AD7195. I'm using this ADC to convert the signal from a loadcell to a digital value. The loadcell has the following specs/configuration:

 Rated load 500 kg Rated output 2 mV/V (AC) Excitation voltage 10 V

I am trying to find out how the ADC is converting the analog value to the digital value. I searched the datasheet and found these statements:

- Code = 2^(N – 1) × [(AIN × Gain/VREF) + 1]

- The offset calibration coefficient is subtracted from the result prior to multiplication by the full-scale coefficient

So I checked my coëfficients and they are:

- offset calibration coëfficient = 8388608

- full-scale calibration coëfficient = 5585472

The ADC is configured with ACX enabled, CHOP enabled, Gain=1 and bipolar operation.

I am using these voltages to power the ADC:

 Vref 5 V A_VDD 5 V A_GND = D_GND 0 V D_VDD 3,3 V REFIN+ = A_VDD 5 V REFIN- = A_GND 0 V

When I put 0 kg weight on the loadcell, the digital value I get is around 13974xxx. Is it coincidence that this is almost the same value as the offset calibration coëfficient + full-scale calibration coëfficient (8388608+5585472=13974080)?

My question is, which statement for conversion does apply to the ADC, because it seems to me that they can't both be true at the same time. Maybe I am missing something? Can somebody please enlighten me?