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AD7195 conversion question

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 load500kg
Rated output2mV/V
(AC) Excitation voltage10V

 

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:  

Vref5V
A_VDD5V
A_GND = D_GND0V
D_VDD3,3V
REFIN+ = A_VDD5V
REFIN- = A_GND0V

 

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?

 

Thanks in advance

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