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AD7124-4 offset and gain calibration equations

Question asked by Frank83 on Mar 5, 2018
Latest reply on Mar 14, 2018 by Frank83

Hi, I'd like to use the AD7124-4 as a mutichannel fully integrated front-end for strain gauges based sensors.

I'm reading the datasheet and I'm having issues understanding the calibration formulas used to set the values of the gain and offset registers.

As reported at page 51 the equations are:

 

1) Unipolar Data= [((0.75*VIN/VREF)*(2^23))-(Offset-0x800000)]*[(Gain/0x400000)*2]

2) Bipolar Data= [((0.75*VIN/VREF)*(2^23))-(Offset-0x800000)]*[(Gain/0x400000)+0x800000]

 

It is stated that the default value of the offset register is 0x800000 while the default value for the gain register is  0x5XXXXX.

I assume that with default calibration values these equations should yield the same results as the default output code equations reported on page 47 of the datasheet:

 

3) Unipolar Code=[(2^N)*AIN*Gain]/VREF

4) Bipolar Code=(2^(N-1))*[(AIN*(Gain/VREF) +1]

 

Now if the default offset register value is 0x800000 indeed the (Offset-0x800000) correction factor goes to zero.

But what happens with the gain register?

Is VIN=AIN*Gain(PGA)? 

Are the X nibbles in the 0x5XXXXX value don't care conditions (why should there be don't care digits in a default calibration value?) or is the default code actually 0x555555? I'd expect the default value of the (0.75*Gain)/0x400000  factor to be 1 but for that to happen the gain should be = 0x555555

 

Thanks for your help and support.

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