Hi,
We are carrying out resistance measurements with our ADS1261 setup using AC-excitation current on a 4-wire system and feeding the resultant voltage into the ADC of ADS1261. We are observing non-linearity in the sense that the calibration resistance reading may be a little over the calibration reference but at the other end of the measurement range, the measurement result is below the actual value or vice versa. Carrying out system valibration (adjusting reference resistor value in SW), the error at the other end of the measurement range is still too much and of the opposite sign than at the end where the calibration was carried out.
As an example:
Test resistor | Result | Error referenced to test resistor
100ohm 99.699ohm -0.301%
1200ohm 1201.189ohm 0.099% (system calibration point)
The same happens e.g. at higher resistances such that the calibration result may be -0.034% of the actual reference value but at the other end of the measurement range the error is +0.102%. The test resistors are precision resistors and very reliable, I'd say. Environment temperature shouldn't be a factor in any way as the measurements are carried out on in a lab and quick succession.
The question is, where is (or could) the non-linearity/skew be arising from? ADS1261 has non-linearity to it but shouldn't it be somewhat systematic in a device? Granted that there is noise in the system but there is a clear indication of the error changing sign when moving to the opposite end of measurement range.
Many thanks in advance!
