ADS1256: ADS1256 internal full scale calibration question

Hi Sunny,

You are correct, the ADS1256 can perform a self-gain calibration for any given gain setting. On most other ADC's you can get away with manually doing the same thing (connecting VREF to the inputs and performing a system gain calibration); however, this would only be an acceptable input condition for a PGA gain of 1 V/V.

The short answer to this question is the ADS1256 has a different kind of input circuit that is able to scale the reference voltage accordingly, so as not to violate the PGA's differential input voltage range. While other devices do require some kind of external calibration signal, I would point out that in many cases the uncalibrated gain error (of the ADS1261, for example) is almost as good as the calibrated gain error of the ADS1256...

The ADS1256's self-gain calibration is most effective at a gain of 1, resulting in 50 ppm FS gain error after calibration; however at gain of 64 V/V, the gain error is still typically 300 ppm FS after calibration. On the ADS1261, the typical gain is 500 ppm FS before calibration, but after calibration is on the level of the noise (which can be less than 1 ppm FS).

Additionally, there is only so much value to performing a self-calibration. In general, for customers that want to achieve the best performance, we highly recommend performing a SYSTEM gain calibration to account for any gain errors that may occur outside of the ADC (in the signal conditioning or input filtering, for example).

Providing an accurate full-scale input signal (with more accuracy than the uncalibrated ADC) can indeed be difficult, which is why in may cases gain calibration is not necessary; which just goes to show that our devices are very accurate "out-of-the-box", or "out-of-the-tape"?... :)