About nonlinearity of PGA (Programmable Gain Amplifier)
What does 'FSR' refer to, input or output?


Gain non-linearity (whether as ppm or as a percentage of FSR) refers to the output signal non-linearity compared to the input signal should that input signal have been amplified perfectly by the same amount.

Both methods of specification are directly interchangeable so I don't see it (personally) as a big deal. If you are looking for why someone should use % FSR then it's not a sound engineering reason - it boils down to whatever the guy writing the specification is most comfortable with. There is no engineering reason to specify it one way or the other.

Looking at the two data sheets for gains of unity, the PGA204 specifies 0.0004% of FSR and the PGA280 specifies 1.5 ppm. If the output signal level is the same (say 10 V) then 1.5 ppm = 15 uV whereas 0.0004% of 10 V is 40 uV.