Hi Oba,
What I am saying is there shouldn't be any measurable difference for any input combination on a single ADS124S08 device. There may be device to device variation and lot to lot variation, but within a single-device the construction is the same for each input channel. Offset will be determined by the PGA and ADC itself and not the input connections.
What may be the true concern by the customer is how the leakage/bias currents may affect measurement error with respect to input filter resistances. The leakage/bias currents would also be the same for each channel, but external resistances may cause error if the resistance is very large. This would be especially true if the PGA is bypassed. We have included graphs of the input current characteristics in the datasheet in Figures 8 through 15.
The ADS124S08 is designed to have equal channel characteristics which may not be true for all devices. One such example is the ADS1220 where the input bias/leakage current is much different with respect to input channel selection. The design of the ADS1220 has a low-side switch connection on the AIN3 input. The leakage on the AIN3 input is greater at higher operating temperatures due to device sizing to keep the low-side switch resistance low. In turn the leakage on this AIN3 pin is larger. The ADS124S08 also has a low-side switch but the position of the switch differs greatly between devices to eliminate the effect on the analog input channels.
So it would be helpful to fully understand the customer's concerns and what issues the customer has seen previously to create these concerns.
Best regards,
Bob B
