I am having trouble understanding the function of the optional buffers between the PGA and ADC on the ADuCM360. It seems the buffers cannot be used at all for Gain=1 when the PGA is off due to a silicon error so I think that the ADC sees the MUX output completely buffered. For PGA gain >2 you can enable the buffer but doesn't the PGA already serve as a buffer between the input and the ADC? What purpose does adding the additional buffer serve. Is the PGA output impedance too high to drive the ADC? It is not clear from the datasheet and manual when you would want to use this feature which increases power consumption.
Your understand is right. For Gain=1, the buffer cannot be used, but this feature is enabled in ADuCM362/3. So if you use this feature, you can choose ADuCM362/3.
For PGA gain>2, the PGA can serve as a buffer, so the buffer isn't necessary. If you use the buffer, it will add noise, but the linearity will be a little better. The recommended way is disable the buffer when PGA gain >2.