I am using the [MCP3248 4 input ADC with built in PGA and reference](http://www.microchip.com/wwwproducts/en/MCP3428) in a design and have determined that it has a small gain error versus a precision volt meter. The error is within the tolerance from the datasheet. The strange part is that the gain errors are different for the 4 channels. My design is such that gain errors cancel out if they are consistent across all channels, so the non-matched gains are an issue. I would like to avoid having to do calibration.

Is different gains on different channels standard for all MUX/PGA type ADCs or is it just the way that this part is designed? I would have thought that the chip would be designed with a 4:1 MUX followed by a PGA and ADC, but it seems with the different gains that it might actually have 4 PGAs followed by a 4:1 MUX to the ADC. Is there some reason why it would have been designed that way?

In order to give you an idea of the scale of the difference, 1.0029 and 1.0075 are the correction multipliers required for two different channels to match the value read by a precision volt meter.