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Unexpected ADC offsets using ADuC847, where to look?

Question asked by OliverSedlacek on Jun 24, 2015
Latest reply on Jul 13, 2015 by OliverSedlacek

Having had great results using the ADuC845, I was surprised to find much larger than expected ADC offsets on my latest PCB. On the AIN1-AIN2 pair I measure 227uV and on the AIN3-AIN4 pair I measure 38uV. The ADC is configured in buffered bipolar mode with SF = 171. With the input shorted, the PGA is set to 0 (20mV scale). What can I try to reduce these offsets? I've attached schematics.

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