33.4.3.2 Offset Reduction by Sign Chopping

Sign Chopping is a feature that can mitigate offset error in the ADC. It is enabled by writing a ‘1’ to the Sign Chopping bit in the Control D register (CTRLD.CHOPPING).

The feature works by swapping the inputs to the ADC every other sample and then subtracting the inverted sample (-V_IN) from the non-inverted sample (V_IN). Since the offset error (V_OFFSET) of the ADC is not proportional to the input voltage, this operation effectively cancels out the offset. This is illustrated in the following equation:

$(V_{IN}+V_{OFFSET})-(-V_{IN}+V_{OFFSET})=V_{IN}+V_{OFFSET}+V_{IN}-V_{OFFSET}=2\times V_{IN}$

The figure below illustrates the Sign Chopping feature. For even samples, the ADC operates as normal. For odd samples, the ADC swaps its inputs to invert the input signal without inverting the offset.

Sign Chopping can be used with either single-ended or differential conversions. It may only be used in accumulation modes (Series or Burst).