TMS320F28335: ADC Offset calibration Issue

Steve,

stevenh said:

I am not able to connect ADCLO to each pin as specified in section 10 of SPRU812A. 

You only need to connect ADCLO to ONE ADC input.  The offset error will then be corrected to within the channel-to-channel variation of the ADC (specified in Table 6-50 of the datasheet sprs439m as +/-4 LSB TYP).

stevenh said:

Instead of using ADCLO, can I just tie my board's input to ground and then perform the steps outline in section 10?  Will it give the correct results for the ADCOFFTRIM register?

This will introduce any offset error of the external signal chain components (e.g. the op-amp you mentioned) into the correction.  There is only a single offset correction register for all the ADC channels.  So, the offset will be compensated perfectly on the input channel you used to perform the offset correction calculation, but if the external offsets on this channel differ from the offsets for the other channels, the other channels may not be corrected well.  In particular, if a channel has smaller offset than what was corrected, you could end up with a dead spot near zero on that channel.

If having to use the board inputs for offset calibration is something that you need to just do now during your current development (for whatever reason), then perform the exercise using each input.  Use the one with the largest determined OFFTRIM value (most positive) in the OFFTRIM register (this is actually the smallest (or largest negative) actual offset).

Regards,

David