Why is there an offset on the BenADDA board (DAC to ADC) of the XtremeDSP Development Kit/-II/-II Pro/-IV?
There is an offset on the BenADDA board.
The reason for the offset is that the output signal being driven out of the DAC is a differential signal that is passed to a differential amplifier, which converts the differential DAC output to a single-ended output before coming off the board.
The offset appears worse when doing a loop-back test, because you have the an additional offset as the signal is then passed to the ADC, then through an op-amp which is also biased to allow for input into the ADC. The effect of these stages is that some offset is added.
A DAC to ADC loop test has been run as part of Nallatech production tests; normally, approximately 10mV offset is considered within specification. This is when the test is done using the DAC and ADC on the same board. In a case where two different boards are used, the offset may also include a grounding difference between the two boards.
It is advisable to allow for a small offset when measuring the output of the ADC or DAC.