We use an ADC from Cirrus Logic on the DAC board. It's there to check the requirements for noise and linearity of the DAC (and subDACs).
It's the same ADC that we used in the Telis project. It's a nice piece of work, very accurate. For Telis it was used to measure in the hundreds of nanovolts.
On the DAC board however, there seem to be problems with linearity. Around zero volts, it seems to jump up and down a little. It wasn't just this sample; the ADC was lifted off the board and replaced with another -- the same problem was seen.
The gain on the ADC was lowered and the effect was still seen in the same way, so it's not the DAC that's causing the trouble. This indicates that the ADC or its placement on the board is the problem.
It's probably that last thing since with Telis it worked wonderfully, but that isn't conclusive because back then, the voltage around zero was looked at, but not very carefully.
What we'll now do is firstly use external measurement equipment and secondly, fire off an e-mail to Cirrus Logic support.
Another problem is that the ADC now gives off some REAL funny numbers after some changes in the FPGA. After a full day of analysis with three people (software engineer, FPGA engineer and analogue designer), we come to the conclusion that... the power must be shut down. You'll ask why wasn't this done right away. We did -- except the command that powers off the digital and analogue parts of the board do NOT affect the power of the ADC... That was one of the requirements.
However it remains to be seen whether this fixes the problem of non-linearity around zero.