ADS7066EVM-PDK: GUI results

Roee,

1. The accuracy of the ADC can be limited by the accuracy of the signal generator itself. I don’t think you will be able to achieve full accuracy on the ADS7066 using this source as a signal generator.  Nevertheless, you should get much better accuracy than what you are currently measuring.  My point here is that once we eliminate the large errors you are seeing, you may need a more accurate / stable DC source to get full performance of the ADS7066.  For now, please just verify the input using a precision multimeter. 
2. In your case, I suspect what you are seeing is settling error. Settling error will happen when the internal ADC sample and hold capacitor doesn’t have enough time to fully charge.  A similar settling concern is related to switching the MUX.  That is, the parasitic capacitance on the output of the MUX needs to fully charge when the mux is switched.  The easiest way to see if you have settling issues is reduce the sampling rate (i.e. increase the acquisition time).   As an experiment, please reduce the sampling rate and see if your accuracy improves.  The GUI provides a simple way to directly enter the sampling rate at the top of the GUI (see below).
3. Settling errors are common when large source impedances are used. This EVM has 1kohm resistors in series with each input.  This is a fairly large resistance from a settling perspective.  I believe that this resistance magnitude was used to provide some protection to the inputs, but it does limit the sampling rate.  If you see improved accuracy by reducing the sampling rate, but want to run at a higher sampling rate, you will need to change the input resistors to a smaller value.   I can help you with the component selection once we determine if the issue is a settling error.

I hope this information is helpful to you.

Art