Is this limit cycle caused by the residual of the digital AI filtering at the half sampling freq and that his the threshold?
Or is this some nonlinear effect? If this is a linear effect associated with the zero-padding, the absolute
value of the DC may affect the amplitude of the oscillation. (Or equivalently the range of the DC where we get this oscillation.)
This is a good question. We may be able to test if it's a linear effect if we have enough DAC range to get the oscillation to be more than a single sample.
You pointed out the ringdown of the digital AI filter in the sample-hold case (i.e. no-zero-padding case).
How does it look like in the conventional zero-padding case?
In the zero-pad case the oscillation just continues indefinitely at the half-count value, so it never dies out (at least as far as I can tell).