It would be helpful to share what you tried that didn't work when you tried the dnl() function in the the waveform calculator. Per the manual (Virtuoso Visualization and Analysis XL User Guide, Calculator Functions, Special Functions, dnl), the calculation is pretty much what you have said. dnl[k (or maybe k+1)] = (sig[k] - sig[k-1])/ideal_step - 1. Part of the trick in using this function is how to specify the time points. Make sure you read that part carefully. You either have to supply the sampling interval, a list of time points, or a clock waveform whose edges define the sampling points to the function. In addition, you need for your simulation to step the DAC through its codes.
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