I have a question concerning how to get the DNL plot of my 9 bit ADC.
I have a ramp input that goes to fullscale.  The output is a plot of all the codes from 0 to 511 with time on the X-axis.  Each step of the output is 1 LSB (1V).  My question is how do you get the DNL from this plot?  Every reference material I see concerning the DNL of an ADC has the input voltage on the X-axis.  So what I need, I guess, is each step of the output as a function of the input voltage instead of time.
Because if I take the DNL of the plot of the output versus time, it is 0 because each step is exactly 1 LSB!

Mischivis,

 Sorry, the nuclear power plants melting down have got me a little
distracted. Some corrections:

1)   Connect an ideal ADC and DAC in series
     I usually just edit the ahdlLib 8 bit ideal components to create
     whatever ideal components I need.

2)  Set the DAC lsb =1V, so the full scale range in your case will
    be 511
    This setup will generate an analog waveform, a staircase, with
    values proportional to the digital codes.

Another option would to use a slow ramp, 0s --> 511s, then you could
directly plot the dnl vs. time.

                                                                    Best Regards,

                                                                        Sheldon

I'm not sure, but I think the answer you are looking for is to measure the amount time each code is present. Ideally (if you have 512 levels) each level should be present for 1/512 of the total time. If the ADC has some DNL error the amount of time each code is present will be proportional to the DNL+1.

I think I had a clever way of doing this with a ramp input using the cadence calculator, but I can't remember it! Perhaps I just sampled the digital output and exported it to Matlab. From there you do a histogram of the codes and compare it to the ideal histogram.

Rob,

 The ability to change the X-axis has been in Virtuoso for a long time,
look at the x-axis properties and change the variables. As for sending
data to the calculator in IC61, hover the mouse over the data you
want to plot and click the right mouse button [RMB]. While holding the
RMB down select calculator from the menu.

                                                              Best Regards,

                                                                Sheldon

sheldon wrote on Mar 22^nd, 2011, 5:19am:

Thanks, but did you have to point out that "long time" part  ;D I actually think I figured it out and then forgot it now that you mention it.

Quote:

Not sure what you are describing, but I want to send the data to an external file and process it with Matlab. I think it was the "printvs" command that used to do it.  I'm sure it is something simple... I just haven't had enough time to search it out. Maybe the OP needs to know too.

Wanna give a quick primer on what the dnl function does?

Rob

you can also pull the time domain data over into Matlab or even Excel and deal with it.