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6th January 2019, 13:22 #1
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DAC design for 14 bit SAR ADC
Hi All,
I am designing Split DAC for 14bit SAR ADC. I wanted to analyze the effect of Mismatch and process variation of the capacitor on the overall SAR ADC. For that, I wanted to run a Monte Carlo simulation on the DAC unit and I am using UMC 180nm Technology. However, I am not getting the Monte Carlo model file for MIM Cap in my PDK. If anyone can help me, it would be appreciable.
Till I am not getting the required model file, I want to write a Monte Carlo model file for MIM cap by my own, just to get a feeling of the effect of Mismatch and process variation on the performance of overall SAR ADC. I want to write this model file and simulate it in cadence. How should I proceed? Please help.

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6th January 2019, 20:33 #2
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Re: DAC design for 14 bit SAR ADC
Maybe you have the PDK for a related, but not proper,
flowvariant (like, "plain digital" low cost flows might
omit the MIM steps to save a fractional penny per die).
You can get gross MIM scatter from PCM specs although
these are "sandbagged", you could get them from a
foundry modeling report / document if you asked nicely
and signed / paid as necessary. The mismatch stuff will
not appear in production test data almost certainly.
You can pretty much assume that 14 bits is not going
to happen without error correction or trimming or both,
in this style of DAC.
You might, in the absence of data, assert some range
of capacitor mismatch at the size in question (itself,
a modeling chore that may or may not have led to
realistic L, W, area dependences of mismatch in the
model). Say, run at 0.01%, 0.1%, 1% mismatch (use
a gaussian multiplier to the base C value expression)
and may as well use an analogLib capacitor as you
are not asking about systematic variation. You could
of course also make the generic C value include other
variables.

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7th January 2019, 11:58 #3
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Re: DAC design for 14 bit SAR ADC
Thanks, dick_freebird for the reply.
I have already modeled as u said, in MATLAB. But now I wanted to do the same in Cadence. I wanted to write the monte carlo model file by adding an additional variable to the nominal value of the capacitor. This variable must follow Gaussian distribution so that each capacitor in the array can assume a different value and I can notice the effect of mismatch.
But, I don't know how to write the model file for monte carlo simulation.
Please help

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7th January 2019, 19:07 #4
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Re: DAC design for 14 bit SAR ADC
Look at gauss() and agauss() and maybe track() functions.
You can make each capacitor's C property something like
(100p+1p*gauss(1.0, seed, blahblah)) and see statistical
behavior.
The sensible scaling of mismatch variation of course is the
key to relevance, and this you'd have to extract from the
vendor or secondhand from technical publications from same
or similar foundry technologies.

26th January 2019, 14:34 #5
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Re: DAC design for 14 bit SAR ADC
Thanks, @dick_freebird for the reply.
I have one more query, initially, I was calculating DNL and INL as:
1. Applying ramp input and had taken 2^N samples for Nbit.
2. Giving the sampled input to the ADC and obtained Digital output corresponding to each input.
3. Calculating the equivalent analog output for each digital output of ADC using ideal DAC.
4. DNL was calculated as:
DNL(i)= ((Vout(i+1)Vout(i))/mean(Vout)) 1 and INL as cumulative summation of DNL.
Is this method is correct? Because I am getting some peaks in the DNL curve of around 250LSB(image is attached).
I recently came across DNL and INL calculation using the histogram. Can anybody explain me, how this works?
Please help.
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