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DAC Design and PSS/PAC in Cadence Spectre (Read 515 times)
repah
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DAC Design and PSS/PAC in Cadence Spectre
Apr 25th, 2020, 3:18pm
 
If I design a DAC - Current Steering DAC to be precise and run PSS/PAC on the DAC, which is clocked at 10GHz, with a 100MHz input frequency,
then plot the results, will PSS be plotting the SNDR or SFDR or the DAC or if not what will it be plotting ?

Thank you.
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Ken Kundert
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Re: DAC Design and PSS/PAC in Cadence Spectre
Reply #1 - Apr 25th, 2020, 5:49pm
 
PSS does not plot anything.  It just computes the periodic steady state solution and then as a post processing step you plot something.

So we really cannot answer your question. Perhaps you mean to ask: "If I plot X, what will I be seeing?". That might be something we could help you with if you told us what you plotted, or what X is.

Also, why are you using PSS? DACs don't store state for more than a clock cycle, so you should probably just use a simple transient analysis and throw away the first few cycles.

-Ken
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repah
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Re: DAC Design and PSS/PAC in Cadence Spectre
Reply #2 - Apr 25th, 2020, 8:41pm
 
Hello,

Sorry I was not clear.

Say I take the output voltage of the clocked DAC, and plot is using PSS/PAC and plot the output spectrum of the output voltage like in the attached picture.

My goal is to obtain the SNDR, SFDR and THD of the DAC.

I am plotting the output voltage of the DAC, using PSS spectrum, dB20.

So, if I plot the output voltage of the DAC in this way, what will I be seeing ?



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PSS_001.png
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Ken Kundert
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Re: DAC Design and PSS/PAC in Cadence Spectre
Reply #3 - Apr 25th, 2020, 10:38pm
 
PSS computes the continuous time spectrum of the output voltage. This differs from a DFT or FFT, which is probably what you want.  In other words, what you are seeing is what you would see if you had an ideal spectrum analyser connected to the output. In this case, when I say ideal I mean it has infinite resolution bandwidth.  What I mean  by the continuous time spectrum is that all time points are used to compute the spectrum, and so all the little artefacts and imperfections present in the analog signal contribute to the computed spectrum. If you used a DFT or FFT, you would get the sampled spectrum, meaning that the waveform would be sampled at equally spaced points in time and only those values would contribute to the computed spectrum.

What you want depends on what your DAC drives. In other words, your testing procedure should observe the output in the same manner that the surrounding circuitry would. So for example, if you DAC drives a continuous time anti-alias filter, then measuring the continuous-time spectrum makes the most sense. However, if you DAC is followed by discrete time sampling circuits, then generally a DFT of FFT would be more appropriate where you take one sample point per clock cycle, and the sample point falls at the end of the cycle when the signal has settled and all the artefacts and imperfections have settled out.  In general the sampled-spectrum should have lower distortion levels than the continuous time spectrum.

The DFT/FFT option is provided by  the calculator.

Once you have the spectrum you want, you can compute the various distortion metrics (SNDR, SFDR, THD) by hand or using the calculator.

-Ken
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