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Simulators >> RF Simulators >> Noise analysis of asynchronous SAR
https://designers-guide.org/forum/YaBB.pl?num=1478278025

Message started by deba on Nov 4^th, 2016, 9:47am

Title: Noise analysis of asynchronous SAR
Post by deba on Nov 4^th, 2016, 9:47am

Hi All,

For asynchronous SAR ADC noise analysis, I am using transient noise analysis to verify the circuit performance. But it doesn't list out the noise contributors as other small signal noise analysis prints out.

Is there a way by which one can get noise contributors using transient noise?

Regards
Debasish

Title: Re: Noise analysis of asynchronous SAR
Post by Ken Kundert on Nov 5^th, 2016, 10:30pm

Perhaps you should try PNoise analysis rather than using transient noise.

-Ken

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 7^th, 2016, 1:54am

Hi Ken,
Pnoise will not work for the asynchronous SAR. The comparator is not clocked synchronously.

There is a noiseon parameter, I guess I need to run multiple .trannoise sims, and keep turning off noise sources one by one to find the dominant one.

Title: Re: Noise analysis of asynchronous SAR
Post by Ken Kundert on Nov 7^th, 2016, 4:57pm

Quote:

Pnoise will not work for the asynchronous SAR. The comparator is not clocked synchronously.

Why does that matter?

-Ken

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 8^th, 2016, 7:49am

Hi Ken,
I am doing noise analysis on the complete SAR. Since the comparator is clocked asynchronously, it has no relation to the sampling clock. It's non-periodic, thus PSS will not converge.

Title: Re: Noise analysis of asynchronous SAR
Post by Ken Kundert on Nov 9^th, 2016, 6:58pm

I am assuming that if the input is the same on each conversion, and the fundamental period equals the conversion interval, then the response on each conversion will be the same and so the waveforms will be periodic. But perhaps there is some aspect about the architecture that I do not understand.

-Ken

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 9^th, 2016, 8:32pm

Hi Ken
The SAR uses monotonic switching down sequence. The input common mode of the comparator keeps going down as the SAR conversion progresses. Thus, the comparator conversion time varies from cycle to cycle, resulting in non-periodic behaviour.

Thanks for your inputs.

Title: Re: Noise analysis of asynchronous SAR
Post by Ken Kundert on Nov 10^th, 2016, 5:19pm

Wouldn't the whole ADC behave periodically in N cycles, where N is the number of cycles needed to do a conversion?

-Ken

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 11^th, 2016, 12:02am

Hi Ken,

As the input common mode for the comparator keeps dropping down, the conversion time is different for each conversion cycle. Thus, a non-periodic behaviour.

A 10-bit 50-MS/s SAR ADC With a Monotonic Capacitor Switching Procedure.
IEEE Journal of Solid-State Circuits ( Volume: 45, Issue: 4, April 2010 )

Title: Re: Noise analysis of asynchronous SAR
Post by Ken Kundert on Nov 11^th, 2016, 7:40pm

According to the flow chart in figure 3, a conversion lasts N cycles. If the input is constant, isn't the converter behavior periodic in N cycles?

-Ken

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 12^th, 2016, 7:04am

Hi Ken,

Fig. 4(b) of the paper shows the behaviour of comparator inputs as the conversion progresses. As the common mode keep changing, the input gm of the transistor keeps changing from cycle to cycle. Thus, even with a constant input, the comparator clock is different for each conversion.

Title: Re: Noise analysis of asynchronous SAR
Post by Ken Kundert on Nov 12^th, 2016, 9:28pm

Don't the waveforms in Fig. 4 repeat after N phases, where N is resolution of the converter in bits?

Perhaps we are getting tripped up in our terminology. When I say conversion time, I mean the time it takes for one input to be fully converted to digital. Thus, a conversion requires N phases, where each phase lasts the length of a clock cycle. So for a 10-bit converter running with a 100MHz clock, the conversion time would be 100ns, or 10 times the 10ns clock period). Given this example, wouldn't the waveforms in Fig. 4 be periodic with a period of 100ns?

-Ken

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 14^th, 2016, 5:25am

Hi Ken,

I got your point. Yes the comparator waveforms will be periodic. For your example, period will be 100 ns. I also checked the PSS does converge for the complete SAR. This is great. Thanks.

Now I need to understand how to translate the noise seen at the comparator output, to the ADC noise spectrum. This has opened up a new perspective.

Title: Re: Noise analysis of asynchronous SAR
Post by deba on Nov 19^th, 2016, 7:56pm

Hi Ken and all,

After establishing that the SAR has a PSS for constant DC input(in my sim it is 0). I did a strobed pnoise analysis for the comparator's output. The strobing is such that the comparator waveforms are in the exponential settling regime. Now I get 13-pnoise plots (for a 13-bit converter) for every conversion cycle. I am still trying to figure out how to relate this to the ADC spectrum plot? Do you have any suggestions on how to do this?

On a separate note, it is not related to this thread. But I will ask anyways. For other ADC architectures, like pipeline, flash, algorithmic ADC etc. one can also do a PSS/PNOISE analysis. But in most of the appnotes, I have seen only transient noise is the recommended method for measuring noise performance. There seems to be very few papers which talk about applying SpectreRF analysis to ADC's. Mostly they are limited only to simple cases like noise analysis of a dynamic comparator. Do you have any comments on this?

Thanks

Title: Re: Noise analysis of asynchronous SAR
Post by zeropond on Apr 9^th, 2017, 8:31am

Hello ;D
I am zeropond Iam a new member of this forum site. but I have nothing any answer to your question about noise analysis of asynchronous SAR. so please ask to your friends.
Thank you :)

Title: Re: Noise analysis of asynchronous SAR
Post by BackerShu on May 12^th, 2017, 12:52am

deba wrote on Nov 19^th, 2016, 7:56pm:

On a separate note, it is not related to this thread. But I will ask anyways. For other ADC architectures, like pipeline, flash, algorithmic ADC etc. one can also do a PSS/PNOISE analysis. But in most of the appnotes, I have seen only transient noise is the recommended method for measuring noise performance. There seems to be very few papers which talk about applying SpectreRF analysis to ADC's. Mostly they are limited only to simple cases like noise analysis of a dynamic comparator. Do you have any comments on this?
Thanks

This is an interesting point. My feeling is that although it is not common in papers/application notes, SpectreRF is widely used for clocked circuits. I think it will be a good idea that you open a new post for this discussion.