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Simulators >> Circuit Simulators >> Noise Simulation of Chopper Amplifier using PSS & Pnoise
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Message started by jdp on Aug 29^th, 2015, 2:32am

Title: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by jdp on Aug 29^th, 2015, 2:32am

Hi.

I am trying to simulate a simple chopper stabilized op-amp in Cadence Virtuoso (PSS + Pnoise analyses), and seeing the output noise spectrum. So, it is expected that the chopping and anti-chopping will translate the 1/f-noise spectrum (of the op-amp) to the chopping freq.

PROBLEM: although it seems that the flicker noise has got shifted to the chopper freq (1 kHz), but, some Flicker-like noise seems to have got introduced at baseband

I am not sure if this is some kind of circuit phenomenon; or simulator-settings issue (various permutations of which I have tried out, and also tried to follow whatever literature/tutorials available on internet) :-[

Any suggestion/help in this regard is highly sought.

Thanks and Regards,
Jay

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Ken Kundert on Aug 29^th, 2015, 5:45am

I recommend creating the noise contribution summary to determine which devices are responsible for the low frequency noise.

-Ken

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by jdp on Sep 2^nd, 2015, 3:39am

Hi Ken,

From Pnoise simulation Noise Summary, it seems that 4 MOSes in the op-amp are contributing most of the noise. Please see the attached figure.

Also, I am showing the noise spectrum before and after the anti-chopper, side by side (note the different y-axes scales) - it is evident that there is reduction in noise, but the additional 1/f-noise in the spectrum after the anti-chopping seems strange!...any idea if this is okay??

Regards.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Ken Kundert on Sep 5^th, 2015, 7:22pm

Chopping will only cause the flicker noise to go away if the chopping is perfectly symmetric. Is it possible that the excess noise in your circuit is caused by an asymmetry in the chopping?

-Ken

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by hiSkill_11 on Sep 7^th, 2015, 12:17am

Did you make sure the chopping clock is nonoverlapping?

jdp wrote on Aug 29^th, 2015, 2:32am:

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by jdp on Sep 8^th, 2015, 7:10pm

Hi hiSkill_11,

I have tried with both non-overlapping as well as normal complementary clocks for chopping - but same problem in both cases :(

Hello Ken,

Regarding asymmetry in chopping - I am not able to find any myself (maybe I am overlooking something), like the duty cycle used is 50%, switches are ideal,...

I am sure I am doing something silly that I am not able to find on my own :-[

Thanks...

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by petitidiot on Sep 16^th, 2015, 4:24am

Is your chopping frequency too low?

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by jdp on Sep 16^th, 2015, 5:38am

No.
I have tried with chopping freq of 1 kHz, 10 kHz, etc. Also, have ensured that this freq. is within the op-amp bandwidth.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by saqibshah on Oct 8^th, 2015, 6:00am

I think your circuit is behaving OK. If you take a look at your 1/f noise before anti-chopping it is in the mV range, but has decreased to the uV range after anti-chopping. Correct me if I am wrong.

Before anti- chopping : approx 80mV peak flicker noise
After anti- chopping : approx 800uV flicker noise.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by jdp on Oct 10^th, 2015, 7:42am

Yes, there has been reduction in flicker noise indeed...but the low freq. 1/f behavior still persisting is what is unexpected.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by skylarjs on Oct 15^th, 2015, 3:08pm

I am also working on a chopping amplifier and am seeing a similar effect.

With the chopping switches enabled I do see a significant reduction in IRN and removal of DC offset, but there is still a 10dB/decade rolloff at low frequencies (like OP). I have tried increasing my chopping frequency well beyond any flicker noise, but the 10dB/decade rolloff still remains.

Is this expected? Or do people see a flat curve even to sub-Hz frequencies when they have done chopper design.

If OP or anyone could resolve this, I would be much appreciated.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Ken Kundert on Oct 18^th, 2015, 9:30am

The chopping is never perfect. There is always some asymmetry in the chopping or some feed through around the switches. You can never completely get rid of the 1/f noise.

-Ken

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Raghu9685 on Mar 16^th, 2016, 10:31am

jdp wrote on Aug 29^th, 2015, 2:32am:

Hi Jay

I am trying similar chopper amplifier for noise simulation but not able to see 1/f-noise shifting at chopper frequency PSS & PNOISE simulation result. I have done similar setting of PSS & PNOISE as you mention .
please help me how to simulate this .....I don't what I am missing.

Thanks & Regards

Raghu

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by ULPAnalog on Mar 16^th, 2016, 12:03pm

Hello Raghu

I have not followed this thread before, so here are some thoughts.

1. It is not entirely clear to me on how your output common mode is being set. Have you simulated the amplifier under nominal conditions without introducing chopping? Are the DC operating points as expected? You may want to do this, first to check if the amplifier is indeed working as an amplifier and secondly to see where the flicker noise corner lies.

2. Jay's simulation settings are all good as far as I know. But I would make refsideband parameter = 0 to use the correct conversion gain when the noise is referred back to input (Input referred noise is usually useful figure for amplifiers).

Thanks and regards

PS. It would have been better if you had created a separate thread, with a link to this topic, to be compliant with the forum guidelines.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Raghu9685 on Mar 16^th, 2016, 12:45pm

Hi ULPAnalog

yes I have simulated amplifier with gain of 51 db & bandwidth of 3 Mhz .
and I am getting flicker noise corner frequency from noise simulation about 100 khz.

steps that I followed :-

1. Simulated Folded-cascode ampifer and optimize aspect ratios (W/L) for low noise . Using spice2 model of flicker noise.
2. Simulated modulator and demodulator circuit for chopping .
3. Then simulated chopper amplifier in this I have done analysis are as
followed :
a. DC
b. AC
C. PSS
d. PNOISE

I don't what I am missing ...
please help me out..

Thanks & Regards
Raghu

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by ULPAnalog on Mar 16^th, 2016, 12:52pm

Hello Raghu

I looked at your schematic once again and recognized it is a single ended output structure. Is that correct? Also it is not clear which of the nodes is out? It looks to me as if 2 nodes are labelled out. Not sure if I am reading it right.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Raghu9685 on Mar 16^th, 2016, 10:40pm

Hi ULPAnalog

Yes It is single ended output structure. both label are not same one are "out" and another one is "out-" . I tried both the cases first I given the single output of amp to anti-chopper circuit not getting result so I also tried that double ended output to anti-chopper circuit then also I am not getting result.

Thanks & Regards
Raghu

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by ULPAnalog on Mar 17^th, 2016, 7:54am

Hello

Could you post the resulting plots and key specs including the chopping frequency etc?

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by Raghu9685 on Mar 18^th, 2016, 4:52am

hi
My Institute cadence license got expired it will take at least 10 days for renewing so I will post the result as soon as I get license.

Title: Re: Noise Simulation of Chopper Amplifier using PSS & Pnoise
Post by pandilla on Aug 27^th, 2017, 10:28am

what is circuit there inside the chopper box?
please explain anyone...
thanking you...