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messner

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Flicker Noise in PSD of chopper stabilized amplifier
Jul 20^th, 2010, 5:29pm

Hi,

I am simulating a chopper stabilized amplifier for audio. I am using a 2-stage amplifier & the demodulation is done in between the stages. My pnoise analysis shows that the I/P referred flicker noise has been reduced but when I plot the PSD of the wave at the demodulator O/P (I/P of 2nd stage), I see the signal power in the baseband but I don't see any flicker noise power at the chopping frequency. I am using Cadence, Spectre, BSIM3V3. I have the following noise parameters in my transistor model files:
NMOS: noimod=2, AF = 0.82, KF = 1.23E-31
PMOS: noimod=2, AF = 1, KF = 1.57E-32
Am I missing something?
Thanks!

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HdrChopper

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Re: Flicker Noise in PSD of chopper stabilized amplifier
Reply #1 - Jul 23^rd, 2010, 7:28pm

Where are the demod switches located in your topology? Are you demodulating current or voltage (thus are you looking at the voltage or current noise components?
Connected at the demodulation switches output, do you have the Miller compensation stage?
If so, and if you are looking at the demod SW output then you might not see the modulated 1/f component since you have the big attenuation provided by the dominant pole at that node.

Please give more details on the topology of your chopped amp.

Best
Tosei

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messner

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Re: Flicker Noise in PSD of chopper stabilized amplifier
Reply #2 - Jul 23^rd, 2010, 11:20pm

Hi Tosei,

I really appreciate the time you are taking out to help me.

I am designing a fully differential 2 stage opamp with voltage chopping. Output voltage of the 1st stage is given to the demod switches. Output voltage from the demodulator is given to the 2nd stage. Please see the attached block diagram.

I am plotting the PSD at the demod s/w output from the calculator. I tried zooming in on the plot but it doesn't show any peak at the chopping freq nor its harmonics.

Please let me know if more details are necessary.

Thanks once again.

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HdrChopper

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Re: Flicker Noise in PSD of chopper stabilized amplifier
Reply #3 - Aug 4^th, 2010, 4:43am

Hi,

I'm sorry for the delay in answering your new post.
One reason for not seeing the modulated flicker component could be that you are trying to see the voltage noise on a low bw node (the node where the miller cap is reflected)
As a consequence the HF modulated noise is heavily attenuated at that point and eventually you might not see the contribution.
If this is the case I'd suggest looking at the noise current instead of voltage coming out of the demodulation switches.
If you do not see anything you might want to try just putting an ideal voltage source at the amp inputs and see how the dc olffset (say 1mv) gets modulated at the demodulation switches output. You should see that component.
Let me know what you get out of those two experiments.

Best
Tosei

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