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Chopping inside or outside the feedback loop? (Read 6495 times)
jdp
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Chopping inside or outside the feedback loop?
Sep 22nd, 2017, 11:14pm
 
Hi,

We know of two possible configurations in which the chopper-switches can be placed to realize a chopper stabilized amplifier - either outside the negative feedback loop, or inside the loop (as shown in figure below).

What are the advantage and disadvantage of either of these schemes?

Any relevant reference/paper is also appreciated.

Thank you!
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chopper_config.jpg

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Horror Vacui
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Re: Chopping inside or outside the feedback loop?
Reply #1 - Oct 22nd, 2017, 12:26pm
 
If you put the chopper in the loop than the chopping frequency is amplified by the open loop amplifier, so chopping might work at a higher frequency. Also any imperfections of the down-conversion will be in the loop, thus its input referred effect is reduced by the loop gain. At the same time no DC current will flow through your switches. They see a capacitive load instead of a resistive load. You should contemplate which one is better.

You can also look at it this way:
1.) You design a differential opamp with chopping and use it in the feedback configuration
2.) You implement a (low-gain, low input impedance) feedback amplifier and you use chopping on it.
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deba
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Re: Chopping inside or outside the feedback loop?
Reply #2 - Oct 23rd, 2017, 8:25am
 
Config-1:
1) has the advantage of removing the mismatch among the resistors.
2) The opamp is always in negative feedback configuration.

Config-2:
1) Opamp enters into open loop mode during non-overlapping times.
2) Resistor mismatch is still present.
3) The opamp need to have high gain till the chopping frequency.

Thanks
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Rakesh
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Re: Chopping inside or outside the feedback loop?
Reply #3 - Oct 23rd, 2017, 1:36pm
 
Can you explain this "If you put the chopper in the loop than the chopping frequency is amplified by the open loop amplifier, so chopping might work at a higher frequency".

For both the cases the opamp has to work till the chopping freq+signal bandwidth as the signal is upconverted to higher frequency.

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jdp
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Re: Chopping inside or outside the feedback loop?
Reply #4 - Oct 28th, 2017, 12:10am
 
Hi. Thanks for your effort in answering the query till now...
Below is a plot of the open-loop Vs. closed-loop gain of the opamp.

Can it be said that in the Chopper-config-1, the chopped signal faces the Closed-loop response; while in config-2, the chopped signal faces the Open-loop gain of the op-amp?
Then can it be said that the chopping freq can be (favorably) more when the chopped signal passes through the larger BW of the closed loop case (Config-2)?
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dont-be-fooled-by-your-amplifiers-bandwidth_fig1.jpg

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Horror Vacui
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Re: Chopping inside or outside the feedback loop?
Reply #5 - Oct 28th, 2017, 1:43pm
 
Rakesh wrote on Oct 23rd, 2017, 1:36pm:
Can you explain this "If you put the chopper in the loop than the chopping frequency is amplified by the open loop amplifier, so chopping might work at a higher frequency".


No, because I did not formulated it well, and it is not true Smiley . As the picture in  jdp's next post explains it the chopping frequency can be higher if the chopper is outside the loop. Note, that you have to have a filter which removes the upconverted flicker noise and offset.  So the system bandwidth is always smaller than the chopping bandwidth. The higher is your chopping frequency the harder to filter it out, unless you do not use a switched cap notch filter which puts the the notch exactly at the chopping frequency, but if I remember it good it will introduce some phase shift creating some stability problems to solve. Such a filter should be in your loop, otherwise the low frequency "noise" will appear again at the input of your amplifier.

I guess the main reason why I have bad feelings with chopping outside the loop is because in that case the core amplifier's gain is reduced by its offset, which is one of the things you wanted to avoid. If the chopping is outside of the loop, only the resistor mismatch will be removed.
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Re: Chopping inside or outside the feedback loop?
Reply #6 - Oct 31st, 2017, 9:33am
 
A slightly different perspective here -  Are you using the op-amp in one location or as a macro in multiple locations within the design?

If it is a multi use macro then chopping within the amplifier structure will make for an easier multi use scenario.

Also - What happens with a lot of simulation work on these devices is that you turn the chopping off in order to speed up simulation time.  Consequently, you need to be careful with second order effects associated with the element switching/chopping in the overall performance, by determining what elements of the chopping process feed through your system.

Don't assume an offset free amplifier because of chopping, you may get spectral spurs due to the switching or residual offsets due to an unbalanced switching characteristic.
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Frank_Heart
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Re: Chopping inside or outside the feedback loop?
Reply #7 - Nov 17th, 2017, 12:35am
 

Horror Vacui wrote on Oct 28th, 2017, 1:43pm:
Rakesh wrote on Oct 23rd, 2017, 1:36pm:
Can you explain this "If you put the chopper in the loop than the chopping frequency is amplified by the open loop amplifier, so chopping might work at a higher frequency".


No, because I did not formulated it well, and it is not true Smiley . As the picture in  jdp's next post explains it the chopping frequency can be higher if the chopper is outside the loop. Note, that you have to have a filter which removes the upconverted flicker noise and offset.  So the system bandwidth is always smaller than the chopping bandwidth. The higher is your chopping frequency the harder to filter it out, unless you do not use a switched cap notch filter which puts the the notch exactly at the chopping frequency, but if I remember it good it will introduce some phase shift creating some stability problems to solve. Such a filter should be in your loop, otherwise the low frequency "noise" will appear again at the input of your amplifier.

I guess the main reason why I have bad feelings with chopping outside the loop is because in that case the core amplifier's gain is reduced by its offset, which is one of the things you wanted to avoid. If the chopping is outside of the loop, only the resistor mismatch will be removed.


Hi, Horror,  
 
   I guess all the offeset & low frequency noise, referring back to input, shall be chopped, in outter loop chopping?

-Frank
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Re: Chopping inside or outside the feedback loop?
Reply #8 - Nov 17th, 2017, 12:39am
 
jdp wrote on Oct 28th, 2017, 12:10am:
Hi. Thanks for your effort in answering the query till now...
Below is a plot of the open-loop Vs. closed-loop gain of the opamp.

Can it be said that in the Chopper-config-1, the chopped signal faces the Closed-loop response; while in config-2, the chopped signal faces the Open-loop gain of the op-amp?
Then can it be said that the chopping freq can be (favorably) more when the chopped signal passes through the larger BW of the closed loop case (Config-2)?


Hi, Jdp,

  I might be wrong, but I think you may end up with higher glitch or residue with inner loop chopper, as in non-overlapping time, the opamp is in open-loop.

-Frank
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Horror Vacui
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Re: Chopping inside or outside the feedback loop?
Reply #9 - Nov 19th, 2017, 10:28am
 
Quote:
Hi, Horror,  
 
   I guess all the offeset & low frequency noise, referring back to input, shall be chopped, in outter loop chopping?

-Frank


Yeah, looking from the outer world it is removed. What I was thought about is that the loop gain is degraded by the offset of the core amplifier. Thinking about it again it is always the case, even if the chopper is inside the loop.
An advantage of the in-the-loop-chopping is that the harmonics are reduced by the finite bandwidth of core amplifier and the loop.
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Horror Vacui
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Re: Chopping inside or outside the feedback loop?
Reply #10 - Nov 19th, 2017, 10:33am
 
jdp wrote on Oct 28th, 2017, 12:10am:
Can it be said that in the Chopper-config-1, the chopped signal faces the Closed-loop response; while in config-2, the chopped signal faces the Open-loop gain of the op-amp?
Then can it be said that the chopping freq can be (favorably) more when the chopped signal passes through the larger BW of the closed loop case (Config-2)?


The higher the chopping frequency, the higher your residual offset will be. This might set your upper limit of the chopper frequency, not the amplifier bandwidth. Check this first.
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Horror Vacui
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Re: Chopping inside or outside the feedback loop?
Reply #11 - Nov 19th, 2017, 10:37am
 
Frank_Heart wrote on Nov 17th, 2017, 12:39am:
  I might be wrong, but I think you may end up with higher glitch or residue with inner loop chopper, as in non-overlapping time, the opamp is in open-loop.


Why do you think this? What is your intuition here?
The non-overlapping time is very short, it is 3-4 orders of magnitude shorter than chopper period. Assuming that the amplifier bandwidth is about the same as the chopping frequency, the amplifier will not be able to track this change, or with other words, it will have high attenuation compared to the signal at chopping frequency.
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Frank_Heart
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Re: Chopping inside or outside the feedback loop?
Reply #12 - Nov 19th, 2017, 9:34pm
 
Hi, Horror,

  I got your points and it might be too short for opamp to response during tnov.  

  I might rephase my statement that talking about the residue due to tnov, closed-loop is better.

Regards.
Feng
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Re: Chopping inside or outside the feedback loop?
Reply #13 - Nov 19th, 2017, 9:50pm
 
Hi, Everyone,

  Assuming input is 0, and no mismatch on Rf&Rg, and assuming opamp offset = Vos.

  So in ideal (amplifier gain is very high around chopping frequency)  :

  Vout_peak = Vos * Rf/Rg  (config-1)
  Vout_peak = Vos * (Rf + Rg)/Rg (config-2)
 
  config-2 needs higher LPF filtering, is it true?

Thanks
Frank
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