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Design >> Analog Design >> Offset compensation of buffers
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Message started by manodipan on Dec 1^st, 2009, 2:48am

Title: Offset compensation of buffers
Post by manodipan on Dec 1^st, 2009, 2:48am

Hi all,
I need to design a highly accurate buffer ...the requirement is very stiff so i need to compensate the offset of buffer to very low value....can you guys suggest some good materials on this? ....thanks

regards,
Manodipan

Title: Re: Offset compensation of buffers
Post by thechopper on Dec 3^rd, 2009, 7:01pm

Hi Manodipan,

Have you considered dynamic offset cancellation techniques for your app?

Best
Tosei

Title: Re: Offset compensation of buffers
Post by raja.cedt on Dec 3^rd, 2009, 8:21pm

hi mano,
tough means how much and at the same time please post your schematic...but periodic calibration could be help full in this regard..
Thanks,
rajasekhar.

Title: Re: Offset compensation of buffers
Post by manodipan on Dec 4^th, 2009, 1:30am

Hi ,
The buffer will be used alongwith the Bandgap which needs to be accurate by 0.005% in absolute terms....now for this reason buffer offset compensation is necessary....now i have seen materials on chopper compensation...what do you guys think is the best solution for this app?

Title: Re: Offset compensation of buffers
Post by thechopper on Dec 4^th, 2009, 3:09pm

If the accuracy spec for your bandgap includes the noise levels of your reference voltage, I would suggest going with a chopper compensation since there is essentially no baseband noise aliasing; vs autozeroing cancellation which heavily alias white noise.
On the other hand, for the chopper stabilization you will need some post-filtering means.

Best
Tosei

Title: Re: Offset compensation of buffers
Post by manodipan on Dec 7^th, 2009, 4:49am

Hi Tosei,
I have a doubt in using Chopper compensation for the loop amplifier.....now general scheme works for differential i/p-o/p scheme...but in our case it is single o/p ,so people apply demodulation scheme within the amplifier,i don't understand the operation of that.....another doubt is that how to check whether the amplifier is offset compensated,can you tell me the simulation setup...

The paper mentioned is "Micropower CMOS Temperature Sensor with Digital output" by Bakker and Huizsing,IEEE JSSC vol.31,No. 7,July 1996...

Thanks and regards,
Manodipan

Title: Re: Offset compensation of buffers
Post by thechopper on Dec 7^th, 2009, 6:04pm

Demodulating inside the opamp is a technique that has been used recently and successfully. It has two main advantages:

1) It allows for chopping SE amplifiers as you already suggested
2) It allows for chopping at high frequency, which usually cannot be done if the amplifier is chopped outside the feedback loop.

#1 works since demodulation is still done differential, prior to SE conversion. Although the second opamp stage is not included in the chopper - thus contributing with offset - its offset contribution is small enough such that it can be neglected assuming the chopped first stage gain is high enough. This is the case for most opamp designs. For example assume your AOL is 80dB = G1[dB] + G2[dB], where G1 is 60 and G2 is 20, and your closed loop is 20dB. If the second stage gain generates 1mv of offset, such offset (which is NOT chopped) is referred to the input by dividing it by 1000 (60dB). Thus the 2nd stage contributes with 1uv input offset. That will translate into 10uv output offset (20dB closed loop gain).
From this example you can see that generally speaking there is no need for chopping a second gain stage.

As for #2, just consider the demodulation is performed within the first stage, which is a [b]wideband[b] gain stage. Such wide BW allows for faster chopping compared to the case where the demodulation is performed outside the loop: in that case the BW is the closed loop BW which is ACL times smaller than the GBW of the opamp (ACL is closed loop gain).

In order to check if the amplifier is offset compensated, assuming you chop the amplifier, just add an ideal voltage source in series with one of the opamp inputs - but after the input modulation switches - and without applied signal.
By running the transient analysis you should see a triangular-like ripple (modulated offset) around 0v (or input CM in the case of a buffer). If that output signal is perfectly centered around 0v (or input CM) that means your amplifier is offset-cancelled.

Hope this helps
Tosei

Title: Re: Offset compensation of buffers
Post by Mayank on Dec 7^th, 2009, 8:39pm

Hi all,
Can you please suggest me some papers on these topics which provide a good explanation :--

1. Auto-Zeroing
2. Chopping
3. Demodulation inside the opamp for SE topologies.(heard of prev two, but this is new to me)

thanx,
mayank.

Title: Re: Offset compensation of buffers
Post by thechopper on Dec 8^th, 2009, 6:23pm

In the following link you can find one of the first works where demodulation is performed inside the loop:

http://www10.edacafe.com/book/phdThesis/

In particular chapters 5 & 6.

Also, from the same author (Sanduleanu) you can check:

"Low power- low voltage chopped trans conductance amplifier for noise and offset reduction" ESSCIR 1997 and

"Low power low voltage chopped amp with a new class AB output stage for mixed level apps " Proceedings of ProRISC Workshop on circuits, systems and signal processing.

Hope this helps
Tosei

Title: Re: Offset compensation of buffers
Post by Frank Wiedmann on Dec 9^th, 2009, 12:46am

A pretty comprehensive paper about autozeroing and chopping is http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=542410.

Title: Re: Offset compensation of buffers
Post by Mayank on Dec 9^th, 2009, 1:04am

Thanx Tosei & Frank,
The reading material you guys suggested is quite extensive and appropriate.

--
Mayank.