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How to reduce 1/f noise in BandGap (Read 22693 times)
ic_engr
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How to reduce 1/f noise in BandGap
Apr 18th, 2011, 5:37am
 
Hello,

I have a design of BandGap circuit based on Banba's paper. I am interested in low noise from 0.1Hz-20Hz. Presently the noise is around 30uVrms across 0.1Hz-30Hz and most of it is 1/f noise.
I am wondering if I can make use of chopper stabilizing technique to reduce this noise since this is mostly 1/f ?

Any suggestions ?

Regards,

ic_engr
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raja.cedt
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Re: How to reduce 1/f noise in BandGap
Reply #1 - Apr 18th, 2011, 9:50am
 
hi,
theoretically you can do it but its gives some other problems like clock glitches on bias line through some Cgd or other means, but before doing you can verify the following things.

Opamp and current mirror are main noise sources. So estimate how much current mirror is generating by replacing opamp with an ideal VCVS, if it contributing more then try optimize like by increasing the device size or introducing degeneration. Run separate noise simulation for opamp and try to optimize.

Thanks.
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Rakesh
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Re: How to reduce 1/f noise in BandGap
Reply #2 - Apr 18th, 2011, 10:38am
 
Hi,
  I think if u r going to degenerate with resistors the noise is going to be high. Its better to increase both the length  and width of the device, so that the flicker noise comes down.
Correct me if i am wrong.
Averging will reduce all the high frequency stuff. Flicker noise is maximum at low frequency.
Can some one intutively explain how increasing the size will reduce flicker noise.
Rakesh
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loose-electron
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Re: How to reduce 1/f noise in BandGap
Reply #3 - Apr 18th, 2011, 11:36am
 
Large W and L will drop the flicker noise down,. but not elinminate it.

Intuitive explanation? - A larger portion of the current thru the transistor passes thru the silicon away from edges of the transistors. The catch-release of the electron/hole is less when not along the edge of a junction.

Chopper methods can be used to push flicker to a HF and get rid of (reduce!) mismatch effects, and then  you filter the DC output and get rid of the HF content of the chopper and the up converted flicker noise.

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raja.cedt
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Re: How to reduce 1/f noise in BandGap
Reply #4 - Apr 18th, 2011, 11:51am
 
hi lose electron,
do have any reference/document for BGR with chopper stabilization?

Thanks.
Raj.
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Rakesh
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Re: How to reduce 1/f noise in BandGap
Reply #5 - Apr 18th, 2011, 2:02pm
 
Thanks jerry,
                Please let me know some good material on the chopper stabilization

rakesh
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Lex
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Re: How to reduce 1/f noise in BandGap
Reply #6 - Apr 19th, 2011, 12:10am
 
Use google. There are so many papers with bandgap/choppers out there.
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ic_engr
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Re: How to reduce 1/f noise in BandGap
Reply #7 - Apr 19th, 2011, 8:33am
 
My BW is from 0.1Hz to 20Hz.
Before I posted, I had already checked that even with replacing the opamp with ideal opamp (AHDL), the total noise was dominated by the 1/f noise of the PMOS current sources feeding into the resistors of the core bandgap (e.g in Banba's circuit). Would chooping this help ?


ic_engr
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rfcooltools.com
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Re: How to reduce 1/f noise in BandGap
Reply #8 - Apr 19th, 2011, 5:42pm
 
ic_engr,

The 1/f likely is probably coming from the fact that there are two pfets that feed the bandgap core where the 1/f noise difference (not coherent) is multiplied by the DC gain of the opamp with that said I would try to make a common device (coherent) be a current mirror then split the current with either  resistors or resistors and two cascodes.  It should improve the 1/f, but will likely have other which you will need to resolve.   just a suggestion.

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Re: How to reduce 1/f noise in BandGap
Reply #9 - Apr 20th, 2011, 11:18am
 
A few things.
Increasing the width and length of the MOS will help, but not as much as having leaving the length short and using resistance degeneration for the same amount of Vgs + IR voltage drop that you use. Since you are using Bamba I assume you are already short of headroom so the IR degeneration will use it more efficiently.

... unless your resistors have 1/f! Many of them do.

You can chop the opamp easy enough, but I think your bigger problem will be chopping the mirror. The load resistor will see zero (or 2x) current for short periods of time. Maybe you will be able to filter this out....

You might look into some switched cap bandgaps. Their output can be as small as you want.

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Re: How to reduce 1/f noise in BandGap
Reply #10 - Apr 20th, 2011, 11:30am
 
Lex wrote on Apr 19th, 2011, 12:10am:
Use google. There are so many papers with bandgap/choppers out there.

For those who don't know, probably because you are too old to know better, google scholar is an awesome tool:
http://scholar.google.com/advanced_scholar_search?hl=en&as_sdt=8000000000

Glancing briefly, I didn't see any current mode solutions. I do remember one presented at ISSCC although I'm not sure it was a 1/f solution... I'm guessing it was presented 8-10 years ago but I've been to so many I don't remember the years.

rg
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ic_engr
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Re: How to reduce 1/f noise in BandGap
Reply #11 - Apr 20th, 2011, 1:04pm
 
I had already tried the larger widths, an increase of 4X still does not reduce the noise due to 1/f being higher in the process.

The opamp transistors are only contributing 25%. The major noise contribution is from the PMOS transistors feeding into the Ressitors of Banba's core BG circuit.

Any help would be appreciated.

ic_enger
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RobG
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Re: How to reduce 1/f noise in BandGap
Reply #12 - Apr 20th, 2011, 1:26pm
 
Larger widths by themselves do not help because the gm scales up the same amount that the noise goes down. This is why you degenerate with resistance or make the device longer as you go wider. You want to lower the gm so that the flicker voltage at the gate produces less current at the drain.

Also, in general, the more current you burn the less noise you will get for the same voltage at the gates because (among other reasons) the devices are larger. If you burn a lot of current in the Bamba core and then divide it down for the leg that biases the resistor I bet you will have less noise.
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Re: How to reduce 1/f noise in BandGap
Reply #13 - Apr 21st, 2011, 12:31am
 
loose-electron wrote on Apr 18th, 2011, 11:36am:
Intuitive explanation? - A larger portion of the current thru the transistor passes thru the silicon away from edges of the transistors. The catch-release of the electron/hole is less when not along the edge of a junction.



Jerry,

That doesn't sound quite correct to me. If I recall, 1/f noise in MOSFETs is due to carrier trapping at the Si-SiO2 interface. That's where most of the traps are, not so much at the P-N junctions created at the S/D implants. The surface-dominated behavior of 1/f noise is one of the reasons why bulk devices such as JFETs or BJTs show very little 1/f, which was also true for PFETs in older technologies using buried channels.

A larger gate area will lead to an ensemble averaging of the trapping behavior. In other terms, if you are sending the same amount of current through a larger gate, then the carriers are likely to get stuck in many different, independent traps on their way home. For very tiny devices, you may be unlucky enough to get one single trap which looks rather nasty. I think there's an illustration in Tsividis' book.

Regards,
Vivek
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loose-electron
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Re: How to reduce 1/f noise in BandGap
Reply #14 - May 2nd, 2011, 1:11pm
 
vivkr wrote on Apr 21st, 2011, 12:31am:
loose-electron wrote on Apr 18th, 2011, 11:36am:
Intuitive explanation? - A larger portion of the current thru the transistor passes thru the silicon away from edges of the transistors. The catch-release of the electron/hole is less when not along the edge of a junction.



Jerry,

That doesn't sound quite correct to me. If I recall, 1/f noise in MOSFETs is due to carrier trapping at the Si-SiO2 interface. That's where most of the traps are, not so much at the P-N junctions created at the S/D implants. The surface-dominated behavior of 1/f noise is one of the reasons why bulk devices such as JFETs or BJTs show very little 1/f, which was also true for PFETs in older technologies using buried channels.

A larger gate area will lead to an ensemble averaging of the trapping behavior. In other terms, if you are sending the same amount of current through a larger gate, then the carriers are likely to get stuck in many different, independent traps on their way home. For very tiny devices, you may be unlucky enough to get one single trap which looks rather nasty. I think there's an illustration in Tsividis' book.

Regards,
Vivek


You just said the same thing that I said, with a few more details.  :D
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Jerry Twomey
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