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Design >> Analog Design >> How to size the cap of a sc cmfb?
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Message started by didibabawu on Jun 13^th, 2007, 4:59am

Title: How to size the cap of a sc cmfb?
Post by didibabawu on Jun 13^th, 2007, 4:59am

I have read something about the sc cmfb in the book--'analog integrated circuit design'. there are four cap in the circuit. two of them are bigger and two smaller.
It says that the value of the smaller might be between 1/4~1/10 of the biger. I don't know why.Can anyone

Title: Re: How to size the cap of a sc cmfb?
Post by tony_taoyh on Jun 14^th, 2007, 5:54pm

I think, the big cap acts like a level shifter, and the smaller one is used to maintain the voltage in the "level shifter".

The reason to use small capacitor is to reduce the load for the differential mode.

hope this help you.

Title: Re: How to size the cap of a sc cmfb?
Post by darrenxyz on Jun 15^th, 2007, 12:05am

you can ref to the paper
"analysis of switched capacitor common mode feedback circuit" Ojas Choksi

it include quantitative and qualitative analysis of sc-cmfb circuit,
i think the paper is pratical in design process,

Title: Re: How to size the cap of a sc cmfb?
Post by Berti on Jun 15^th, 2007, 1:01am

The ratio of the two capacitors has an influence on the time required for
the cmfb to reach steady-state. The size of the capacitor connected to the
amplifier on the other hand influences the small-signal bandwidth of the cmfb.

But I also recommend to read the paper from Choksi. It includes a really
good analysis of sc-cmfb circuits.

Cheers

Title: Re: How to size the cap of a sc cmfb?
Post by imd1 on Jun 15^th, 2007, 1:36am

The paper by Choksi is good, but I think it does nto talk about the PSRR of the S-C CMFB.

As for the sizing, it's not complicated. One capacitor recharges the CMFB capacitor which is always connected to the output, so that it does level-shifting. This recharging capacitor can be as small as you'd like, if the start-up time is not an issue. You can make it larger to improve the start-up time, but then you are increasing the loading of the main amplifier (the CMFB adds the sum of the two caps to the capacitive load at the output). I have used equal capacitors, for example, and sized the sum of them to be small with respect to the capacitance at the output.

Title: Re: How to size the cap of a sc cmfb?
Post by Shano on Jun 15^th, 2007, 3:16am

I just think about this question recently.

In the paper"Analysis of Switched-Capacitor Common-Mode
Feedback Circuit", it mentions that the GBW of common
signal loop should be near the GBW of differential signal loop to reduce the
distortion. When C1+C2 increase, the GBW of common signal loop increases
and the GBW of differential signal loop decreases. (Figure 7 of the paper).
C1 is the switched-capacitor and C2 is not switched.

And in Martin book, the C1 would be 1/4 to 1/10 of C2 for loading balance in two
phases.

However, I still have some doubts.
The return ratio in that paper is derived in one of the two phases.
In that paper, it is phase2.
So, the switched capacitor effect is not counted in.
I think the return ratio in that paper would be deviated from true situation.
Is it right?

Shano