cmfb in switched-capacitor amp?

[ling_san]

cmfb charge injection switched capacitor

I'm designing a sc circuit used for MDAC in a 40M/s pipeline ADC

I use a two stage amp ,telescope for the first and common source for the second


Should I use two cmfb loop ? one for the first stage ,and the other for the second?

which kind of cmfb should I use?

I want to use a swiched-capacitor cmfb ,how much frequency of the clock should I

choose? 40M,or more?

should I use other kinds of cmfb?


thanks a lot

 

[sunking]

only one cmfb is ok. either first stage or second stage is good.
the clock of cmfb 40MHz is ok

 

[ling_san]

Thanks

Well,I tried another classical CMFB using differential pair yesterday, It worked well.


but the switched-capacitor CMFB just didn't work.

What should I be attention to ? how much capacitor could be appropriate?

 

[ocarnu]

It depends. You can use a value for the pair which connects ou1-ou2 with Vctrl (say hundreds of fF) and a value 3-5 time smaller for the switched pair. For very small values you will have problems with the charge injection from switches.
Don't forget to simulate enough time (100 clock cycles) and follow Vctrl.