can Switch Capacitor CMFB, redue the over all gain of a OTA

[Shaber_Mezbah]

Hi all,

I am designing a high speed pipe-lined DAC.

I am using a differential folded cascode OTA with S/W capacitor CMFB(common mode feedback) circuit for each bit cell. the AC-analysis is showing a 60bd gain. but transient analysis is showing some gain error. in this case, gain is droping by more than 20bd.

first i thought that it might be due to clock feed through. but, i tried for all the recovery techniques like dummy switch and well designed transmission gate.
but, i couldn't improve that gain..

:|:?:Can the S/W cap. CMFB circuit reduce the over all gain of the differential OTA at high frequency...??

 

[amitbhaiji]

Re: can Switch Capacitor CMFB, redue the over all gain of a

yes sc cmfb circuit can reduce the gain by loading the output. Your switching capacior you can consider as switch resistor (1/C*f) which can reduce the gain.

 

[wsy979]

Re: can Switch Capacitor CMFB, redue the over all gain of a

well, I have met same question
s/c cmfb need time to settle. while ac analysis perform at 0s, before cmfb settled.
I think you can make an ideal cmfb to replace s/c cmfb for ac analysis.

Btw: s/c cmfb can reduce the gain bandwidth, but not so remarkable

regards

 

[walkingsun]

Re: can Switch Capacitor CMFB, redue the over all gain of a

yes, the added capacitor is usually about 200fF.

 

[Btrend]

Re: can Switch Capacitor CMFB, redue the over all gain of a

1. SC CMFB should not affect ur DC gain of OTA
2. SC CMFB neet time to settle down to the correct voltage which will
bias the main OTA at the right bias point.
3. u had better check bias points of OTA, and compare it with SC CMFB & without SC CMFB. if ur result show 20dB degrade, then the bias point must be changed, cause the SC CMFB did not settle down to the correct voltage as in ur simulation of
OTA without SC CMFB.