help! urgent question

[jolia]

hi all,
should the bandwidth of amplifier's common-mode feedback loop be as large as
the amplifier's differential bandwidth? if not,are there any negative effect in the sc circuit? does it affect the amplifier's settling time or the system's THD?

 

[pfd001]

the bandwidth of the cmfb should be as large as that of the differential opamp. In sc circuit, the common mode voltage also need to set. the bandwidth of the cmfb will affect the setting time

 

[jolia]

the bandwidth of the cmfb should be as large as that of the differential opamp. In sc circuit, the common mode voltage also need to set. the bandwidth of the cmfb will affect the setting time

thank you pfd001, I designed one amplifier in which the bandwidth of the cmfb isn't as large as that of differential opamp, simulate it in sc circuit and the differential results (outp-outn)is very good,in other words it settles very well.but only see the single end outp or outn, it seems that it does not settle,and the both single ends (outp and outn)changes the same way.I have also found that the thd of this sc circiut isn't good ,only80dB,my target is 120dB

 

[sethtalk]

by my own experience, BW(cmfb)>=1.5BW(diff) is
more safe due to variation, you can ref to
"Analysis of switched-capacitor common-mode feedback circuit,Ojas Choksi, 2003 "
for qualitative and quantitative analysis

 

[jolia]

thank you sethtalk,I 'll read this paper

 

[jolia]

I have read some paper ,if the cmfb op's bandwidth and slew rate isn't large enough it can affect signal's distortion