About the relationship between pole/zero and settling time

[gdhp]

Hi all

I am designing a Sample hold opamp with cascode compensation. I have known the opamp has a pole and a zero very near. So in order to decrease the influence on settling time of doublet, i push this doublet far away from the loop BW(about 3*BW), And other high fre poles and zeroes are all largger than 3BW.

I thought if the doublet is out of the BW, it will not influence the settling time. But i found although the doulet is set as the above, they also have influence on the settling time and the settling damping.

Anyone has some suggestions about this problemns?

Thanks

 

[leo_o2]

You can't be help until you paste your schematic

 

[LvW]

Re: About the relationship between pole/zero and settling ti

I don´t understand the following part of your posting:

i push this doublet far away from the loop BW(about 3*BW), And other high fre poles and zeroes are all largger than 3BW.

Do you mean "open loop BW" or transit frequency or something else ?

 

[gdhp]

Hi LvW
the loop BW is as (BW_OPEN*Feedback), it is that all other high frequency pole/zero are all high than 3BW

Added after 17 minutes:

leo_o2
The schematic is full-diff two-stage opamp,with first stage is foldedcascode, the second is common souce stage.
Thanks

 

[LvW]

Re: About the relationship between pole/zero and settling ti

Hi LvW
the loop BW is as (BW_OPEN*Feedback), it is that all other high frequency pole/zero are all high than 3BW
Thanks

OK, so I understand that your doublet is beyond the "classical" BW of the open loop but still within the closed loop BW - and that´s the reason for the influence on settling behaviour.It must be outside the closed loop BW if its influence should be negligible.
Is my assumption concerning closed loop correct ?
Regards