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Design >> Analog Design >> chopping frequency and bandgap bandwidth selection issue
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Message started by trashbox on Oct 7^th, 2008, 1:16am

Title: chopping frequency and bandgap bandwidth selection issue
Post by trashbox on Oct 7^th, 2008, 1:16am

Hi all,
I'm designing a chopping-op based bandgap such as attachment and I encounter an issue about the selection of chopping frequency and loop bandwidth:

In attachment, bandgap itself's Loop 1 can suppress the high frequency parts if its bandwidth (open loop unity-gain bandwidth) is lower than chopping frequency. But it will result in bad respondence of chopping signal.

The other way is use Node 1 to suppress high frequency parts with large cap. It seems better.

Which way is better? How to select chopping frequency and bandgap bandwidth? Thanks!

Title: Re: chopping frequency and bandgap bandwidth selection issue
Post by vivkr on Oct 7^th, 2008, 6:31am

Hi,

You already have the answer to the question. You need to use a large cap at node 1.

Vivek

Title: Re: chopping frequency and bandgap bandwidth selection issue
Post by thechopper on Oct 7^th, 2008, 6:43am

Hi,

Please keep in mind when you chop a system, in order to get accurate gains you need the system's BW to be at least 5-6x larger than the chopping frequency. Otherwise, BW limitation will cause modulated signal attenuations that will show up as gain reductions. Everything outside the chopper can (and needs to) be band limited in order to kill the ripple resulting from the chopper operation.
As vivek suggested, large cap at node 1 is the way out (or decrease the chopping frequency to at least 1/5 of the loop BW). That is your post-chopper filtering means.

Regards
Tosei

Title: Re: chopping frequency and bandgap bandwidth selection issue
Post by trashbox on Oct 8^th, 2008, 10:19pm

Hi Vivek and Tosei !
Thanks for your reply.

Regards,
trashbox