The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines.
Nov 27th, 2020, 9:39am
Pages: 1
Send Topic Print
use chopping in a SC sigma delta modulator (Read 2216 times)
xlowen
New Member
*
Offline



Posts: 9
Delft, Netherlands
use chopping in a SC sigma delta modulator
Nov 18th, 2016, 7:40am
 
Dear All,

From the measurement results (attached Fig) of a JSSC paper "A CMOS 110-dB@40-kS/s Programmable-Gain Chopper-Stabilized Third-Order 2-1 Cascade Sigma-Delta Modulator for Low-Power High-Linearity Automotive Sensor ASICs", it seems that choppping at lower frequency (Fs/2), some kind of noise folding happens. I built a similar design (2nd FF SC SDM with chopping used in the 1st stage) and saw similar results in my transistor-level simulation (no mismatch is added into simulation!). In many papers, people just choose Fs/2 as chopping frequency but never explain why. Could someone provide some insight on this issue?

Thanks!
Back to top
 

chopping_in_SDM.jpg
View Profile   IP Logged
deba
Community Member
***
Offline



Posts: 84

Re: use chopping in a SC sigma delta modulator
Reply #1 - Nov 18th, 2016, 8:11am
 
Due to chopping at fx, the shaped quantization is 2fx will fold inband and corrupts the inband noise floor. The noise is minimum at f=0,fs,2fs etc. Thus, if chopping is done at fs/2, the minimum corruption is obtained.
Back to top
 
 
View Profile   IP Logged
xlowen
New Member
*
Offline



Posts: 9
Delft, Netherlands
Re: use chopping in a SC sigma delta modulator
Reply #2 - Nov 18th, 2016, 8:24am
 
Hi deba,

This is true for a continuous time SDM, however, for a switch cap design, if you chop at proper time (eg. middle of sampling phase), I didn't see any direct correlation between chopping action and feedback signal. If you look at this paper"On Chopper Effects in Discrete-Time ΣΔ Modulators", the author concluded that noise folding happens due to unmatched parasitic caps around OTA, but in my simulation, there is no mismatch but I still see noise folding...

Thanks,
Long
Back to top
 
 
View Profile   IP Logged
deba
Community Member
***
Offline



Posts: 84

Re: use chopping in a SC sigma delta modulator
Reply #3 - Nov 19th, 2016, 7:38pm
 
Hi,

I didn't realize its a SC DSM. I just looked at the PSD plots.

Yes for a SC DSM, if the chopping is done at an appropriate time as you had mentioned one should not see this noise foldover issue. But there are voltage dependent caps on the input differential pair of the opamp. I wonder whether that mismatch can result in a noise foldover.

Thanks
Back to top
 
 
View Profile   IP Logged
Pages: 1
Send Topic Print
Copyright 2002-2020 Designer’s Guide Consulting, Inc. Designer’s Guide® is a registered trademark of Designer’s Guide Consulting, Inc. All rights reserved. Send comments or questions to editor@designers-guide.org. Consider submitting a paper or model.