The Designer's Guide Community
Forum
Welcome, Guest. Please Login or Register. Please follow the Forum guidelines. Oct 23rd, 2017, 3:45pm
  HomeHelpSearchLoginRegisterPM to admin  
 
Pages: 1
Send Topic Print
2nd order SDM followed by sinc2 decimation filter? (Read 400 times)
neoflash
Community Fellow
*****
Offline

Mixed-Signal
Designer

Posts: 373

2nd order SDM followed by sinc2 decimation filter?
Aug 04th, 2017, 11:36am
 
This attached diagram shows a sensing scheme that uses 2nd order sigma delta modulator followed by a second order sinc decimation filter.

In this configuration, the out of band quantization noise will be flat instead of rolling off in higher frequency.

In other words, the post filter noise spectrum will be approximately white and quite some noise will alias back.

I think it should be much easier to use 3rd order sinc filter. However, this paper made a different choice made me think that maybe this wide band noise is not a problem when OSR is high enough?

Regards,
Neo
Back to top
 

sinc2.png
View Profile   IP Logged
Berti-2
Junior Member
**
Offline



Posts: 10

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #1 - Aug 7th, 2017, 4:51am
 
The frequency-response is sinc is sin(x)/x-like. For large OSR, the anti-aliasing filtering therefore becomes more efficient. Those filters are typically implemented as CIC filters.

I guess that's the reason why a second order filter is sufficient in this case.
(Your assumption that the quantization noise will be flat is consequently not true).

Cheers
Back to top
 
 
View Profile   IP Logged
neoflash
Community Fellow
*****
Offline

Mixed-Signal
Designer

Posts: 373

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #2 - Aug 7th, 2017, 3:28pm
 
The noise shaped of the 2nd order modulator will be 40dB/dec.

It then appears to me that a sinc-2 filter will not be sufficient to suppress the shaped noise.

Berti-2 wrote on Aug 7th, 2017, 4:51am:
The frequency-response is sinc is sin(x)/x-like. For large OSR, the anti-aliasing filtering therefore becomes more efficient. Those filters are typically implemented as CIC filters.

I guess that's the reason why a second order filter is sufficient in this case.
(Your assumption that the quantization noise will be flat is consequently not true).

Cheers

Back to top
 
 
View Profile   IP Logged
Berti-2
Junior Member
**
Offline



Posts: 10

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #3 - Aug 8th, 2017, 4:03am
 
If the OSR is large enough (>50), I would expect a sinc-2 filter being sufficient (because of the sin(x)/x characteristic, as I wrote in the previous post).

But you can write a simple Matlab script to check whether the decimation filtering is sufficient for a given OSR ... if you don't believe me.  ;)
Back to top
 
 
View Profile   IP Logged
neoflash
Community Fellow
*****
Offline

Mixed-Signal
Designer

Posts: 373

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #4 - Aug 11th, 2017, 9:17pm
 
I believe you.

But that approach of high osr doesn't seem to be an economic choice.

A sinc3 with much lower OSR will do the job better, isn't it a lower cost way?

Berti-2 wrote on Aug 8th, 2017, 4:03am:
If the OSR is large enough (>50), I would expect a sinc-2 filter being sufficient (because of the sin(x)/x characteristic, as I wrote in the previous post).

But you can write a simple Matlab script to check whether the decimation filtering is sufficient for a given OSR ... if you don't believe me. ;)

Back to top
 
 
View Profile   IP Logged
nrk1
Community Member
***
Offline



Posts: 79

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #5 - Aug 15th, 2017, 3:11am
 
Neoflash, Can you share the reference?

Back to top
 
 
View Profile   IP Logged
Berti-2
Junior Member
**
Offline



Posts: 10

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #6 - Aug 17th, 2017, 4:25am
 
Hi neon-flash,

You need to consider the combination of analog modulator and decimation filter.
The chosen OSR is often dictated by the analog part (modulator) and not the decimation filter.
Back to top
 
 
View Profile   IP Logged
neoflash
Community Fellow
*****
Offline

Mixed-Signal
Designer

Posts: 373

Re: 2nd order SDM followed by sinc2 decimation filter?
Reply #7 - Aug 17th, 2017, 4:19pm
 
nrk1 wrote on Aug 15th, 2017, 3:11am:
Neoflash, Can you share the reference?



It is this paper. ISSCC-2017

9.7 A 6.9mW 120fps 2850 capacitive touch sensor with 41.7dB SNR for 1mm stylus using current-driven ΔΣ ADCs
Hyunseok Hwang ; Hyeyeon Lee ; Hongchae Kim ; Youngcheol Chae
Publication Year: 2017, Page(s):170      - 171
Back to top
 
 
View Profile   IP Logged
Pages: 1
Send Topic Print
Trouble viewing this site? Copyright © 2002-2014 Designer's Guide Consulting. 'Designer's Guide' is a registered trademark of Designer's Guide LLC. All rights reserved.

Our colleges are not as safe as they seem. Sexual assault is pervasive and the treatment of the victim by the adminstration is often as damaging as the assault: Campus Survivors, Campus Survivors Forum.

Some of our other sites that you might find useful: QuantiPhy.