Does a delta sigma modulator need an output filter? - diyAudio
Go Back   Home > Forums > Amplifiers > Class D

Class D Switching Power Amplifiers and Power D/A conversion

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 18th January 2010, 08:10 PM   #1
diyAudio Member
 
kipman725's Avatar
 
Join Date: Jun 2007
Location: London/N.lincs
Send a message via MSN to kipman725
Default Does a delta sigma modulator need an output filter?

Simple question before I cause some horrific RF pollution but due to the broadband nature of the noise from a delta-sigma ADC does an amplifier based on this modulation scheme require an output filter? if so can this filter be implemented digitaly on the bit-stream itself?
  Reply With Quote
Old 18th January 2010, 08:24 PM   #2
diyAudio Member
 
kipman725's Avatar
 
Join Date: Jun 2007
Location: London/N.lincs
Send a message via MSN to kipman725
This is for a subwoofer application (nice high voice coil inductance) and the switching fequancy is 2Mhz. After reading TI's documents it seems like a bad idea, however I think I can get away with very small inductors/capacitors as I only really have to start attenuating the output >100Khz looking at my output spectrum. I don't use inductors much however I guess I am looking for a low hystiris core and very high self resonance in this application.
  Reply With Quote
Old 18th January 2010, 08:48 PM   #3
diyAudio Member
 
theAnonymous1's Avatar
 
Join Date: Feb 2004
Location: Anonymityville
A subwoofer amp that switches at 2Mhz? Why?
  Reply With Quote
Old 18th January 2010, 08:53 PM   #4
Eva is offline Eva  Spain
diyAudio Member
 
Eva's Avatar
 
Join Date: Oct 2003
Location: Near the sea
Send a message via MSN to Eva
Note that the voice coil of a speaker is inductive but does not act as a good high frequency inductor due to capacitance, particularly when it has more than one layer or when the former is made of aluminium or any other conductive material. A lowpass filter is always a good idea, although in low power appliations it may be made with ferrite beads and low value capacitors.
__________________
I use to feel like the small child in The Emperor's New Clothes tale
  Reply With Quote
Old 18th January 2010, 10:11 PM   #5
diyAudio Member
 
kipman725's Avatar
 
Join Date: Jun 2007
Location: London/N.lincs
Send a message via MSN to kipman725
Quote:
Originally Posted by theAnonymous1 View Post
A subwoofer amp that switches at 2Mhz? Why?
Thats just the modulators operating frequency to get good SNR. I am going to be performing digital filtering on the bit-stream and the 2Mhz figure is just the maximum switching frequency which should only happen in the case of no input. As for why I just want to test some ideas out and they are getting too complex for spice to be much use.
  Reply With Quote
Old 18th January 2010, 11:22 PM   #6
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
I'm not clear on your topology - a delta-sigma modulator/DAC is a "digital filter" that has increased and radomized the switching times at high frequencies in order to reduce the audio frequency error components of the modulation process

you could choose other modulators or program your own to change the shape of the modulation "noise" frequency response, subject to some fundamental limits - if you are very good a advanced control theory including nonlinear oscillation/limit cycle analysis


if you are refering to ADC then you always have to have anti-alias filtering on the input - for audio this is usaully done with active fliters rather than trying to find good inductors
  Reply With Quote
Old 19th January 2010, 12:04 AM   #7
diyAudio Member
 
kipman725's Avatar
 
Join Date: Jun 2007
Location: London/N.lincs
Send a message via MSN to kipman725
The modulator is described as the ADC. The filter is for the switching output. My question pertains to whether by averaging the delta sigma waveform digitally I can reduce the high frequency components of the output waveform. This is while still feeding back the full 2Mhz sample rate bit stream. I think this will work but I haven't had chance to study discrete time systems yet, but my limited knowledge of digital filters suggests that they work in the same way as analogue and so this should work?

input -> 2mhz delta sigma modulator -> Average of N samples -> output stage
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Sigma Delta Bandwidth d1983 Class D 1 14th November 2007 03:17 AM
Sigma Delta ICs d1983 Class D 11 6th November 2007 08:23 PM
Delta-Sigma Class D contitek Class D 16 10th March 2005 01:10 PM
which clock used in delta-sigma modulator for cs43122? rlim Digital Source 1 16th July 2004 09:34 PM


New To Site? Need Help?

All times are GMT. The time now is 09:54 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2