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Sigma Delta Vs. SAR Converters

Case Incidents:

Sigma-Delta vs. SAR Converters

From: Jeff 
Subject: Re: sigma-delta vs. SAR converters in data acquisition / DSP system
To: Anatoly 

Date: 21 Dec, 2000

-Anatoly 

>If it is possible, answer please the following questions:
>
>1. What is understood under " group delay "?

Group delay == amount of time after physical signal appears on input channel 
until it appears on output channel.  Sigma-delta converters typically have 16 to 
32 sample delay, because of their built-in FIR digital filters.  SAR converters 
normally have only 1 sample delay, but require external anti-alias input filters 
and reconstruction output filters.  Sigma-delta converters do not require 
external filtering, or require only minimal 1-pole stage that is easy and 
inexpensive to implmement on the board.  Additionally, sigma-delta converters 
have advantages that onchip filters are very sharp (equivalent to 12-pole or 
more external filter) and have nearly ideal linear phase response.

Also, DSP processing can add to group delay.  For example, if real-time code 
running on DSP implements a filter or other buffer or frame-based algorithm, 
then signals can be further delayed.

Normally the group delay issue becomes significant in a control system, because 
it is sensitive to phase lag.  Most control systems perform real-time algorithm 
on "sample basis", rather than frame basis.  Most audio/speech systems are on 
frame basis.

>7. Realization of a range +10 volts  and opportunity of FIFO organization ?

+/-10V is the default input range for the A4D4 module.  I'm not sure about SD16, 
but normally it is not a problem to alter the input range by using either 
on-module jumpers, trimpot, or adding voltage divider circuit to input.  More 
information on the A4D4 and SD16 modules, and using them with the M62/67 
boards, is located at:

  ftp://ftp.signalogic.com/documentation/Modules/

Jeff Brower
DSP sw/hw engineer