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
