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Thread: What is a Sigma-Delta AtoD converter? [EE]
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face BY : Scott Dattalo email (remove spam text)



On Sat, 15 Apr 2000, Russell McMahon wrote:

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I was wondering when you were going to chime in Russel.

A pic version of what Russel describes can be found here:

http://www.dattalo.com/technical/software/software.html

I know one person that's trying to use this. I won't mention names, but let's
say that she's pulling her hair out to get it to work.

The point being, there are subtleties with this circuit that aren't immediately
obvious. For one, the circuit has a relatively low input impedance compared to
most A/D converters.  In addition, the output impedance of the signal being
measured is assumed to be low.  But a buffer amplifier like Russel mentions can
fix both of these problems. Depending on the source, you could add a 'large'
capacitor - say 10 times the integrating one - instead of a buffer at the input
of this poor person's sigma-delta converter and some what alleviate problems
associated with high impedance signals.

Another caveat is that the R's and the C need to be tightly coupled with the
loop time of the software. If the RC time constant is too large you run the
risk of not being able to have the pic charge or discharge the capacitor during
a pass through the algorithm. If the RC time constant is too small then every
iteration through the algorithm would cause the digital input to switch
states. The values on my web page for the R and C are 10k and 0.56uF were the
result of convenience - I'm not sure if they're optimum.

Scott

<Pine.LNX.4.21.0004150723320.23899-100000@tempest2.blackhat.net>

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