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



>I never heard of a Sigma-Delta AtoD converter before. Can anyone tell me
how
>it works, and has anyone had experience with this new part?

I am using a software implemented Sigma Delta converter in a current project
(using a Z8 :-))


The SD converter is essentially a tracking integrator.

A comparator is used to compare the input voltage with the voltage on a
capacitor.
Once per clock cycle, if the capacitor voltage is lower than the input then
a positive drive signal is applied via a resistor to the capacitor. If the
capacitor voltage is too high a negative drive signal is applied. The drive
signal continues for the rest of the clock cycle EVEN IF THE COMPARATOR
CHANGES STATE BEFORE THE END OF THE CYCLE. The relative number of highs and
lows of the driving signal allow one to calculate the value of the input
relative to the reference.

Using a Z8 with an internal comparator this essentially allows a converter
to be made with a reference, one capacitor and 2 resistors. In practice I
add 2 opamp sections before this to provide voltage gain and buffering.
Still a cheap and accurate converter. I get about 10 bits of accuracy but
more can be had with longer conversion times.



     Russell McMahon
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