Don,

What is your source for this info on sigma-delta converters?

I am currently working on a project where I am using sigma-delta ADCs and
AFAIK, they are not cheap , but really high accuracy (although, I suppose
that making 16 or 24 bit R2R ladder ADCs would be more expensive). IIRC,
They work by having a capacitor attached to one input of a comparator. The
other input is the input to the ADC. If the voltage on the cap is lower
than the main input voltage, a short pulse is fed to the cap thru a
resistor to increase its voltage. If it is too high, the cap is allowed to
discharge for a short bit through a resistor. The pulses fed to the cap are
considered to be a fast bitstream,and their average over a thousand or so
bits is computed, giving the average value of the input.

So, the converter actually "samples" the input a thousand or more times per
actual code output. Usually the output rate is around 100Hz and the 1-bit
sampling rate is around 20-100kHz. So, if the input were to change suddenly
between output codes, it should be able to handle it at least within the
converter's error specs.

In fact, one of the unusual things about sigma-delta converters is that
they often do not require an input anti-aliasing filter (or a very minimal
one) because they sample MUCH faster than the output rate. If you try to
feed them a signal which is too fast (say, a 1 kHz signal when the output
rate is 60Hz) it will automatically be attenuated by the averaging of the
bitstream. You only need to ensure that the input doesn't have significant
frequency content comparable (in frequency) to half the fast 1-bit sampling
rate.

Sean

At 09:29 AM 2/5/00 -0500, you wrote:
>Phillipe -
>
>IIRC, the most audio ADC's now use inexpensive delta-sigma converters.
>These converters measure CHANGES in the input signal, and can only
>accomodate a small change each sample.  That means that if you have a
>sudden large change (as you might have when switching between two input
>channels), the converter may take quite a few samples before catching up
>to the change.  I don't know how many, but if you want to persue this
>approach, you'd better find out.

>Don
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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