[sdiy] CV input op-amp circuit

Brian Willoughby brianw at audiobanshee.com
Thu Dec 10 04:18:24 CET 2020

On Dec 7, 2020, at 10:21, Ingo Debus <igg.debus at gmail.com> wrote:
> Am 07.12.2020 um 05:56 schrieb Brian Willoughby <brianw at audiobanshee.com>:
>> All 24-bit ADC chips use delta-sigma conversion, which makes it impossible to place an analog multiplexer in front of the ADC to expand the channel count. Since ∆Σ functions on the difference between each sample, you can't swap in an unrelated channel without getting erroneous conversions.
> Sure about that? I have used a 16 bit Delta-Sigma ADC with an analog multiplexer in front of it. Worked great.
> Of course the multiplexer does not switch with every new sample for the Delta-Sigma algorithm (this would be way too fast).

I've read data sheets which discuss running several samples to prime the delta-sigma engine, and also mentioning that the first few samples after a channel change must be thrown out.

Instead of saying "impossible" without qualification, I should have clarified that it's not possible to run at the maximum sample rate without losing a few intermediate samples when changing channels.

>> ADC chips that have built-in multiplexers and/or are compatible with external multiplexers are limited to 8-bit, 10-bit, 12-bit, or 16-bit at the most. I don't think I've seen anything with more than 16 bits of accuracy, and far more often 12 bits is a practical limit.
> Look here:
> https://www.ti.com/product/ADS122C04
> 24 bits, Delta-Sigma, multiplexer built in.

The ADS122C04 only runs at 1 kHz, not 1 MHz, and it mentions oversampling. It's entirely possible that every one sample that comes out of the A/D represents a group of samples taken at the oversampling rate. In fact, the chip may automatically clear out the delta between samples, so that changing the mux would not mix the signals.

In any case, I'd be very careful about using delta-sigma for multiplexed channels, unless high sample rates are not required.


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