[sdiy] CV input op-amp circuit

Brian Willoughby brianw at audiobanshee.com
Sat Dec 5 22:50:57 CET 2020

On Dec 5, 2020, at 13:42, Mike Beauchamp <list at mikebeauchamp.com> wrote:
> On 12/5/20 4:01 PM, Brian Willoughby wrote:
>> Even with all of that potential complexity taken care of, there's still the possibility that hitting a chip with all it can take will affect sensitive peripherals like the ADC. Personally, I prefer off-chip ADC, even going to the extent of placing the stand-alone ADC chip near the signals being read - such as pots and faders - so that ground and voltage references are more controlled than they might be near a digital processor switching at tens or hundreds of megaHertz.
>> Brian
> That sounds very smart Brian. I've had good luck using the internal ADC on some Teensy 3.2's but only when averaging many samples, etc.

After lots of problems with noise related to pots and faders, I decided to stick to dedicated ADC chips. After making that decision, I started to notice that lots of modern synths do the same. There's invariably a separate PCB for the front panel controls like pots and faders, and you'll probably notice that many of them have a dedicated 8-channel (or more) ADC, and then use a serial interface from the front panel PCB to the main processor. Any ground loops or other ground / reference issues are eliminated because the analog voltages are all local to a single board, where the only digital component is the ADC.

> Regarding protecting ADC inputs, is anyone using external BAT54S schottkey diodes? It seems like it might be easier to slap these in rather than rely on the internal uC's diodes and the issues youve raised of sharing that current if multiple pins are overloaded etc.

That sounds like a good idea. I briefly tried using external clamping diodes for a Euro switch panel, but something didn't work out and I ended up not populating those parts.

One thing you might have to look out for is that clamping activity will still cause spikes on the supply rails.


> Mike

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