Strangest behavior with my AXO and MCP3208

#1

Hello all, I'm just posting this here even if I believe my issue is so singular it might not resonate with anybody.

So I have an AXO enclosed with two screens, two encoders and 16 knobs/faders.
Those 16 potentiometers are hooked up to the AXO using two MCP3208.
Now I seem to have trouble with one of the two MCP3208, the reading of the potentiometers are sometimes erratic and the values are either jumping a little by two units or even going quickly to 0 when turned to 64.
I noticed this behavior while rehearsing in a studio with a lot of other midi gear connected to the AXO.

So I brought it back to my house to check it up and there, everything was working 100% fine, no odd readings.

Then I thought something in my whole setup was messing with the AXO and brought it back to the studio in its full setup. The problem appeared again.
So I unplugged everything, connected the AXO solely on one computer (power supply too) on another table to check it. The problem was still there.
I swapped the two MCP3208 chips to no avail.

I then brought it again to my house to check it further. AGAIN, there is NO problem at my house.

It does not make sense that the problem appears in one location and not another, even while being only connected to a computer (the one I'm using in the studio is a laptop so I don't think there is any problem with the mains at the studio…

#2

I am by no means an expert, and I don't have any experience with ADCs, but whenever I am using a 4051/4067 multiplexer and my pots are going nuts, it's almost always a ground issue.

#3

Hey! Thanks for you answer!

I thought there was something going with the power or ground. Could it be possible that it impacts the two ADCs for a certain period of time, even after unplugging evrything from the main power and using the AXO solely with an external battery?
Because once the problem appears, it seems to stay for a while. But only when at this particular place, when I bring the Axo to my house (10mn away), the problem disappear.

Out of despair, I decided to add decoupling caps on every pots I have, I do not know if this will change anything…

It is really frustrating that I cannot reproduce the problem and that I can't know for sure this won't happen when i play live shows…

#4

From what my amateur self has heard on all the synth diy places: every IC needs a decoupling cap (two, if it's a bipolar powered IC like an op amp)

The other thing that I wanted to say was parasitic capacitance, although I know almost 0 about it.

I was making a module using an Arduino and hadn't set pull-up resistors for the gate input, so they were floating and giving me weird readings all over the place. I think it's because the module's parameters were dependent on the gate input, so I'm not sure that the capacitance was causing my pots to go nuts, but it's something else to consider.

If you haven't, maybe try asking about this on www.reddit.com/r/synthdiy . People there are very helpful with the electrical side of things.

#6

It could be a question of your design being susceptible to electric interference, and there's more of it in your studio than at home, for whatever reason.

I haven't seen the design of course so I can't comment (and I likely wouldn't spot it directly anyway), but decoupling caps on the lines to the potentiometers can be one solution. Or it could be something with the digital lines between the MCP3208's and the Axo.