Teensy LC paired with multiple mux - PCB project.

C

chLarraby

New member
Hi all -

I recently put together a Teensy LC with 32-channels, using four 4051s wired up to 12 digital pins. It works, but I had a few questions about next steps that I wanted to ask the experts over here. Thanks for your time and consideration, in advance.

1. It looks like I could squeeze another mux in there pretty easily on dedicated digital pins, but I would be dipping into analog pins after that. Is there any crosstalk danger in using some of the analog-designated pins as digital switching for the muxes?

2. Secondly, this all seems like this would all run better if I went and got some PCBs printed. What I recently noticed, though, is that there are a number of open-source projects that already aim to do this kind of thing - and I'm curious if folks here had a standard favorite. It might not be necessary to reinvent the wheel, if there's already something pretty close. In the same token, if there isn't anything, would anyone here be interested, if I made it available via github, etc?

3. Finally, one thing I've noticed, looking at some other versions, is that they generally don't include an RC filter at the analog input of the Teensy. I was thinking that would be pretty standard issue for mux outputs, but maybe it's not necessary? I was hoping to hear some folks weight in on that issue, as well.

Thanks again!
 
If you had an RC filter at the analog input of the Teensy you'd have to wait about 8 or 9 time constants after changing the multiplexer before taking a reading to allow it to settle to 12 bit accuracy. You can have individual RC filters on each channel before the multiplexer without this problem however. In an ideal world each input to the multiplexer would have its own brick-wall anti-aliasing filter tailored to its sampling rate, leading to minimum noise as well as avoiding aliasing.

In practice is easier to sample fast enough so a simple RC filter is enough to reduce noise and aliasing to acceptible levels, and perform some averaging in the digital domain if required to further improve noise.
 
Ah, very interesting, thank you - so RC filters on the incoming pots before they get to the mux is an improvement?
 
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