[sdiy] Keyboards good for scrap / repurposing?

rsdio at audiobanshee.com rsdio at audiobanshee.com
Tue Feb 20 04:04:46 CET 2018

On Feb 19, 2018, at 3:18 AM, sleepy_dog at gmx.de wrote:
> Oh, and analog mux for reading pots, seems a bit noisy so far, but there's no metal enclosure and only perfboard with wild wires, no PCB with ground layer yet.

Make sure to ground the partial shields that nearly all pots have around them. This will go a long way towards keeping out unwanted noise.

An analog multiplexer is standard, sometimes even more than one. Some “multi-channel" ADC chips even have an analog mux inside, with only a single ADC circuit. Such combo ADC chips save on I/O pins because the ADC can internally drive the mux address.

For this (hybrid synth control panels), I recommend placing the ADC chip on the same board as the pots, then using serial digital I/O to connect from the pot board to the main CPU board. This way, any problems with ground offsets or power supply or reference voltages differing between boards will be moot, since the ADC will be operating from the same voltages as the pots. It requires fewer GPIO pins than parallel ADC interfaces.

You’ll do well to avoid the on-board ADC of the CPU itself, because those are notoriously noisy unless you can afford to shut down the CPU for every conversion (some chips actually have this feature, but they can’t really handle a lot of processing while also scanning pots frequently). That means you’ll want an external ADC chip anyway, which fits nicely with the advantages of placing the ADC chip directly on the pot board.

I’ve read papers from analog design masters who actually recommend converting analog signals to digital before transmitting them between boards - even when the final signal usage is analog. So many ground and noise problems can be avoided by going digital, at least for inter-board signal routing, that it makes up for the negative aspects of digital. In your case, the pot positions are going to be converted to digital anyway, so you might as well take advantage of the option to transmit these signals in the digital domain between boards. Note that I’m assuming a front panel board with mostly user interface that is separate from a main board with CPU and sound generation circuits.

Brian Willoughby
Sound Consulting

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