[sdiy] Protection Device?

[O Gillet]

The CV input circuitry of pretty much all my modules look like this:
https://imgur.com/a/Xz5YN0X

The op-amp has R2R outputs and is single-supply, powered by 3.3V.

Benefits:

- The V- input of the op-amp is a summing point - you can attach there
as many CV inputs or pots as you want. It's very helpful in some
situations to have both a CV input and a pot to control a parameter!

- If you connect an external signal straight to an MCU ADC input, the
range of the CV will be necessarily equal to the range of the
microcontroller ADC input. It's probably OK if you use a +5V
microcontroller and want 0V to correspond to the minimum value of the
parameter, +5V to the maximum value of the parameter... But what if
you want the range of the parameter to be -5V to +5V ; or 0V to +8V?
Or what if your microcontroller is powered by +3.3V? The schematic I
have posted covers all these cases - you just change the resistors to
get different scale/offset values.

- The capacitor acts as a 1-pole low-pass filter which removes some of
the high frequencies in the CV signal - providing cleaner readouts.

- ADCs do not like being driven from sources with high ouptut
impedances. For example, the ADCs on the AVR want a source impedance
of 10k or lower - to rapidly charge the S&H capacitor which is part of
the ADC circuit. The output of the op-amp works as a very low
impedance source for the ADC - and will simultaneously leave you in
control of the input impedance of your module (100k is an implicit
standard for Euro modules). Standardizing all input impedances to 100k
is great - it allows consistant behaviour when using passive modules.

- Of course the op-amp input will never "see" extreme external
voltages, because you're not directly exposing any gate to the
external world - every input voltage goes through the 100k resistor.
If you connect +50V to the CV input, well, that's only 0.5mA flowing
through the input resistor (and 25mW dissipated by the resistor).

So yes, an op-amp might look a bit too much, but it kills a whole
flock of birds with one stone.