[sdiy] Protection Device?

O Gillet ol.gillet at gmail.com
Tue Jun 19 23:48:55 CEST 2018

> The inputs must be clamped that range.

The circuit I have posted clamps all voltages below -8V to 3.275V, all
voltages above +8V to 0.025V, and is linear in-between. You can change
the resistor values to get other scale/offset ratios.

Unless you're building something like a development board, I don't see
a case for directly exposing MCU pins to the external world. What
you're sending to those MCU pins is very likely to need some
conditioning anyway (filtering, scale/offset) - so the idea is to use
a single-supply rail to rail op-amp powered by the same voltage as
your MCU to get the clamping "for free".
On Tue, Jun 19, 2018 at 11:40 PM Tim Ressel <timr at circuitabbey.com> wrote:
> the issue is this: the uP input cannot tolerate voltages much beyond its
> rails, so the max range is -0.3V to +3.6V (in my case). The inputs must
> be clamped that range.
> --timmers
> On 6/19/2018 2:22 PM, O Gillet wrote:
> > 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.
> > _______________________________________________
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> >
> --
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
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