[sdiy] CV input circuit

O Gillet ol.gillet at gmail.com
Mon May 14 18:57:15 CEST 2018


>  guess I'm having a bit of difficulty figuring out exactly what the
voltage will be at the MORPHADC and SPREADADC pins.

For this you have to know the input impedance of the MORPHADC pin.

I'm not a big fan of this kind of circuit. I prefer the way I do it (with a
rail to rail op-amp in inverting configuration): the op-amp has a low
output impedance, which is good for driving ADC inputs; the input impedance
can be set to 100k precisely, which works nicely with some passive modules;
you can do some low-pass filtering too. Costs an op-amp per input, and a
voltage reference for your whole board.

On Mon, May 14, 2018 at 6:25 PM Corey K <coreyker at gmail.com> wrote:

> I've been studying a few open source circuit designs for digital eurorack
> modules, and I've come across one that I'm having a little difficulty
> analyzing. It is for the 4ms spectral multiband resonator (SMR). In
> particular, I'm looking at circuits for the MORPHCV and SPREADCV inputs,
> which are on sheet 4 of the schematic (left hand side):
>
> https://github.com/4ms/SMR/blob/master/hardware/smr-1.0.1-schematic.pdf
>
> I understand the basic parts of the circuit: the input passes through and
> op-amp buffer (gain=1), and the output is connected to a 5v source via a
> trim pot. The input to a microcontroller is clipped to an acceptable range
> [0, 3.3V] with two diodes.
>
> I guess I'm having a bit of difficulty figuring out exactly what the
> voltage will be at the MORPHADC and SPREADADC pins. I tried to do some
> nodal analysis, but things weren't making sense to me.
>
> My intuition is that this is some type of "summing circuit" (the current
> from the 5v source gets added to the current from the op-amp). If so, why
> wouldn't the designer have used a traditional op-amp summing circuit? What
> might be preferable about this configuration?
>
> Thanks,
> Corey
>
>
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