[sdiy] CV input circuit

[Tom Wiltshire]

+1 agree. I’d do it Olivier’s way too, for the reasons given. One op-amp mixer per CV input isn’t a big deal and a reference voltage is not hard to rustle up either.

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 14 May 2018, at 17:57, O Gillet <ol.gillet at gmail.com> wrote:
> 
> >  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 <mailto: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 <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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