[sdiy] CV input circuit

[Corey K]

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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