[sdiy] CV input op-amp circuit

Tom Wiltshire tom at electricdruid.net
Fri Dec 4 15:30:04 CET 2020

Hi Chris,

I’ve seen various approaches to this problem including:

1) Half-wave precision rectifiers to remove inputs below zero volts, with zeners or similar to limit the highest voltage

2) Op-amp clipping with op-amps run from +/-12V that then have their maximum output scaled down and offset to the correct range.

3) Op-amp clipping on single-ended op-amps run at the processor voltage (like yours)

I’m sure there are others. Honestly, I think (3) is pretty simple and straightforward, although I like the precision rectifier circuit too (just because it’s cool!) but the limitation there is that it doesn’t control the higher voltage as neatly.

I notice you have the MCP6002 on 5V supply, but you said you wanted 0-3.3V output. Is that right?
100K input impedance is typical for CV inputs on modular gear, so you might want to double your resistor values round the op-amp.


       Electric Druid
Synth & Stompbox DIY

> On 4 Dec 2020, at 13:31, Christian Maniewski via Synth-diy <synth-diy at synth-diy.org> wrote:
> Hi all!
> I’m trying to come up with an op-amp design for a CV input. I want to transform a signal ranging from -5V to +7V to a more MCU digestable 0-3.3V. I came up with the circuit you’ll find attached.
> I have seen other approaches, where an offset reference is injected in the feedback loop, while the positive op-amp input is grounded. Are there any disadvantages to my approach or is it also valid?
> Thank you so much!
> I’ve been following this email list for some time now. This is my first question and first email entirely. Please bear with me.
> Chris
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