Synth cv circuit help

Hi. I’m trying to use a mutable instrument control voltage input circuit as an example for my diy project. Could someone help me?

I want a jack input to accept a synth cv and then send it into a microcontroller’s ADC pin. I’m using a single sided 3.3V power supply and the example circuit looks like an inverting op amp circuit. I’m not sure on the details on how this circuit works. My ADC pin can only accept 0 to 3.3V. Synths could end up sending -30V to 30V if you really tried to patch to the extreme. :slight_smile: My question is what will happen to this circuit with that extreme synth cv range? Usually the op amp pins try to stay at 0V, but will they go higher with that cv range? Will I fry the op amp or fry the ADC microcontroller?

See attached picture.

Thanks!

The circuit you have posted won’t work, because it won’t accept positive voltages. Check the schematics of Mutable Instruments products, you’ll see that there’s always a negative voltage summed to the V- input of the op-amp through a large value resistor.

> I’m not sure on the details on how this circuit works.

Then you’d better read the first chapters of a book about op-amp. Being able to compute the response of such a circuit is a really simple task. You should know how to do it!

> My question is what will happen to this circuit with that extreme synth cv range?

The op-amp will clip and the output will stay at either 0V (if the input voltage is too high), or 3.3V (if the input voltage is too low)

> Will I fry the op amp

No. The V+ input is grounded, and you are not applying any voltage directly to V-, so it’s free to stay at 0V (normal conditions), or roam between 0V and 3.3V when the output clips.

> or fry the ADC microcontroller?

No. The op-amp output is guaranteed to stay between GND and +3.3V.

The worst case scenario is that too much current will flow through the 100k input resistor, this will happen when V^2 / R_in exceeds the power rating of your resistor. With an 1/8 or 1/16 resistor, the input voltage can go well above 30V.