# Negative AREF value in CV inputs protection

Hi, I’m trying to desing a circuit for reading CV values with a RPi.
I’ve looked at the Braids schematics as a reference, and I think I finally understand how it all works, except I’m not sure where the AREF_-10 is coming from. Is it just from the -12V rail with a voltage divider?
As the RPi only has positive voltage rails, I imagine I can just use the 5V rail that I’ll invert with another opamp, or am I missing something fundamental?

I know the CV input question is asked often, but I’ve looked and didn’t find this particular problem. Thanks :3

1. Braids schematics’ are old, please look at any other recent modules (Plaits, Tides…).
2. The circuit for the AREF_-10 is located on the same page as the power connector. Using just the -12V rail with a voltage divider is bad design, since any noise or fluctuation on your -12V rail will bleed in your circuit.
3. If your op-amp is single-supply, it won’t invert +5V into -5V, but will just clip at 0V (or higher).
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Hmm thanks, looks like the section is a bit simplified in Plaits.
Now I’m a bit stuck but not willing to give up. Do You know if there’s any way to transform the range from negative to positive, without negative rail available?

My idea is something like this, but I’m worried it’ll fry the RPi if I use it as a 3v3 supply:

At worst I’ll try to figure out how to create a negative power supply or add a battery, but the design starts to become cluttered with that :T

I’m not sure what this circuit achieves, where is the input, where the output, and what is a supply rail, but I don’t think it’s doing what you expect it to do.

which means?

What kind of output do you expect with an input of -50V, -10V, -5V, -1V, 0V, +1V, +5V, +10V, +50V?

I apologize, I should really learn to comment things. Here’s the annotated version, I’m trying to convert ±10V to 0-5V:

I think it’s supposed to work like a combined voltage divider and voltage/resistor summer.

If I make it into a simulation in spice it works nicely, but I don’t think I should go just by that.
Alternatively, I’ve found out that a summer opamp might work, but I feel like it has the same issue of 5V rail and Vin being connected which could fry the RPi (which from what I gathered could be fixed with a zener diode similarly to what I used in the circuit above).

What’s the input impedance of the ADC on the RPi? If it’s like any microcontroller (in the 10k range), this won’t work.

This circuit is sound. It requires a rail-to-rail op-amp (MCP600x is fine).

What is this issue?

The inverting circuit (second answer) would be my preferred approach.

Thanks a ton, I’ll go with the second approach then. The RPi (Zero at least) doesn’t have any ADC on its own. I was thinking about using something like MCP3204.