Reading -12V into analog input

Hi Everyone, I've read a few different posts on how to read -5V using a voltage divider. Or even using an inverting op amp with gain of 1 to be able to read -12V at the input. I have tried setting up an inverting rail to rail op amp. However, my dual power supply reads -12V and positive 11.8V. So at the output I only read 11.8 instead of 12 when the input is -12.

Is there a better way to do this? Any other ideas I could try?

The project is that I have a monitor setup and as I vary the power the monitor output varies from -12V to -9.5V. I would like to have that monitor go into the arduino.

If your power supply is close to -12, is it hard to get an op-amp to work all the way to the rail. Put a voltage divider on your -12 V signal to get it to -6 to -4.75 before the op-amp, then the op amp can handle it easily.

You could also create an inverting op amp with a gain of -0.5, that should do it, too.

Thanks! I will try that out.

Voltage divider.

10k from +5volt to analogue pin, 25k from analogue pin to -12volt.

12volt will give an A/D value of ~29. 0volt will give an A/D value of ~731

Code can invert that. Leo..