Negative to Positive Voltage Converter

Is there an easy circuit or IC that will convert negative voltage to positive voltage and retain the initial value? Context below.

I am trying to add a volt meter to my power supply using some old parts I have lying around. My power supply is two 2-12V channels, one positive and one negative. I have an adc that can take anywhere from 0 to 5v. To start, I have a voltage divider on my channel output to my ADC so that my max voltage on both channels is just under 5v. However, I don't think my ADC can take negative voltage. I only have one 7 seg display, so I am using a switch to decide which voltage I am measuring.

On my ADC there is a VCC, +VREF, and a -VREF. I have VCC coming from my rectifier and my VREFs are coming from my regulated output. Initially I was using the switch to swap both VREFs depending on the channel. For example, my positive channel would have +VREF on my regulated positive output, and -VREF would be ground. On my negative channel it would switch so my +VREF was ground and -VREF was my negative output. This would still provide up to a 5v drop to my ADC for measurement. However, it doesn't seem to work that way.

My theory is that the 5V VCC doesn't change, so when I switch to my negative channel, it becomes much more than 5V relative to my VREF. So I want to put a negative to positive voltage converter on my negative output so that the ADC will receive a 5v drop relative to the initial ground. I just have no idea how to do that and all google seems to give me are positive to negative converters. Any help is appreciated! Thanks!

Answer

How about a simple inverting amplifier:

$$ Vout = - \frac{Rf}{Ri} Vin $$

^{simulate this circuit – Schematic created using CircuitLab}

Ri at around 10Kohm can be a good starting value. Choose Rf to scale the output voltage such that it is always within the output voltage limit of the opamp and the input limit of the ADC.