Measuring negative voltages

[HenrikOlsson]

Hi Sam,
Attached is a schematic of an inputstage I've designed and used succesfully with the internal ADC. It's an OP-based rectifier with adjustable gain.

Apply your -12V signal to the input and adjust P1 untill you get your desired output (+5V or +4.096 or whatever). (Do this before you connect the output to the ADC)

The sign output will be high or low dependig on if the input voltage is positive or negative and you can use a digital input to read the sign and 'add' one bit of resoultion to the ADC resulting in +/- 10bit. Since your voltage is always negative you can leave that part out if you don't need it.

There's one drawback though. You need to feed the OP-amp with +/-15V.

Hope it helps.

/Henrik Olsson.