Measuring negative voltages

[sammy]

Hi Charles:
That is exacly what i was looking for. do you have any formulas as to how to calculated different resistor values and to know how much offset you have and what not.
thank you
sam

[Charles Linquis]

You build a voltage divider from Vref. For example:

If you are trying to measure a -5V (full-scale) voltage and you have a 5V reference:

Build a voltage divider with a 4K resistor from Vref to the input of the A/D and another 4k from that pin to the voltage to be measured (the INPUT).

If the INPUT voltage is at "0" volts, the A/D pin will be at 2.5V (or 512 counts if you are using a 10-bit A/D).

At -5V input, the voltage at the A/D pin will be at "0" volts.

So, to obtain a reading, run the converter and SUBTRACT the count from the "0" voltage input reading (512) and scale the result.

If you still need more help, I can show you some more.