We often need a simple way to measure a voltage which is both positive and negative using the ADC. As long as we impose a few simple rules we can do this using nothing more than three resistors. The rules are...
1) The voltage range is symmetrical. ie the maximum positive voltage is equal in magnitude to the maximum negative voltage.
2) The voltage range in centred in the ADCs range. ie zero volts in will read half full-scale.
3) The voltage source must be able to both source and sink current.
4) The voltage source must have a low output impedance.
With this in mind let's design an input stage to take a +/-40V signal and transform it into a 0V-5V signal to feed to the ADC.
So, we take this signal...
...and turn it into this...
How? Using this simple circuit...
How do you calculate the resistor values....
And as a PDF...