adc on 18f4550

[krodkey]

Does anyone know what the conversion equation is to convert the value obtained from read_adc() on a 18f4550 chip to volts DC?

Thanks

[Ttelmah]

It depends on your Vref.
(Vref/1024)*ADC_READING

Note that the divider is 1024, though the output of the ADC, is 0-1023. Also that this will need to be done in floating point arithmetic (slow and bulky). Using a scaled integer is much quicker fo most applications.

Best Wishes

[krodkey78]

I used the equation and it seems to be working but the voltages will not swing into the negative range. I am collecting the analog data from strain guages that are connected to a fiberglass arm. When the arm bend the resistance changes. Bend it one way and the voltage should go positive and bend it the other is should go negative. I am not sure how to read the negative values from adc. It just returns to 0. Below is the code I am using.

[treitmey]

page 399
http://ww1.microchip.com/downloads/en/DeviceDoc/39632c.pdf

It shows range from VREFlow to VREFhigh
and VREFlow can only go down to Vss-0.3 ie the PICs ground.

I think you would need and opamp of some configuration to shift and scale
your reading to fit inside the 0-5V range.