ADC Help

hi, im using a pic a6f877a and i need to use the ADC to measure -0.7V.
for as far as i know, ADC does not read negative value. what should i do?


ps: im testing for transistors turn on volatges for both npn n pnp..
building a transistor and diode tester..


pls do help.. ty^^

maydayrocks said:

hi, im using a pic a6f877a and i need to use the ADC to measure -0.7V.
for as far as i know, ADC does not read negative value. what should i do?
ps: im testing for transistors turn on volatges for both npn n pnp..
building a transistor and diode tester..
pls do help.. ty^^

Click to expand...

hi,
The PIC adc only accepts +V from 0V thru +5V.

How are you generating the -V in order to test the -Vbe for the pnp's.?

Your question is far too vague to answer, but one possible solution would be an inverting opamp - but you need to give FAR more details.

my tester needs to read -0.7V.. in order to identiy its a PNP transistors..
im using PORTA that's a sure thing..

im also currently building a current source board with 1mA, 10mA, 100mA, n 100uA..

if u need more info pls do tell me exactly wat should i need to supply.. ty^^

There is nothing at all that says the PICs ground reference has to be a true ground reference. You could bias it's VCC/GND pins so that it's GND was -2.5 volts and it's +VCC was +2.5 volts, it'll still be running at 5 volts but could read an external signal bellow 'ground' Coming up with a solid virtual ground would be the hard part, and you'd have to keep in mind that it's I/O logic would become -2.5 for 0 and +2.5 for 1. Which may or may not bode well for external circuits. If this PICS sole jobs was as an ADC though you should be able to get way with it.

so what does it suppose to mean? how do i make it to -2.5v?

With an inverting voltage regulator... This is the first thing you should understand with ANY micro controller project you EVER undertake, the micro controller itself is only PART of the project, you're going to need a mix of analog, digital, or hybrid ICs to complete your project. You could search for a hundred years and never find exactly how to do what you want with just a PIC, add a few extra ICs or components in there and a lot more features become available. While micro controllers are a godsend as far as general purpose use goes you still need to fill in individual gaps in needed features to complete the project. I think they call it engineering.