ADC Help - Electronic Circuits Projects Diagrams Free

maydayrocks · 21st July 2008, 07:06 PM

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^^

ericgibbs · 21st July 2008, 07:23 PM

Quote:

Originally Posted by maydayrocks

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^^

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.?

**Nigel Goodwin** · 21st July 2008, 08:59 PM

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.

maydayrocks · 22nd July 2008, 03:03 AM

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^^

Sceadwian · 22nd July 2008, 04:28 AM

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.

maydayrocks · 22nd July 2008, 04:51 AM

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

Sceadwian · 22nd July 2008, 06:25 AM

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.

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21st July 2008, 07:06 PM	(permalink)
maydayrocks	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^^

21st July 2008, 08:59 PM	(permalink)
Nigel Goodwin	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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

22nd July 2008, 03:03 AM	(permalink)
maydayrocks	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^^

22nd July 2008, 04:28 AM	(permalink)
Sceadwian	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. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har." Last edited by Sceadwian; 22nd July 2008 at 04:29 AM.

22nd July 2008, 04:51 AM	(permalink)
maydayrocks	so what does it suppose to mean? how do i make it to -2.5v?

22nd July 2008, 06:25 AM	(permalink)
Sceadwian	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. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har." Last edited by Sceadwian; 22nd July 2008 at 06:28 AM.