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rocky79
- 30th September 2005, 07:00
Is it possible to use a negative voltage on the A/D input for the pic12f675 ?
Thanks
mat janssen
- 30th September 2005, 07:09
Yes it is possible to put a negative voltage on the input of a 12F675. BUT the chip can die, and the result of your measurement is wrong.
To read the real value you have do swap the wires and measure again.
When it is not possible to swap the wires just add an opamp wired as a one time inverting amplifier.
rocky79
- 30th September 2005, 07:41
sure i can put an inverting element but i am trying to avoid adding extra parts.
mat janssen
- 30th September 2005, 07:45
Then swap the wires !!
Acetronics2
- 30th September 2005, 07:56
To read the real value you have do swap the wires and measure again.
When it is not possible to swap the wires just add an opamp wired as a one time inverting amplifier.

Hi Mat,

classical op amps can do an inverting or non-inverting amps, just by connecting "+" input to ground or to the signal input ... ( good thing with a Pic, no ???? )
Consider an inverting amp ( G = -1 R's = 100k ) and simply connect the "+" input via a 100//100 k resistor to ground or signal.

of course a negative supply for the AOP is needed ...

that is used in R/C systems to trim gain and reverse on the Tx...
I look for the scheme ... as oooooold as I .

from Elektor July/Aug 1983 p 7-62


Alain
rocky79
- 1st October 2005, 00:35
Then swap the wires !!

Well if this is the solution i wouldn't be asking this question. it's not that simple
languer
- 1st October 2005, 06:40
Well, you have your answer:

Q. Is it possible to use a negative voltage on the A/D input for the pic12f675 ?

A. Yes is it possible (careful what you ask for).

Is it recommended? -> No it's not. You could damage the chip and besides your measurement will not go under Vss (GND).

You could sense the signal even without a negative supply, but here are the guidelines (if you want to operate from positive supply only):

1) You need the supply rail (Vdd) to extend somewhat above your signal range (i.e., if your signal can swing from +3V to -3V your supply needs to be +6V with some margin). Why is this?
2) To measure this you need to "reference" the measurement. For this you need an OPAMP.