Could you tell me if the PIC16f877 input to the a/d converter work on voltage or current, or do both. For example if I had 5volt input to the a/d converter would the digital represntation for an 8bit converter be 256 and for 2.5volt be approx 128.

Dean Matthews
Reliability Engineering
Ford Engine Plant Bridgend
South Wales, U.K
Tel No:(01656)672597
Fax No:(01656)672558
Email: dmatth14@ford.com
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-----Original Message-----
From: Olin Lathrop [mailto:olin_piclist@EMBEDINC.COM]
Sent: 24 August 2001 19:38
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: High impedance inputr to PIC A/D


> B. To use a cheap two-or-three transistor amplifier.  I get transistors
> three for a nickel, and resistors four for a penny, so any concievable
> combination of bipolar  transistors and resistors would be plenty cheap.
> FETs (the other obvious choice) are probably out of my price range.  So
far,
> I've not been able to get anything to work so well.

If you're so far just looking at the voltage from a photo transistor with 2M
collector resistor, you obviously don't care about speed or a few 10s of mV
offset.  In that case you can make a "do it yourself" op amp with a few
transistors and resistors.  The attached schematic shows what the op amp
front end looks like.  This is a comparator between the IN and FEEDBACK
signals.  You will still have to take one of the OUT signals, and amplify it
by a large voltage gain, then feed the result back into FEEDBACK.  The
result will then be a reasonably good replica of the IN signal at much lower
impedence.

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