Does 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: Dan Michaels [mailto:oricom@USWEST.NET]
Sent: 27 August 2001 15:40
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: High impedance inputr to PIC A/D


Lawrence Lile wrote:
>DING DING DING DING DING!  A WINNER!
>
>Bob, you get the "THE BEST DESIGN HAS ZERO PARTS" award for approaching that
>lofty ideal most closely.  What a good idea!
>

Hi Lawrence, at first I thought so too, regarding Bob's solution.
However, as Harold pointed out, the leakage current out of the pin
- whether configured in analog or digital mode - is going to run into
the high source impedance and ultimately charge the cap to an incorrect
value. Bob's switching method is not going to change this.

The spec shows [digital] I/O ports have a max leakage of +/-1 uA.

1 uA * 2M --> 2v offset, cap or no cap.

High source impedance only works accurately with truly "tiny" leakage
currents.

best regards,
- dan michaels
www.oricomtech.com
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