Something you might try is measuring using current from the shack
side.  Set up a constant current source (LM317 or similar) and then
measure the voltage at the output of the regulator.  The voltage will
vary based on the position of the pot in order to make the current
constant.

It should help significantly, based on the description of the problems
you're having.

-Adam

On 5/7/08, John Ferrell <johnferrell@earthlink.net> wrote:
> Yes but that requires access at the tower top.
> Also, I don't think the pot is a significant factor in the problem.
>
> Essentially I am trying to duplicate an ohm meter. It is interesting that
> MicroChip offers special purpose chips for Ohm meters.
>
> John Ferrell    W8CCW
>
> "All that is necessary for the triumph of evil is for good men to do
> nothing." -- Edmund Burke
> http://DixieNC.US
>
> ----- Original Message -----
> From: "Michael Rigby-Jones" <Michael.Rigby-Jones@bookham.com>
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
> Sent: Wednesday, May 07, 2008 5:11 AM
> Subject: RE: [PIC]Analog signal conditioning
>
>
> >
> >
> >> -----Original Message-----
> >> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
> > Behalf
> >> Of John Ferrell
> >> Sent: 06 May 2008 20:31
> >> To: Microcontroller discussion list - Public.
> >> Subject: Re: [PIC]Analog signal conditioning
> >>
> >> Until I fell into the trap I would have agreed with you. The AP Notes
> >> regarding ADC are serious reading.
> >> If you keep the pot onboard with the PIC you can get away with a lot.
> >> Reading the wiper of a 5K pot across Vcc and ground is not a problem.
> >>
> >> My Pot is at the end of about 75 feet of cable. The cable contributes
> > some
> >> unknown reactance in the configuration. The troublesome noise is a
> >> byproduct
> >> of the conversion within the PIC.  The current requirements are likely
> > to
> >> be
> >> damaging to the Pot (500 ohms) if I simply put Vcc across it... I have
> > a
> >> 4.3K resistor in series with it.
> >>
> >> The conclusion that I draw at this time is that remote sensing is not
> > a
> >> simple matter even if it is just a resistor.
> >>
> >
> > Did you see my original post on differential measurement?
> >
> > Mike
> >
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