Peter, David, Mike, Neil,

Thanks for all the replies, folks. From what everybody's advised, I guess
the best way to do this would be to use an op-amp to match the impedences to
levels at par with the sensor and the AD input. If one is to do this, which
would be the best (cheapest, simplest to use) op-amp? The frequency response
is not an issue at all. And preferably with a quad configuration, as I need
to use this scheme on 3 channels.

By the way Neil, Mike was right about your suggestion. I ran the values
thru' a spreadsheet, and although the solution looks temptingly simple, the
changes in the effective value of the sensor would be too insignificant.

Regards,

Anand Dhuru



----- Original Message -----
From: "David Minkler" <Mink@LUXTRON.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, June 13, 2002 9:46 PM
Subject: Re: [PIC]:strange analog in problem


> Hi,
> The problem with these voltage divider schemes is that you will only be
> able to use a comparatively small fraction of the ADC range thereby
> limiting your measurement resolution.  Adding additional "parallel (in
> the Thevenin sense)" resistors to the network without scaling the
> network to compensate will only make the problem worse.  The voltage
> non-linearity of these resistive networks may actually turn out to be a
> benefit if the sensor changes its resistance in an exponential or
> pseudo-exponential fashion (as in a thermistor) as the resistive divider
> will (sort of) compensate in the reverse direction.
>
> In order to provide a good solution to the problem we need to know more
> about the sensor characteristics and the required measurement resolution
> and accuracy.  Tell us more.
>
> Best regards,
> Dave
>
> Pic Dude wrote:
> >
> > For simplicity, could you not form a voltage divider
> > w/2 fixed resistors, and the large variable resistor
> > to be measured placed in parallel across one of the
> > fixed resistors?  Of course you now need some math
> > since the voltage measured at the AD port is no longer
> > linearly proportional to the variable resistance.
> >
>
> --
> http://www.piclist.com hint: The PICList is archived three different
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>
>
>

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