LM324 is easy to use and very inexpensive.

Paul

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Anand Dhuru
> Sent: Friday, June 14, 2002 7:48 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC]:strange analog in problem
>
>
> 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.
> > >
> >
> >
> >
>
>

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