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.

Cheers,
-Neil.




-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Anand Dhuru
Sent: Thursday, June 13, 2002 2:36 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]:strange analog in problem


Along these lines...

What happens if I need to measure a resistance to the tune of 100K to 1M
using the ad on the 16F877?

I had been toying with the idea of using a fixed resistance and the variable
forming a voltage divider, with the common point being fed to the PIC AD.
Following this thread's indicated that the max. impedence for reliable
operation should NOT be more than 20K. Are there any suggestions?

The resisatnce to be measured is in fact a sensor, and therefore I do not
require the reading in ohms, as such. A look up table can give me the values
to be indiacted on the diaplay.

Regards,

Anand Dhuru

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.