> Basically, I need to interface a couple of sensors to a PIC with A/D. The
> PIC and A/D side I can take care of, but these sensors are setup such that
> they are a variable resistance to ground. For example, one of them is an
> automotive pressure sensor with a 10-187 ohm resistance to ground, and I
> would like to convert that to a variable voltage. Oh, and most of the
> sensors are linear.

What resolution do you need?  For example, if 8 bits is enough you can use
1/4 range of a 10 bit A/D, which only requires 0 to 1.25 volts.  Even less
if you only need 6 bits, for example.

What is the maximum current or power rating of the sensors?  For example, if
you wanted the 187 ohms to result in 5V, it would require 28mA and would
dump 134mW into the sensor.  If this is acceptable, you could run a fixed
current thru the sensors and get the desired 0-5 volts out directly.  Since
you have software that can apply corrections, this could be as simple as a
200 ohm resistor to are regulated 10V supply, for example.  If you use the
output of the sensor directly, then be sure to put a 10K ohm resistor
between the sensor and the PIC pin.  Otherwise an open sensor will blow up
the PIC.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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