Hello,

I'm hoping to use an AD592 temperature sensor to *accurately* measure =
temperature (+/- 1 degree C).  The device outputs 1uA/K and has a rated =
accuracy of (+/-0.5 degree C).  Now, to measure the current produced by =
the AD592 I am measuring the voltage over a known resistance value =
(~10K) using a PIC.  Unfortunately, I didn't account for the fact that =
the PIC AtoD really gives you a relative voltage (i.e. what percentage =
is the measured voltage compared to Vref+).  I'm using a LM2940 to =
achieve +5V from a 9V source, but the LM2940 actually outputs 5.4V =
(beyond the maximum value specified in the datasheet) and will decrease =
as the 9V battery voltage decreases.  Thus, I am left with no accurate =
way to measure the absolute value of the voltage across the resistor =
connected to the AD592. =20

Note, this is for a one-off design so I am more than happy to do any =
tricks.  For example, the resistor I am using is just a 10K resistor I =
grabbed out of my junk box and used a DMM to get a precise value that I =
use for calculations on the PIC.  If there a similar type 'trick' I can =
use to determine absolute voltage.  I'm thinking something along the =
lines of using a diode to get an absolute voltage value (0.7V) so I can =
determine the voltage across the resistor (of course, a diode is not =
precisely 0.7V and changes with temperature so this probably isn't a =
good approach). =20

Any suggestions?

Regards,
Donovan Parks

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