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'[EE]: AD592 Temperature Sensor (coverting relative'
2002\03\24@004107 by Donovan Parks

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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.  
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).  
Any suggestions?

Regards,
Donovan Parks

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2002\03\24@024710 by Stuart Meier

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Compare the voltage against a precision voltage ref, available at
precision of 1%. I use loads of ZRB500F01s, cost < #0.40, Temp coeff
50ppm/degC.

Stuart
{Original Message removed}

2002\03\24@032449 by Donovan Parks

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Thanks Stuart,

Knew there had to be something out there.

Regards,
Donovan Parks



> Compare the voltage against a precision voltage ref, available at
> precision of 1%. I use loads of ZRB500F01s, cost < #0.40, Temp coeff
> 50ppm/degC.
>
> Stuart
> {Original Message removed}

2002\03\24@120644 by Olin Lathrop

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>>
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.
<<

You could use a zener or two to create a reasonably accurate Vref for the
A/D converter.  A better but costlier approach would be to use a voltage
reference chip to feed Vref.


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

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