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'[EE]: AD592 Temperature Sensor (coverting relative'
2002\03\24@004107
by
Donovan Parks
|
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
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
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
>>
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_OUTolinTakeThisOuT
embedinc.com, http://www.embedinc.com
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