Hi Yan,
>>> The document that Jeff quoted shows two ways to wire a three-wire
>>> setup, both on page 7. I can't quite figure out what the difference
>>> is, which one I should use, and why there are 2 resistors in parallel,
>>> one 499 1% and the other 22K 5%.... Are they just trying to adjust
>>> the range of the sensor with the 22K resistor?
Here's my interpretation of the circuit on p7 on the left;
* The LM34 puts out 10mV/deg-F.
* Dropping 10mV/F over 499-Ohms gives you
20uA/F supply current.
* The 20k in parallel with 499-Ohm in parallel gives you
another approximately 499-Ohm resistor, which acts as
a current sense resistor for the current going to the sensor.
* The circuit basically converts the voltage output sensor
into a current-based circuit.
* You'll need a differential input amp (instrumentation amp),
or two ADC channels to correctly measure the temperature of
(current sourced by) the sensor.
* In addition to the sensor current due to temperature, you've
now got the inaccuracy of the supply current (read the numbers
in the data sheet).
If the circuit meets your requirements, then by all means use
it.
Cheers,
Dave
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