On Thu, 30 Jun 2005 15:25:07 +0100, you wrote:

>I've had a look through the FAQ and drew a blank on this.
>
>I'm a relative newbie to PIC and electronics, but for my home brewery, I need some temperature control.  PT100 sensors seem to be the chosen standard for food safe applications from what I've found 'googling'.
>
>Can anybody suggest some further reading, perhaps with a circuit (3 or 4 wire) which will perform the ADC (say 0.1 deg accuracy over 0-100C range) with a PIC?
>
>Regards,
>Mark

The easiest way is to do 4-wire, using an ADC (or adc+amp) with a differential input. 
You pass a current through the PT100 (a milliamp or two to avoid heating), and have a reference
resistor in series with the excitation supply. You measure the voltage across the PT100, using the
voltage across the reference resistor as the reverence voltage. If the ADC doesn't have a
differetial reference, put the ref resistor in the ground leg of the PT100 so you get a reference
voltage referenced to ground.
This method avoids the need for a regulated current source or voltage reference - the ADC reading is
the ratio between the PT100 and reference resistors, and therefore is directly proportional to the
PT100 resistance.

3-wire is slightly more fiddly - you need to measure the voltage difference between the excitation
and sense wires, and add this to the voltage you measure across the PT100 element.

Have a look at appnotes for high-performance ADCs from LT or Maxim, as PT100s are a commonly used
example application.

 


 

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