Re: PIC based Pt100 RTD temp sensor circuit

MarkMc wrote:
(snip)

I think I understand what's required at a high level, but my
electronics isn't up to it at the low-level.  I need a range of 0C to
100C and 0.5C accuracy/resolution for my application.

High level things I think I need (high-level block diagram)
4-wire PT100 RTD
"Something" to generate the excitation current - I think this needs to
be extremely accurate and invariable.
Something to amplify the small voltage across the RTD in to the range
of an ADC.  Looks like NS ML4140A-2.500 is used for this in the links
above?


Except for the transposition of LM, yes. It is a precision 2.5 volt reference regulator. In combination with the opamp, it forces the voltage drop across the grounded 2.5K resistor to be a constant 2.5 volts, so the RTD current is forced to be a constant 1 mA.

An ADC to convert the analogue signal in to a format for my PIC to use.
 Are there not PIC's which have on-board ADC's that I can use for this
purpose or is this not a good idea?

Most PICs have an internal 10 bit (1024 level) ADC. You just need to amplify and shift the signal into the ADC measurement range. You may also want to reference the ADC full scale voltage to a precision reference, instead of using the 5 volt supply as the full scale value, or else the accuracy of the PIC supply gets involved in the measurement.

I think I know how to convert from the digital voltage in to a
temperature - divide the known voltage range up in to chunks based on
the bit resolution of the ADC.  Say - 0-5v ADC and 10-bit, I would then
 5/((2^10)-1) to get the voltage increment per bit.  Then map the
'voltage' reading back to resistance (known excitation current), and
use a look-up table to 'linearise' and then interpolate to give a
reasonable approximation of temperature.

One thing I'm not sure about is how to calibrate this process.

You might use a distilled water ice bath and boiling point as a two point calibration. Otherwise, you buy a high accuracy liquid thermometer and use it in a stirred water or oil bath as a temperature reference.

> Another is that IIRC 0C is 100R and 100C is 138.5R (for Platinum PT100) 
which @ 1mA gives a voltage reading range of 100mV-138.5mV.  I'm sure I
can scale this up so that the 100C resistance maps somewhere near 5
volts, but the 0C will not be near 0v.  so some of the 10-bit signal is
wasted.  Could I not get better accuracy (perhaps it's not needed with
10-bits) by translating the 0C voltage/resistance somewhere near 0v and
then scale/amplify so that 100C is near 5v so I'm using a larger
portion of the bit range?

You can add an arbitrary offset to the resistance measurement by connecting R1 in figure 1 to an adjustable voltage, instead of to ground. This voltage should be a low impedance source (not just a pot hooked up between two voltages) like an opamp follower, in order to not upset the subtracter's ability to measure the voltage across the RTD. The subtracter circuit is A4 and the resistors around it.

You can use a pot between the 2.5 volt reference and ground to feed the follower.

When you have zero degrees just above ground and 100 degrees just below the ADC positive reference voltage (full scale voltage) you are ready to calibrate and linearize the circuit with stored constants in the program.

Regards,
Mark

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