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Need RTD Pt1000 Signal conditiong circuit & Temp Vs Resistance

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rajnaren5

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Hi, friends

I am using two wire RTD Pt 1000, in my circuit. Can any body give the signal conditioning circuit for this two wire RTD Pt1000. Please send me the circuit as early as possible. I am waiting for ur response.

Thank u so much.
 

grieblm

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Simple, just put another precision 1K (0.05% -- if possible) in series, apply a voltage and use an A/D to monitor the voltage change of this divider circuit. The change in voltage should be proportional to the change in temperature. The voltage at 25 deg C will typically be 1/2 your applied voltage (although this can be calibrated).

The response is also fairly linear over a fairly large temperature range but the data sheet should give you the RTD characteristic equation if necessary for better accuracy over larger temperature ranges.
 

FvM

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The change in voltage should be proportional to the change in temperature
Because the divider output voltage is proportional to the ratio Rx/(Rx + R0), it's not linear in Rx respectively temperature. In addition the non-linear Pt characteristic has to be considered, as you mentioned. Nevertheless, using a simple ratiometric RTD "signal conditioning" circuit is a good suggestion for an ADC frontend, because it reduces the number of sensitive components considerably.

But for acceptable accuracy, Rx has to be calculated from the measured voltage, which involves a divide operation. For correction of the Pt characteristic, either a polynomial or a look-up table based interpolation gives good results. Analog Devices AN-709 presents C code and a table code generator for the latter.
 

grieblm

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Because the divider output voltage is proportional to the ratio Rx/(Rx + R0), it's not linear in Rx respectively temperature. In addition the non-linear Pt characteristic has to be considered, as you mentioned. Nevertheless, using a simple ratiometric RTD "signal conditioning" circuit is a good suggestion for an ADC frontend, because it reduces the number of sensitive components considerably.

But for acceptable accuracy, Rx has to be calculated from the measured voltage, which involves a divide operation. For correction of the Pt characteristic, either a polynomial or a look-up table based interpolation gives good results. Analog Devices AN-709 presents C code and a table code generator for the latter.
It is possible to combine both calculations into the polynomial or table to minimize the total computations. Your accuracy requirements will dictate the degree of the polynomial.
 

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It is possible to combine both calculations into the polynomial or table to minimize the total computations.
Have you tried to include the calculation into a polynomial? If so, you'll know why it isn't a good idea. You'll need e.g. two orders more to compensate for the additional error. with a table, it's no problem of course.
 

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These days computation is fairly cheap even on 8-bit micros. It's pretty simple to implement 4th order polynomials in real-time.
 

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