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# Need to measure PCB temperature

Category: Datasheet/Specs
Product Number: MAX31865

The first step in over temperature protection circuit is to measure the temperature on the PCB. The range of temperature on the PCB to be measured can be between 0 degC to +200 degC.

To measure the temperature on the PCB using PT RTD, I found MAX31865 which is RTD to digital.

https://www.analog.com/media/en/technical-documentation/data-sheets/max31865.pdf

I have some questions.

If I use four wire routing on the PCB then Rcable is the resistance of the copper traces, right ? I do not need to mount any SMD physical resistor for Rcable. Is that true ?

According to datasheet.

The capacitor C1 is 0.1 uF. The de-coupling capacitors for Vdd are 0.1 uF. These are the physical SMD capacitors on the PCB.

Datasheet also says that PT100 uses a 400 Ohm reference resistor (Rref), and a PT1000 uses a 4 kOhm reference resistor (Rref).

In my case I am planning to use PT100. Therefore, I will put 400 Ohm reference resistor (Rref).

I am wondering about the reading or converting the "temperature" from ADC code which I assume read by the I2C bus. The microcontroller will read ADC code and convert using the following formula into degC. The formula is mentioned on page 11 of 26.

Temperature (C) ≈ (ADC code/32) – 256.

There is a Table 9 on page 20 of 26. This table describe the values for Temperature (C), RTD Resistance (Ohm), RTD DATA Reg (01H-02H) (hex), ADC Code (dec), and the last column is (ADC code/32) – 256.

I was calculating if I read the ADC code 15901 (decimal) or 7C3Ah (hex), than

Temperature (C) ≈ (15901/32) – 256

Temperature (C) ≈ 240.91 degC

This value is written in the last column in Table 9 with the ADC code 15901 in the same line.

To calculate the RTD Resistance (Ohm). There is a formula on page 19 of 26.

Resistance_RTD = (ADC Code x Rref)/2^15
Resistance_RTD = (15901 x 400)/2^15
Resistance_RTD = 194.10 Ohm

This value also written in the second column of Table 9 in the same line where ADC code 15901 is written.

I am actually concerned about the correction of the measurement. The last column of Table 9 says 240.91 degC which match to my calculations. How do we get 250 degC, the first column value ? Is that corrected value ?

I tried to use the Callendar Van Dusen equation from page 10 of 26.

R(T) = R0(1 + aT + bT^2 + c(T - 100)T^3)

where,
a = 3.90830 x 10^-3
b = -5.77500 x 10^-7
c = -4.18301 x 10^-12 for -200 C < T < 0 C
c = 0 for 0 C < T < +850 C

The value of c parameter will be zero for positive temperature. The equation will be reduced to.
R(T) = R0 x (1 + aT + bT^2)

If I put the values of a and b parameters and the temperature T as 240.91 degC. Then I get 190 degC. How do I get 250 degC. ?