4.1.7.3.1 RTD sensor and pull-up resistor selection
For measuring the temperature using an RTD, the sensor’s voltage needs to be adjusted to a specific
temperature range by adding a pull-up resistor.
The analog input can be configured to read the input voltage or input resistance. Subitem 0x2038:3 defines the
value of the pull-up resistor. Two modes for measuring the analog input value can be distinguished:
Pull-up resistor (0x2038:3) = 0
The measured value will be the exact value of the analog input voltage.
0 = 0 volts and 4096 = full scale (according to the full scale voltage of the analog input)
Pull-up resistor (0x2038:3) > 0
In this mode, the firmware will calculate the resistor value (in Ω) connected to the input terminals. This feature
is useful when using RTD sensors or another resistive sensor.
Note
RTDs with a minimum of 3 Ω resistance change per °C result in an acceptable accuracy.
According to the curves below, PT1000 and KTY-130 are good fitting sensors, both sensors are almost linear in
the desired range.
PT1000: 3.9 Ω per °C
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