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App Notes

Software & Driver


14.1.3 RTD


Index: 40-42

AIN Extended Feature RTD computes the temperature of a resistance temperature detector.


When AIN_EF_READ_A is read the T7 reads the analog input and calculates the resistance of the RTD. Temperature is then calculated using the rational polynomial technique. The resistance calculation relies on several settings which are determined by the excitation circuit the RTD is a part of.

Excitation Circuits:

There are six supported excitation circuits. Three based on a current source and three based on a voltage divider.

Current source: Current source circuits are the simplest of the supported excitation circuits. The RTD is connect to the current source and to GND. The top of the RTD is then connected to an analog input. More than one RTD can be connected in series. When using a series of RTDs both sides of the RTD need to be connected to AINs and those AINs need to be set to differential, care must also be taken to ensure that the total voltage drop across the RTDs can not exceed the current source's maximum output. Following, are the 3 supported current source excitation circuit indices. 

Circuit index 0: 200 microamp current source.  This index uses the stored factory calibration value of the 200 microamp current source.
Circuit index 1: 10 microamp current source.  This index uses the stored factory calibration value of the 10 microamp current source.
Circuit index 2: X microamp current source.  This index uses a supplied value for the  current source (CONFIG_D).

Voltage divider:  Voltage divider circuits rely on combinations of supplied voltage and a fixed resistor in series with the RTD.

Circuit index 3: Shunt resistor. In this circuit the RTD is connected to an unknown voltage source and a fixed resistor. The resistor is then connected to GND. The analog input matching the AIN_EF number to be used is connected to the positive side of the RTD and a second AIN is connected to the negative side of the RTD. Specify shunt resistance in ohms (CONFIG_E) and a second AIN# from which to measure the shunt voltage (CONFIG_C).
Circuit index 4: Fixed voltage source. Circuit 4 is similar to 3 except it only requires one AIN and the voltage supplied must be specified (CONFIG_D). The voltage source must be low-noise. Using a Vs terminal will result in a lot of noise. The shunt resistance need to be specified as well (CONFIG_E).
Circuit index 5: 

RTD feature index values:
40: PT100
41: PT500
42: PT1000

Configuration Registers:
CONFIG_A: RTD_Options – Bitmask containing additional options. Bits 0 and 1 change the units of the output of the calculations. The default is kelvin. Bit 0: 1 = Report in ºC, Bit 1: 1 = Report in ºF.
CONFIG_B: Excitation Type – This option tells the T7 how the RTD is connected. There are several options:

  • 0: Current source, 200 µA, use factory cal. value.
  • 1: Current source, 10 µA, use factory cal. value.
  • 2: Current source, specify amps in CONFIG_D.
  • 3: Shunt resistor, specify Rshunt ohms and AIN# to measure shunt voltage.
  • 4: Voltage source, specify R2 ohms and Vexc volts.
  • 5: Voltage source, specify R2 ohms and AIN# to measure Vexc voltage.

CONFIG_C: Excitation AIN # - Channel number of the AIN line used to measure the RTD's excitation.
CONFIG_D: Excitation detail - Volts-Amps
CONFIG_E: Excitation detail - Ohms

Result Registers:
READ_A: Final calculated temperature.
READ_B: Resistance of the RTD.
READ_C: Voltage across the RTD.
READ_D: Current through the RTD.


Shunt resistance, circuit #3:
Using the shunt resistor circuit with shunt resistor value = 2700 Ω and RTD type 100

AIN_EF Configuration:
AIN0_EF_INDEX = 40 -- Set AIN_EF0 to RTD100.
AIN0_EF_CONFIG_A = 0 -- Set result units to Kelvin.
AIN0_EF_CONFIG_B = 3 -- Set excitation circuit to 3.
AIN0_EF_CONFIG_C = 1 -- Set the second AIN to AIN1.
AIN0_EF_CONFIG_E = 2700 -- Set the shunt resistance.

Getting a Reading:
Read AIN0_EF_READ_A -- When this register is read the T7 will measure the voltages on the AINs and calculate temperature.

Analog input configuration:
AIN settings can be used to reduce noise. A differential reading can be used across the RTD by setting AIN0 to differential. If the voltage across the RTD will not exceed 1V then gain should be used to increase resolution. Increasing settling time can also help when the resistances are large.