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Datasheets and User Guides

App Notes

Software & Driver

 

14.1.5 Thermistor

 

Index: 50, 51

The AIN extended features system can automatically perform the necessary calculations for thermistors. Index 50 calculates temperature using the Steinhart-Hart equation. Index 51 uses the beta equation.

 

Operation:

Reading from the AIN#_EF_READ_A register begins the conversion process. The T7 will measure the voltage on the AIN and computes temperature using the Steinhart-Hart equation. The temperature, resistance, and voltage are then available to be read from the result registers.

 

Configuration:

Both the Thermistor's Steinhart-Hart constants (A, B, C and D) and information about the excitation circuit need to be provided. Information about the excitation circuits can be found in Section 14.1.0.1. Steinhart-Hart constants are normally provided in the thermistor's datasheet or obtained from the manufacturer.

The beta function can also be used to calculate temperature. The Beta works well over a limited range of about 50 ºC. Typical error is ~±0.5 ºC. The Steinhart-Hart is usually better than 0.01 ºC across a larger rage. When configuring a T-series device for beta thermistor calculations both the beta and the nominal temperature need to specified.

 

Configuration Registers: 

When using index 50 (Steinhart-Hart) the four values G-J are the Steinhart-Hart constants.  When using index 51 (beta) value G and H contain the ß value and the temperature at which beta was calculated. 

AIN#_EF_CONFIG_A: Thermistor Options - Requires firmware 1.0221. Selects temperature units. 0 = C, 1 = K, 2 = F. 
AIN#_EF_CONFIG_B: Excitation Type
AIN#_EF_CONFIG_C: Second AIN
AIN#_EF_CONFIG_D: Excitation Detail Volts / Amps
AIN#_EF_CONFIG_E: Excitation Detail Ohms
AIN#_EF_CONFIG_F: R25
AIN#_EF_CONFIG_G: Steinhart-Hart A / ß
AIN#_EF_CONFIG_H: Steinhart-Hart B / ºC where ß was calculated.
AIN#_EF_CONFIG_I: Steinhart-Hart C / 0
AIN#_EF_CONFIG_J: Steinhart-Hart D / 0

 

Result Registers:

AIN#_EF_READ_A: Thermistor Temperature
AIN#_EF_READ_B: Thermistor Resistance
AIN#_EF_READ_C: Thermistor Voltage

 

Example:

In this example we will configure a T7 to read from a Vishay NTCLE100E3103 10k Thermistor using a LabJack LJTick-Resistance to complete the excitation circuit.  The LJTick-Resistance is connected to the AIN0/1 terminal block. The thermistor is connected between the Vref and INA terminals on the LJTick-Resistance.

  • AIN0_EF_INDEX = 50
  • AIN0_EF_CONFIG_A = 0 -- No options, will output results in degrees Celsius.
  • AIN0_EF_CONFIG_B = 4 -- Excitation circuit #4.
  • AIN0_EF_CONFIG_C = 0 -- Second AIN, not used for excitation circuit #4.
  • AIN0_EF_CONFIG_D = 2.5 -- 2.5 V provided by the LJTick-Resistance
  • AIN0_EF_CONFIG_E = 10000 -- 10 kΩ shunt resistor provided by the LJTick-Resistance-10k.
  • AIN0_EF_CONFIG_F = 10000 -- R25 The nominal resistance of the thermistor at 25 ºC.
  • AIN0_EF_CONFIG_G = 0.003354016 -- Constants from the thermistor's datasheet.
  • AIN0_EF_CONFIG_H = 0.000256985
  • AIN0_EF_CONFIG_I = 0.000002620
  • AIN0_EF_CONFIG_J = 0.00000006383

AIN0 settings such as resolution, range, settling, and differential may need to be configured before continuing.  In particular, if you know your voltage will stay below 1.0V, you can use the 1.0V range for improved resolution and accuracy.

Results:

  • AIN0_EF_READ_A = 23.19 -- Temperature of the thermistor. (C)
  • AIN0_EF_READ_B = 10829.4 -- Calculated resistance. (Ω)
  • AIN0_EF_READ_C = 1.299774 -- Voltage across the thermistor. (V)