14.1.3 RTD [T-Series Datasheet]

Overview

AIN#_EF_INDEX values:

40: PT100
41: PT500
42: PT1000

This RTD Extended Feature automatically performs calculations for a Resistance Temperature Detector (RTD). RTD types are listed above.

When AIN#_EF_READ_A is read, the T-series device reads an analog input and calculates the resistance of the RTD. Temperature is then calculated using the rational polynomial technique.

An RTD (aka PT100, PT1000) is a type of temperature sensor. See the Temperature Sensors App Note.

An RTD provides a varying resistance, but the LabJack measures voltage, so some sort of circuit must be used to convert the varying resistance to a varying voltage. This AIN-EF supports various excitation circuits. The best option is usually the LJTick-Resistance, which would be excitation circuit #4.

The resistance to temperature conversion is done using the RTD Rational Polynomial technique. The polynomial coefficients are fixed and assume the most common RTD characteristics, where a PT100 (for example) has a resistance of 100.0 ohms at 0 °C and a coefficient of 0.00385. Math for non-standard RTDs will have to be handled by the user. PT500 is assumed to have 5 times the resistance of a PT100, and a PT1000 is assumed to have 10 times. More information about the polynomial can be found here: http://www.mosaic-industries.com/embedded-systems/microcontroller-projec...

Configuration

To configure, write to the following registers.

AIN#_EF_CONFIG_A - Options: Selects temperature units:

0 = K
1 = °C
2 = °F

AIN#_EF_CONFIG_B - Excitation Circuit Index: The index of the voltage divider excitation circuit to be used.

See 14.1.0.1 Excitation Circuits for circuit indices.

AIN#_EF_CONFIG_C - 2nd AIN: Channel Number to Measure Vresistor: For excitation circuits 3 and 5 this is the extra AIN used to measure the voltage across the fixed resistor. Ignored for other excitation circuits.

AIN#_EF_CONFIG_D - Excitation Volts or Amps: For excitation circuit 2 this is the fixed amps of the current source. For excitation circuit 4 this is the fixed volts of the voltage source. Ignored for other excitation circuits.

AIN#_EF_CONFIG_E - Fixed Resistor Ohms: For excitation circuits 3, 4 and 5, this is the ohms of the fixed resistor.

Remarks

The normal analog input settings are used for negative channel, resolution index, settling, and range.

T7: If the voltage will stay below 1.0V, use the 1.0V range for improved resolution and accuracy.

T8: For improved resolution and accuracy, use the lowest voltage range that is appropriate for your signal's voltage min/max. For example, if the signal's min/max is -0.0064V/0.0549V, use the T8 voltage range of ±0.075V.

Results

Retrieve the results by reading the following registers.

AIN#_EF_READ_A: Calculated temperature.
AIN#_EF_READ_B: Resistance of the RTD.
AIN#_EF_READ_C: Voltage across the RTD.
AIN#_EF_READ_D: Current through the RTD.

Only reading AIN#_EF_READ_A triggers a new measurement, so you must always read A before reading B, C or D.

Example

The LJTick-Resistance-1k is the best and easiest way to measure an RTD, but if you don't have an LJTR the 200UA source on the T7 is a quick way to get readings.

200UA current source, circuit #0:

Connect 200UA to AIN0 and connect a PT100 RTD from AIN0 to GND:

AIN0_EF_INDEX = 40 // Set AIN_EF0 to RTD100. AIN0_EF_CONFIG_A = 0 // Set result units to kelvin. AIN0_EF_CONFIG_B = 0 // Set excitation circuit to 0.

Now each read of AIN0_EF_READ_A will measure the voltage on AIN0, use that to calculate resistance based on the factory stored value for 200UA, and use that to calculate temperature.

LJTick-Resistance-1k, circuit #4:

Connect a PT100 RTD from Vref to VINA on an LJTick-Resistance-1k that is plugged into the AIN0/AIN1 block.

AIN0_EF_INDEX = 40 // Set AIN_EF0 to RTD100. AIN0_EF_CONFIG_A = 0 // Set result units to kelvin. AIN0_EF_CONFIG_B = 4 // Set excitation circuit to 4. AIN0_EF_CONFIG_D = 2.50 // Vref on the LJTR is 2.50 volts. AIN0_EF_CONFIG_E = 1000 // We are using the 1k version of the LJTR.

Now each read of AIN0_EF_READ_A will measure the voltage on AIN0, do the voltage divider math to determine the resistance of the RTD, and use that to calculate temperature.