14.1.6 Resistance [T-Series Datasheet]
Overview
AIN#_EF_INDEX: 4
This Resistance Extended Feature will calculate the resistance of a given circuit element.
Configuration
To configure, write to the following registers.
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
For results, read the following registers.
AIN#_EF_READ_A: Resistance of sensor
AIN#_EF_READ_B: Voltage of sensor
AIN#_EF_READ_C: Current through sensor
Only reading AIN#_EF_READ_A triggers a new measurement, so you must always read A before reading B or C.
Example
200UA current source, circuit #0:
An unknown resistor to be measured (a 10 kΩ resistor is used in this test) is connected in series with the 200UA current source available on the T7.
AIN0_EF_INDEX = 4
AIN0_EF_CONFIG_B = 0 -- Excitation circuit 0 (200 µA current source)
Typical reading results:
- AIN0_EF_READ_A returns 10089.7 Ω
- AIN0_EF_READ_B returns 2.012 V
- AIN0_EF_READ_C returns 199 µA
LJTick-Resistance-1k, circuit #4:
Connect a resistor to measure from Vref to VINA on an LJTick-Resistance-1k that is plugged into the AIN0/AIN1 block.
AIN0_EF_INDEX = 4 // Set AIN_EF0 to RTD100.
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 and do the voltage divider math to determine the resistance of the resistor.