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14.1.0.1 Excitation Circuits

Excitation Circuits:

AIN_EF modes that measure sensors usually need an excitation circuit. The Excitation circuit provides the conditions that the sensor needs to operate and controls how the output from the sensor is measured. There are currently 6 excitation circuits used by AIN_EF. Individual AIN_EF modes may only support a subset of the circuits listed here.




Circuit 0 – 200 µA Current Source:

This excitation circuit uses the 200 µA current source available on the T7 to power a sensor. The current source is not exactly 200 µA, so the value measured during calibration will be used for calculations. Current source excitation circuits are useful for resistance based sensors like RTDs or bridge circuits.


Circuit 1 – 10 µA Current Source:

This excitation circuit uses the 10 µA current source available on the T7 to power a sensor. The current source is not exactly 10 µA, so the value measured during calibration will be used for calculations. Current source excitation circuits are useful for resistance based sensors like RTDs or bridge circuits.


Circuit 2 – Custom Current Source:

This excitation circuit uses a current source external to the LabJack. The current provided by the source is specified during configuration of the AIN#_EF. Current source excitation circuits are useful for resistance based sensors like RTDs or bridge circuits.


Circuit 3 – Shunt Resistor:

This excitation circuit uses a resistor in series with a sensor. When using this circuit the LabJack will first measure the voltage across the sensor, then the voltage across the shunt resistor is measured. The second measurement uses a second analog input which is specified in the AIN#_EF configuration. The shunt resistance is also specified during configuration. After measuring both voltages the current through and the resistance of the sensor can be calculated.

Differential readings can reduce noise. If using and even numbered AIN_EF then the analog input can be set to differential. The differential reading will be less susceptible to noise. The odd AIN can then be used as the second AIN which is used to measure the voltage across the shunt resistor.

This circuit attempts to compensate for power supply noise. If better readings are needed then


Circuit 4 – Known Voltage Source:

This excitation circuit uses a voltage source and a shunt resistor. Values for the output of the voltage source and the resistor must be provided during AIN#_EF configuration. When using this circuit the LabJack will measure the voltage between the sensor and the resistor then calculate the resistance of the sensor.


Circuit 5 – Voltage Source:

This excitation circuit is set up the same as circuit 4, but it is measured differently. Only the value of the shunt resistor needs to be specified. When using this circuit the LabJack will measure the voltage across the shunt resistor then measure the voltage across the sensor. Resistance of the sensor can then be computed. If the analog input is set to differential then multiple sensors can be used in series. The voltage source should be low-noise.