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14.1.0.1 Excitation Circuits

Overview

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.

Three types of excitation circuits are based on a current source and three are based on a voltage divider. Individual AIN_EF modes may only support a subset of the circuits listed here.

Current Source Excitation Circuits

Current source circuits are the simplest of the supported excitation circuits.

Circuit 0 – 200 µA Current Source - T7 Only:

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 - T7 Only:

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. 

Voltage Divider Excitation Circuits

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

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. 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.

T7 only: differential readings can reduce noise. For more information, see the Differential Readings note below.

This circuit attempts to compensate for power supply noise.


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.

The voltage source needs to be stable and low noise.



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. The voltage source should be low-noise.

T7 only: If the analog input is set to differential, then multiple sensors can be used in series—for more information, see the Differential Readings note below.

Differential Readings - T7 Only

Multiple sensors can be connected in series. When using a series of sensors, both sides of the sensor 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 sensors can not exceed the current source's maximum output.

For circuit types 3 and 5 above, differential readings can reduce noise. To configure, the AIN_EF channel must be set as the positive AIN channel and the negative AIN channel must then be used as the second AIN (which is used to measure the voltage across the shunt resistor).

See 14.0 AIN for more on differential readings.