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In constant current mode, the controller forces the output current, as determined by the voltage across the sense resistor, to match the setpoint, regardless of the actual output voltage. A true current source has an output impedance of infinity. Because of the finite bandwidth of the control loop, the output behaves like a true current source only at DC. At higher frequencies, the output impedance approaches the value of the series resistance. The output behaves like a current source in parallel with a capacitor.
The capacitance described above can be called virtual capacitance, because it represents a capacitance synthesized by the action of a control loop on a resistor rather than from an actual capacitor. In addition to virtual capacitance, components and interconnections in the device add real capacitance. Generally, this real capacitance is smaller than the virtual capacitance caused by the operation of the control loop, especially in high current ranges. However, some devices include large values of real output capacitance to improve performance for certain use cases.
To help reduce noise and ripple when the device is operating in a high-current range, NI recommends setting the niDCPower Output Capacitance property to High or the NIDCPOWER_ATTR_OUTPUT_CAPACITANCE attribute to NIDCPOWER_VAL_HIGH for devices that support this feature. Remember that a larger capacitance results in a slower output response. Refer to Load Regulation for more information about capacitive loads.
|Note The NI PXI-4130 supports High or Low settings on channel 1. For all other NI-DCPower devices, the niDCPower Output Capacitance property is not supported. For more information about reducing noise in high-current ranges with the NI PXI-4130, refer to NI PXI-4130 Output Capacitance Selection.|