Output Impedance

NI power supplies and SMUs include output amplifiers that drive their outputs through series resistors. The resistors enable the measurement and control of output current. The value of the resistor is larger for low-current ranges and smaller for high-current ranges.

Depending on whether the device is in constant voltage mode or in constant current mode, feedback can make the output behave like a true voltage or current source at DC. At higher frequencies, there is no feedback, and the output behaves like a voltage source in series with the selected output resistor.

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. The slew rate of the instrument to a new setpoint will be limited by output capacitance in constant current mode.

In constant voltage mode, the controller forces the output voltage to match the setpoint, even when there is a voltage drop across the resistor. The slew rate of the instrument to a new setpoint will be limited by output inductance in constant voltage mode.

Output Capacitance

  • Virtual Capacitance—Represents a capacitance synthesized by the action of a control loop on a resistor rather than from an actual capacitor. 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 value of the virtual capacitance increases as the output current decreases in percent of full-scale range.
  • Real Capacitance—Capacitance added by components and interconnections in the device. 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.

    • Output Inductance

  • Virtual Inductance—Represents an inductance synthesized by the action of a control loop on a resistor rather than from an actual inductor. A true voltage source has an output impedance of zero. Because of the finite bandwidth of the control loop, the output behaves like a true voltage source only at DC. At higher frequencies, the output impedance approaches the value of the series resistance. In general, the output behaves like a voltage source in series with a parallel combination of the series resistance and an inductor.
  • Real Inductance—Inductance added by components and interconnections in the device. Generally, this real inductance is smaller than the virtual inductance caused by the operation of the control loop, especially in low current ranges.

    • Decreasing Output Capacitance

    Output capacitance has an effect on the output slew rate. You can decrease output capacitance and increase the speed of the PXIe-4112.

    Decreasing Virtual Output Capacitance

    Virtual output capacitance can significantly limit output slew rate. For example, consider the PXIe-4112 stepping from 1 V to 2 V in the 1 A range with a 20 mA compliance limit. Even in the absence of a load, the 20 mA compliance current charging the virtual capacitance limits the output slew rate. You can adjust the settings of NI-DCPower to decrease the effect of virtual output capacitance.

    You can decrease output capacitance by increasing the compliance limit. In that case, the real output capacitance does not decrease, but the current available to charge it increases. Increasing the compliance limit to full-scale value of the current range in the preceding example effectively removes the output-current-related slew rate limit.

    Decreasing Real Output Capacitance

    Real output capacitance can limit slew rate. To decrease output capacitance, you can use shorter length cabling to reduce the actual capacitance of the load.

    When slew rate is limited by the current available to charge a real output capacitance, changing ranges or GBW settings has no effect. Changing ranges or GBW settings affects only the virtual output capacitance.

    Using NI-DCPower to Decrease the Impact of Output Capacitance

    You can increase the compliance limit in NI-DCPower to decrease the impact of output capacitance. The real output capacitance does not decrease, but the current available to charge it increases. Increasing the compliance limit to 100 mA in the preceding example effectively removes the output-current-related slew rate limit.

    Note The current ADC does not measure the current that charges the virtual output capacitance. Therefore, when the output slew rate is limited by the available charging current, that current may not be measured by the current measurement circuitry.

    Decreasing Output Inductance

    Cable inductance has an effect on the output current slew rate. You can decrease cabling inductance and increase the speed of the PXIe-4112.

    You can perform any of the following actions to decrease output inductance:

    • Use shorter length cabling.
    • Reduce the loop area between the + and - terminals.