PXIe-4154 Source Instability with Reactive Loads

A typical connection of a PXIe-4154 to a battery-operated device includes a cable and some type of test fixture, each element of which adds impedance.

This impedance includes:

  • The source and sense leads have impedance per unit length;
  • The test fixture has some parasitic impedance; and
  • The battery-operated device presents a different level of impedance depending on its mode of operation.

These impedances all become part of the power circuit, which can induce instability.

Source Instability: Reactive Loads in Constant Current Mode

This instrument may experience instability with inductive loads in certain circumstances in constant current mode.

When operating in constant current mode, especially in higher current ranges, some inductive loads may cause instability.

Instability manifests as oscillating or unregulated behavior across the output terminals, which can result in excessive measurement noise, erratic behavior, or thermal shutdown.

If you notice abnormalities, you can attempt the following to troubleshoot:

  • Verify the behavior of the channel by inspecting the voltage across the output terminals with an oscilloscope
  • Use the Output Capacitance property to toggle the output capacitor

Source Instability: Reactive Loads in Constant Voltage Mode

The PXIe-4154 may experience instability on channel 0 with reactive loads in certain circumstances in constant voltage mode.

If channel 0 is configured for fast transient response (wide-bandwidth), the channel is more susceptible to excessive ringing and/or oscillation in the presence of high-reactance loads, such as low-ESR capacitive loads and inductance in the sense and source leads.

Instability manifests as oscillating or unregulated behavior across the output terminals, which can result in excessive measurement noise, erratic behavior, or thermal shutdown.

PXIe-4154 Improving Voltage Stability for Reactive Loads

You can design connections from the PXIe-4154 to your DUT to minimize instability during constant voltage mode operation with reactive loads.

NI recommends the following options to improve the stability and performance of the PXIe-4154 voltage control loop for reactive loads while operating in constant voltage mode.
  • Reduce the impedance of the leads connecting the PXIe-4154 to the DUT
    • Reduce the length of the cabling used.
    • Reduce wire inductance by using twisted-pair wires; this also minimizes noise pick-up. Use one twisted pair for power sourcing leads and a separate twisted pair for voltage sensing leads; do not twist output and sense leads together.

      The following figure demonstrates the use of twisted pair wires to connect the power supply channel to the load.

      Figure 7. Connecting the Power Supply Channel to the Load


    • Reduce the transient response voltage dip:
      • Reduce the resistance in the path by using low-resistance relays
      • Reduce the number of relays in the connection path
      • Use low-gauge wire, such as 16 AWG or 18 AWG
  • Add an RC network (a resistor RS in series with a capacitor CS) connected across the load to improve stability.

    The inductance of the wires and capacitance at the load present a reactive load to the PXIe-4154 channel. The RC network damps the resonance of these reactive elements in the power circuit.

    Equipment that meets the following requirements will remove oscillation in most cases:

    • Resistor: 250 mΩ, rated to ≥1 W
    • Capacitor: 10 µF to 22 µF (higher capacitance further improves stability)

    The following figure illustrates the use of an RC network with the PXIe-4154.

    Figure 8. Connecting the Power Supply Channel to the Load with an RC Network