Solution
A noisy or oscillating constant current output can be caused by a multitude of different factors. This article will list some of the commonly found causes for instability, and the main considerations to make when troubleshooting.
Possible Causes for Unstable Outputs
Instability in a DC output of a SMU could be caused by the following:
- Improper grounding or signal wiring.
- External devices that introduce electromagnetic or power line interference.
- Damaged cables.
- Reactive loads.
When analysing the noisy signal, consider the shape and consistency of the noise:
- If the noise is oscillating at a regular interval such as 50 or 60 Hz, the power lines to the system may be introducing power line noise.
- If the signal is oscillating consistently when other equipment is switched on or used, external devices may be introducing electromagnetic interference.
- If the noise is visible only at the start of testing, high-start up currents from intensive equipment may be interfering.
- If there is any vibration or movement in the system, the signal instability may be caused by a continuously varying resistance (such as a damaged cable or moving load).
Troubleshooting Steps
The following tests will help to isolate the cause of the oscillation or noise:
- Check the connections on the SMU.
- Check the ground connection.
- From the device's User Manual and Pinout diagram, determine whether or not the output should be connected to ground.
- Most of NI's PXI(e) SMU cards will have a floating output with an optional connection to chassis ground.
- Check for the possibility of ground loops. Refer to Grounding Considerations for further details.
- Verify that the noise can be seen on multiple measurement instruments.
- To ensure that the measurement instrument is not introducing noise, measure the SMU output with several devices.
- Common measurement devices are Oscilloscopes and Digital Multi-Meters (DMMs).
- Test the system with another PXI(e) SMU card.
- If the problem persists with another known working SMU, then the noise will be caused somewhere else in the system.
- If the problem stops after changing the SMU, then the initial SMU card is at fault.
- Test the SMU card in another PXI(e) chassis.
- This will determine whether or not the chassis was introducing noise to the system.
- Connect a different cable to the SMU output.
- This verifies whether or not internal damage to the cable was compromising the signal integrity.
- Check the external environment for possible noise sources.
- Equipment that introduces high levels of electromagnetism can interfere with the SMU's output.
- If any motors or generators are within the vicinity, remove them and retest the system.
- Power the system from an alternative power source.
- If the behaviour changes when connecting to power sources further away, instruments in nearby proximity may be introducing power line noise.
- Connect a purely resistive load to the output of the SMU.
- Some SMUs can be sensitive to reactive loads, particularly inductive loads.
- Most of NI's PXI(e) SMU cards are equipped with SourceAdapt technology. This allows for customisation of the SMU's response to any load so that the response displays minimal overshoot, rise time and oscillation.
- Check whether the SMU supports SourceAdapt technology at PXI Source Measure Unit.