PXIe 4139 signal randomly becomes very noisy

[Basjong53]

I am trying to measure the current of a photosensitive device using a PXIe 4139. The current is in the order of a few nA. When I turn on a light, the current increases and I need to record the trend of the current. I'm having difficulty in setting up the SMU to get a clean signal. It only works sometimes, for some reason the signal switches between clean and super noisy randomly, without interfering in the measurement at all. 

Here is an example. After 6 seconds, the noise starts. I didn't change anything in the setup/software.

Here the signal starts noisy and suddenly became clean.

Using instrument studio, I don't see any noise.

Here is the code of the above examples:

What could be the problem here?

[santo_13]

To be clear, this happens only with your DUT (photosensitive device)?

InstrumentStudio settings are not the same as your code.

A few best practices,

1. Disable measurement auto range
2. Choose the smallest current range that works
3. Try Local vs Remote sensing
4. Set aperture time to #N PLC instead of seconds to ensure you eliminate power line noise