PXI
Sourcing+Measuring over Channels / Measuring-Issue aperture time
Hello PXI and LabVIEW community,
We're currently facing an issue with our SMU setup and would appreciate your insights. Here's the setup we're using:
- PXIe-1083 chassis
- PXIe-4140 4-channel SMU
- PXI-2503 relay multiplexer
Our objective is to apply pulses across 1 to 12 channels and perform measurements for each pulse generated by the SMU. The target pulse width is relatively short, ranging from 15 to 60 µs. We aim to achieve the highest possible measurement resolution. The same pulse is repeated across all selected channels. In the small VI snippet I provided, the program currently pulses voltage. However, in the main application, our goal is to pulse current instead.
The diagram below illustrates the intended pulsing behavior:
While testing our program, we observed some irregular behavior. It's important to note that although the trigger lines are implemented in the software, the switch is not physically connected to the SMU. Instead, the SMU's sourcing and measurement cables are directly wired to a simple resistor. We monitored the pulsing behavior using an oscilloscope and noticed an unexpected trend: the pulse duration increases as the measurement record length becomes longer.
To illustrate this effect, we've prepared a few examples demonstrating how the pulse width changes with different record lengths:
- We used an aperture time of 0.5 µs, but the issue persists even when increasing it to 1 µs.
- In each oscilloscope screenshot, the white lines represent the cursor positions. On the right-hand side, you can see the corresponding Δt value between the cursors, which indicates the measured pulse duration.
In this case, the pulse width corresponds exactly to our expected value.
As shown on the oscilloscope, the observed pulse width is significantly longer than expected. While we anticipate a pulse width of 25 µs, the actual measurement shows a duration of approximately 68 µs.
The final pulse example I’d like to present is approximately three times longer than expected. While we anticipated a duration of around 50µs, the measured pulse width is significantly extended.
In summary, we’ve observed that increasing the measurement record length leads to an unintended extension of the pulse width. Maintaining a short pulse duration is critical for our application.
Additionally, we prefer not to adjust the aperture time for each pulse segment, as doing so would compromise the integrity of the measurements.
We hope someone can help us with this issue. Unfortunately, we haven’t been able to find any documentation that addresses it. There might be a simple fix, but we suspect our method of generating the pulse is somewhat unconventional. Additionally, the pulsing VIs are disabled for the PXIe-4140, which limits our options.
As you already know, the right way to do this is to use a SMU with pulsing capability such as 4139 where you get a very accurate pulse.
You seem to use the advanced sequencing. To keep things simple, start with preparing the core logic to control SMU to generate predictable pulses, once you achieve that, then you can include sync with relays to sweep over channels and profiles.
