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I'm doing a voltage sweep in the 2450smu using SCPI commands from my laptop (with Matlab), from -1V to 1V with 0.1V steps.
In some voltage points the the actual voltage at the SMU doesn't follow the ones I defined and it seems to randomly jump to different value. In every sweep its a bit different, and sometimes it does doing good sweep.
It usually happens when the current is small (in the range of nano ampers and above).
NPLC is configured to be 5
I'm using the *OPC? command after setting different voltage to make sure it has been set and after taking a measurement.
I also added a 1sec pause after setting different voltage to let my system relax.
What can be the reason for this and how can I overcome this?
- Keithley Applications
- Posts: 1659
- Joined: October 15th, 2010, 10:35 am
- Country: United States
Since you mention low currents, I suspect you might be in range compliance.
If last current measurement used 10nA range and now you step to new voltage, the current will be limited to the value of the measure range. This can cause the requested voltage level to not be achieved if it needs more than 10nA in this example.
This compliance limiting condition is typically indicated on the front panel. Any yellow text in the voltage sourcing?
If you are using auto range, try taking a dummy current reading rather than delaying 1 second after sourcing. If the last used range was the choke point, it should uprange and remove the range induced current limit.
Here is what range compliance looks like. Notice the measurements are not continuous (Manual Trigger mode) and last measurement used 10nA range. I connect to a 10KΩ resistor. The Current Range and LIMIT annunciators turn yellow to signal that current limiting is occurring due to the range.
The actual voltage will be the limited current (~10nA) * the 10KΩ load.
If I press trigger button, a new measurement is performed and the instrument up ranges to 1mA. The desired 1V is again achieved since current limiting is no longer occurring.
I also not sure this is the case because it doesn't happens in all the measurements, just in specific ones, though all measurements at some point reaches the nA ranges and can stay there for large range of voltages (diode dark current measurements). Yet when it does happens it does that for a range of voltages. It usually start at 0V or 0.1V up to 1V or almost 1V.
I will try ,as you suggested, to perform dummy measurement every time the voltage is wrong.
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