Why is my Source Measure Unit (SMU) Sequence Only Stepping Every 15 ms?

Updated Jul 26, 2018

Reported In

Hardware

  • PXI Source Measure Unit

Issue Details

I have created a sequence in DCPower for my Source Measure Unit (SMU) to output and have reduced the source delay time, however, it seems that the steps always advance every ~15 ms. My SMU is specified to have a much faster update rate.
  • How do I achieve faster update and sampling rates?
  • Is there a way to speed up my SMU?

Solution

By default, NI SMUs take a measurement every time the output updates to a new value in a sequence.  The Aperture Time property dictates how much time is required to take a single measurement.  The default aperture time for our SMUs is one Power Line Cycle (PLC) or 16.66 ms (1 / 60 Hz = 16.66ms). The amount of time each point in a sequence takes is equal to the sum of the programmed aperture time and source delay with an additional few microseconds of overhead while the device updates to the next setpoint.

The aperture time can be adjusted in LabVIEW by either using the Configure Aperture Time VI or Aperture Time Property. If using the property, you can specify the time units using the Aperture Time Units property. You can use the NI-DCPower Advanced Sequence Changing Aperture Time.vi shipping example as a reference.

The aperture time can be adjusted in text-based by either using the niDCPower_ConfigureApertureTime function or the NIDCPOWER_ATTR_APERTURE_TIME attribute. If using the attribute, you can specify the time units using the NIDCPOWER_ATTR_APERTURE_TIME function. 

Users can choose whether to specify aperture time in seconds or power line cycles by changing the Aperture Time Units property.  

For more information on aperture time, and its programming functions, properties and attributes please see the NI-DCPower Help linked below, or found at Start»Programs»National Instruments»NI-DCPower»Documentaion»NI DC Power Supplies and SMUs Help if you have the NI-DCPower drivers installed.

If you do not wish to take measurements every time the output updates during your sequence, you can effectively turn off the measurement engine by changing the Measure When property from the default value of Automatically After Source Complete to On Measure Trigger.

Additional Information

Aperture time is defined as the time that the analog to digital converter (ADC) on the SMU takes to read a voltage or current value. It is important to note that measurement resolution, measurement speed, and frequency rejection are all functions of aperture time. Therefore, you will have to examine the specifications for your particular SMU to understand the effects of changing the aperture time. In general longer apertures times offer higher measurement resolution, and shorter times improve the measurement speed. 

Aperture is not adjustable on the PXI-4110, PXI-4130, or PXIe-4154. Use the Samples to Average property to change the reading rate on these devices.

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