Configuring Aperture Time for Optimal Power Line Noise Rejection with PXIe-4081 DMM

The PXIe-4081 DMM is a versatile instrument capable of high precision measurements. Users often face challenges in configuring the aperture time to balance noise rejection and measurement speed.

This article provides guidance on configuring aperture time settings on the PXIe-4081 Digital Multimeter (DMM) for effective 50 Hz or 60 Hz power line noise rejection. It also addresses the trade-offs between measurement speed and noise reduction.

1. Understanding Aperture Time and Power Line Noise Rejection: Aperture time is the duration during which the DMM integrates the input signal. For effective noise rejection at 50 Hz, the aperture time should be set to a cycle or multiple of the power line frequency, equivalent to 20 ms. For effective noise rejection at 60 Hz, the aperture time should be set to a cycle or multiple of the power line frequency, equivalent to 16,67 ms.

2. Setting Aperture Time: To suppress 50 Hz noise:

• Set PowerLineFrequency: Configure the DMM’s powerLineFrequency setting to 50 Hz.
• Adjust Aperture Time Units: Use the DmmApertureTimeUnits.PowerLineCycles setting.
• Configure Aperture Time: Set the aperture time to a multiple of 20 ms (e.g., 1 PLC = 20 ms).

To suppress 60 Hz noise, set PowerLineFrequency to 60 Hz and configure Aperture time as a multiple fo 16,67 ms (1 PLC = 16,67 ms) accordingly.

3. Trade-off Considerations:

• Noise Rejection vs. Measurement Speed: Increasing aperture time enhances noise suppression but may significantly reduce the number of samples per second, particularly if the DMM is configured for faster measurements by selecting a lower resolution (fewer digits). For example, using a 5 1/2 digits resolution results in a reduction in sampling rate from approximately 1000-2000 samples per second to around 25-50 samples per second due to the extended aperture time required for effective noise rejection. It's important to evaluate the specific requirements of your application to find the optimal balance between noise rejection and measurement speed.
• Measurement Cycle Impact: The measurement cycle duration can extend 1-2 times the aperture time, affecting overall speed.

Proper configuration of aperture time settings on your PXIe-4081 DMM is crucial for balancing power line noise rejection and measurement speed. Adapting these settings according to your application needs ensures optimal performance and data accuracy.

Example Configurations:

• Reducing Noise: Use longer aperture times for enhanced noise rejection in environments susceptible to 50 Hz interference.
• Improving Speed: Opt for default settings if high-speed measurements are critical, but can accept more noise.

C++-Style Pseudo Code Example:

MyDMM.powerLineFrequency = 50;

MyDMM.ApertureTimeUnits = DmmApertureTimeUnits.PowerLineCycles; // Default setting

MyDMM.ApertureTime = 1.0; // Equivalent to 1/50 Hz or 20 ms for noise suppression.

See NI-DMM Help for the exact usage of commands. You can find this file on your computer with a NI-DMM Driver installation, typically in this folder:

C:\Users\Public\Documents\National Instruments\NI-DMM\documentation

LabVIEW Example:

In LabVIEW, there is an example Improve DC Noise Rejection which shows a way to implement it. You can access this example via the NI Example Finder.