NI PXIe-4144/4145 DC Noise Rejection

NI DC Power Supplies and SMUs Help (NI-DCPower)

Edition Date: January 2018

Part Number: 370736U-01

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DC noise rejection is a configurable noise reduction feature available for NI PXIe-4144/4145 voltage and current measurements. Each reading the device returns is actually an average of multiple high-speed samples if the selected aperture time is greater than the device’s 1.67 µs minimum aperture time. Valid aperture times are integer multiples of the minimum aperture time. By adjusting the relative weighting of those samples, you can adjust the sensitivity to different interfering frequencies.

Note Note  The NI PXIe-4144 does not support second-order DC noise rejection.

The following table describes the NI-DCPower DC noise rejection settings.

DC Noise Rejection Setting Lowest Frequency Rejection Notch High-Frequency Noise Rejection
Normal 1/taperture Good
Second-order 2/taperture Better

Normal DC Noise Rejection

When you select Normal DC noise rejection, the NI PXIe-4144/4145 weights all samples equally. This setting mimics the behavior of most traditional SMUs. The following figure shows normal weighting, where all samples are weighted equally.

The following figure shows the resulting noise rejection as a function of frequency.

Notice that you can obtain good rejection only very near multiples of 1/taperture. To achieve the fastest possible readings with good powerline noise rejection, set the aperture to 1 power line cycle (PLC), and set the DC noise rejection to Normal.

Second-Order DC Noise Rejection

When you select Second-order DC noise rejection, the NI PXIe-4145 applies a triangular weighting to the measurement samples, as shown in the following figure. In second-order DC noise rejection, the samples taken in the middle of the aperture time are weighted more than samples taken at the beginning and the end of the measurement.

The following figure shows the resulting noise rejection as a function of frequency.

Notice that the device provides superior noise rejection near even multiples of 1/taperture, and noise rejection increases more rapidly with frequency than with normal sample weighting. Also, the notches are wider than they would be with normal weighting, resulting in less sensitivity to slight variations in noise frequency. Use second-order DC noise rejection if you need better powerline noise rejection than you can obtain with normal DC noise rejection or you need better high-frequency noise rejection than normal DC noise rejection offers. To achieve the fastest possible readings with excellent powerline noise rejection, set the aperture to 2 PLC, and set the DC noise rejection to Second-order. In this configuration, one measurement is produced in the first full aperture followed by two measurements per subsequent aperture times, resulting in approximately the same measurement rate as normal filtering for large measure records.

Configuring DC Noise Rejection

Use the niDCPower DC Noise Rejection property or the NIDCPOWER_ATTR_DC_NOISE_REJECTION attribute to configure DC noise rejection.

Related Topics

Aperture Time

Noise

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