Noise is unwanted signals present on the output channels that can affect devices connected to the output channels.

Noise can be characterized as normal-mode noise or common-mode noise. Regardless of its characterization, noise is meaningful only when it is specified with an associated bandwidth.

  • Common-mode noise—Noise present between the Output common LO terminal and the chassis or earth ground. In this sense, the equivalent circuit is a current noise source connected across these two terminals. When you connect an impedance between the output common/ground and chassis or earth ground, a noise current can flow in the impedance, resulting in an unexpected offset or other undesirable error.
  • Normal-mode noise—Noise present between the Output HI terminal and the Output common LO terminal, appearing either in series (constant voltage mode) or parallel (constant current mode) with the output of the device. Normal-mode noise can be expressed as voltage noise or current noise, depending on the control mode of the output channel.

    AC-to-DC rectification causes ripple, a type of periodic normal-mode noise.

    • Verifying Output Noise Specifications

    Exercise care when verifying the noise specifications of an output device, such as a power supply or SMU. When verifying the specified wideband noise of a device, the effects of ground loops, unnecessarily long probe ground leads, and electrically noisy environments can combine and skew your measurements.

    Observe the following recommendations when verifying the output noise specifications of a power supply or SMU:

    • Connect the probe directly to the terminals of the power supply or SMU. Do not use long leads, loose wires, or unshielded cables.
    • Limit the probe ground lead to 2.54 cm (1 in.) at most. Connect this lead directly to the output common/ground terminal of the appropriate channel.
    • Set the bandwidth of the measurement device to the bandwidth of interest.
    • Exercise caution when making measurements in a modern laboratory environment—with computers, electronic ballasts, switching power supplies, and so on—to avoid measuring the environment noise instead of the device noise.

    AC and DC Noise Rejection

    You can manipulate the aperture time of measurements made with SMUs and power supplies to reject specific AC noise frequencies in DC voltage and current measurements.

    Each measurement that an NI-DCPower instrument returns is an average of one or more higher-speed samples. All instruments return a multiple of 50 Hz and/or 60 Hz to enable rejection of power line noise.

    You can reject AC noise by adjusting the measurement aperture time to be a multiple of the AC noise period.

    You can reject the frequency of noise by adjusting the aperture time to be a multiple of an AC noise frequency with Period = 1/f.

    Normal DC Measurement Noise Rejection

    With normal noise rejection, the instrument assigns equal weight to each sample. This setting mimics the behavior of most traditional power supplies and SMUs.

    Normal noise rejection is the default behavior for all NI-DCPower instruments.

    The following figure shows normal weighting, with aperture times on the x-axis and relative weighting on the y-axis.

    Figure 20. Normal Noise Rejection


    The following figure shows the resulting noise rejection as a function of frequency, with multiples of 1 / Aperture Time on the x-axis and magnitude response, in dB, on the y-axis.

    Figure 21. Normal Noise Rejection by Frequency


    The best frequency rejection is available only near integer multiples of 1 / Aperture Time. You can achieve the fastest possible readings along with good power-line noise rejection by setting the aperture to one power-line cycle (PLC) and noise rejection to Normal.

    Second-Order DC Measurement Noise Rejection

    With second-order noise rejection, the instrument assigns a triangular weighting to measurement samples. Samples taken in the middle of the aperture time have more weight than samples taken at the beginning and end of that measurement.

    The following figure shows second-order weighting, with aperture times on the x-axis and relative weighting on the y-axis.

    Figure 22. Second-Order Noise Rejection


    The following figure shows the resulting noise rejection as a function of frequency, with multiples of 1 / Aperture Time on the x-axis and magnitude response, in dB, on the y-axis.

    Figure 23. Second-Order Noise Rejection by Frequency


    With second-order noise rejection, the instrument provides superior noise rejection even near multiples of 1 / Aperture Time, and noise rejection increases more rapidly with frequency compared to normal noise rejection. Notches are also wider than they would be with normal weighting, which results in less sensitivity to slight variations in noise frequency.

    Use second-order noise rejection if you need better power-line noise rejection or better high-frequency noise rejection than you can obtain with normal noise rejection.

    You can achieve the fastest possible readings with second-order noise rejection, along with excellent power-line noise rejection, by setting the aperture to two power-line cycles (PLC) and noise rejection to Second-Order.

    In this configuration, one measurement is produced in the first full aperture, followed by two measurements for each subsequent aperture time. This results in approximately the same measurement rate as normal filtering for large measure records.

    Choosing an AC Noise Rejection Profile

    You have a choice of AC noise rejection profiles: normal and second-order. Normal noise rejection is the default noise rejection behavior for all NI-DCPower instruments, while second-order noise rejection can provide better frequency rejection in some situations.

    The length of the measurement aperture time affects which noise frequencies are rejected. The noise rejection profile changes how frequencies are rejected with respect to the measurement aperture time and affects the minimum time required for the instrument to make a single measurement.
    Choose the AC noise rejection profile that suits your application based on the following criteria.
    Lowest Frequency Rejection Notch High-Frequency Noise Rejection Minimum Measurement Time Required Recommended Noise Rejection Profile
    1 / Aperture Time Good Shorter: Aperture Time Normal (default)
    2 / Aperture Time Better Longer: 2 × Aperture Time Second-Order

    Rejecting AC Noise in DC Measurements with Aperture Time

    Directly adjusting the aperture time of your measurements allows you to reject specific AC noise frequencies in your DC measurements with NI-DCPower.

    Complete the following steps to reject AC noise frequencies by adjusting the aperture time of your measurements.

    1. Choose the noise rejection profile that suits your application.
      • Normal
      • Second-Order
    2. Based on the aperture time units and the noise rejection profile you intend to use, calculate the aperture time required to reject the frequency f (Hz) you need to reject.
      • Aperture time units: seconds
        Noise Rejection Profile Target Aperture Time (s)
        Normal (default) Aperture Time = 1 / f
        Second-Order Aperture Time = 2 / f
      • Aperture time units: power line cycles (PLC)
        Noise Rejection Profile Power Line Frequency Target Aperture Time (PLC)
        Normal (default) 60 Hz Aperture Time = 60 Hz / f
        50 Hz Aperture Time = 50 Hz / f
        Second-Order 60 Hz Aperture Time = 2 × (60 Hz / f)
        50 Hz Aperture Time = 2 × (50 Hz / f)
      Note Each NI-DCPower instrument supports discrete aperture times: an instrument-specific minimum value and integer multiples of that value. When you set an unsupported aperture time, NI-DCPower coerces the value to the nearest longer supported value for your instrument.
    3. Configure the aperture time you calculated.
      1. Set the aperture time and the appropriate units with Configure Aperture Time.
      2. If using power line cycle units, provide the frequency of the AC power line for your system to Configure Power Line Frequency.
    4. Use DC Noise Rejection to set the noise rejection profile you chose.