Measure the Noise on a Power Supply Output
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Measuring the AC noise on a DC output is one of the most common tests performed on a power supply. The measurement techniques we will be describing are primarily applicable for linear power supplies and switching power supplies with frequencies less than 50 kHz. A linear power supply uses an AC to DC transformer to convert the line voltage to a stable DC output. Hence, the voltage output of a linear power supply generally has a 50/60 Hz noise ripple, as well as any additional noise due to the power supply components.
A switching power supply converts the 50/60 Hz current (line voltage) to a much higher frequency. These power supplies are more compact, but are often very susceptible to high frequency noise due to magnetic losses and component parametrics that limit filter efficiencies.
Noise on these power supplies can be measured and expressed as root mean square (RMS) or peak to peak. RMS gives you a power equivalent of what the noise ripple on voltage output looks like, but doesn’t give you all of the frequency information. For example, RMS measurements do not expose transients because their energy contribution is often too small.
The type of power supply you're testing will determine which measurement techniques is appropriate. However, many test systems today use both measurement techniques, which requires the following measurement capabilities:
The FlexDMM is ideal for power supply testing because it can operate as both a full-featured 6 1/2 Digit Multimeter and high-voltage, isolated digitizer. Hence, you no longer need two separate measurement devices, a multimeter (RMS measurements) and a oscilloscope (peak-to-peak measurements), to accurately analyze a power supply output. When operating as a isolated digitizer, the FlexDMM has the ability to acquire up to 300 V waveforms at a max sample rate of 1.8 MS/s. The FlexDMM has 10 bits (noise-free) at the 1.8 MS/s sample rate, but as you decrease the sample rate it achieves up to 23 bits of resolution. Once you have acquired the power supply voltage waveform with the FlexDMM, you then use LabVIEW analysis VI’s to perform the required noise measurements, as shown in Figure 1.
Related Links:
Configure the NI PXI-4070 FlexDMM to acquire high-speed waveforms
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