Ripple and Noise

There’s no universally-accepted method of measuring ripple and noise. Some sellers include external circuitry in making the measurements, so to duplicate their results, you will need to contact them how they make their measurements. The easiest measurement is to connect an AC-coupled oscilloscope to the output of the power supply. The measurement can be made of common-mode noise (noise on both + and – outputs of the power supply with regard to the AC power ground) or normal-mode (also called differential-mode) noise, which is the noise seen between the + and – terminals of the power supply. Note: since the outside of the BNC connector on many scopes is connected to the power ground, you’ll have to use an isolation transformer to power the scope or use a differential amplifier to measure the normal-mode noise.

Ripple for linear power supplies is usually measured at twice line frequency. For switching power supplies, you’ll want to examine higher frequencies and may see voltage spikes. Ripple can be defined as the portion of unfiltered AC voltage and noise present at the output of a filtered power supply when operated at full load and is typically specified in volts RMS. Noise on the other hand is is typically specified as peak-to-peak AC voltage and can be defined as the portion of unfiltered and unshielded EMI noise present at the output of a filtered power supply when operated at full load.

It can be important to know what bandwidth the noise is specified over. Often it is 20 MHz, as an oscilloscope is used to measure it. Note: sometimes ripple and noise is specified as PARD, which
is an acronym for “periodic and random deviations”.

Most linear power supplies should have less than 3 mV RMS ripple and less than 50 mV peak for switching supplies

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