By John Papiewski | Updated March 24, 2022
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In modern electronics, the purity of a DC power supply can be the difference between reliable operation and erratic behavior. While some devices tolerate a little ripple, precision equipment—such as instrumentation amplifiers or analog-to-digital converters—requires ripple levels below a few millivolts. Over time, electrolytic capacitors in a supply degrade, and the ripple rises, potentially degrading performance or even damaging sensitive circuits. The most reliable way to assess ripple is with an oscilloscope set to AC coupling, which filters out the DC component and reveals the remaining AC fluctuations.
Power on both the supply and the oscilloscope. Set the oscilloscope input to AC coupling to reject the DC bias. Configure the trigger to line mode and choose a sweep time of about 0.1 s (1 Hz). Set the vertical scale to 100 mV/div and zero the horizontal axis.
Using a multimeter set to DC volts, record the supply’s nominal output voltage. If the supply is adjustable, set it to the desired value before measuring. Note the reading, then disconnect the meter to avoid loading the circuit during the oscilloscope measurement.
Attach the oscilloscope probe’s ground clip to the supply’s ground. If the probe offers a 1× sensitivity switch, leave it on 1×. Connect the probe tip to the positive output rail.
Look for a ripple waveform on the screen. If present, adjust the vertical sensitivity so the peak-to-peak trace spans a few divisions. Count the number of divisions occupied by the ripple, then multiply that count by the vertical scale to obtain the actual peak-to-peak ripple amplitude.
Divide the ripple amplitude by the DC voltage measured in Step 2 and multiply by 100 to express ripple as a percentage of the supply voltage.
Precision supplies typically exhibit ripple well below 1% of their output voltage. Use a clean, low-noise environment to avoid masking the true ripple with external RF or mains interference. For supplies rated at 10 V or less, placing a 100 Ω, 5 W resistor between ground and positive output during measurement can simulate a load and reveal worst-case ripple.
