To measure power-supply output ripple, probe the specified output point with a safe connection and the shortest practical ground return, verify the DC output, then isolate and measure the AC variation. Report the result as peak-to-peak (Vpp) or RMS, together with the probe, bandwidth, coupling, load, and measurement location. A long probe ground lead, unsuitable bandwidth, or unsafe ground connection can produce a misleading reading—or damage equipment.
What output ripple voltage means
Output ripple is the residual AC voltage superimposed on a supply’s nominally DC output. It may appear alongside random noise, switching spikes, transients, or oscillation, so a waveform that looks “dirty” does not necessarily represent one phenomenon.
- Ripple is usually periodic or quasi-periodic variation associated with rectification or switching. Linear supplies often have a component near twice the AC line frequency—about 100 Hz on 50-Hz systems or 120 Hz on 60-Hz systems. Switching supplies can have ripple at the switching frequency, often hundreds of kilohertz or higher, plus harmonics.
- Noise refers to random or broadband components, which may be mixed with switching spikes and electromagnetic interference.
- Transient response is a temporary output deviation caused by a change in load or input; it is not the same as steady-state ripple.
- Output oscillation may indicate control-loop instability and can show up as a sustained low-frequency or growing waveform.
- PARD means periodic and random deviation, a broader supply-quality term used in some specifications.
Supply ripple is commonly stated as peak-to-peak voltage, RMS voltage, or occasionally a percentage of the DC output. These describe different properties; use the form required by the specification you are checking.
Vpp, RMS, peak, and frequency
- Vpp is the maximum observed voltage minus the minimum observed voltage in the measurement window. It is common in ripple specifications.
- RMS describes the effective AC component and is useful for noise and power calculations. Its value depends on measurement bandwidth and time window.
- Peak amplitude helps identify spikes that might affect sensitive downstream circuitry.
- Frequency or spectrum can help distinguish line-frequency ripple, switching components, harmonics, control-loop oscillation, and EMI.
For a sine wave only, VRMS = VPP ÷ (2√2). Switching-ripple waveforms are not generally sinusoidal, so that conversion cannot be assumed.
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Check measurement safety before connecting a probe
A conventional bench oscilloscope’s probe ground is connected to protective earth. Connect a standard single-ended probe’s ground only to a circuit point that is safely at earth-referenced ground, such as the intended return of an isolated, grounded low-voltage supply. Clipping it to a live, floating, or high-side switching node can short that node to earth, damage the device under test (DUT), probe, or scope, and create shock or fire hazards.
For a floating output, a high-side node, or a measurement where neither point is safely at earth potential, use a properly rated differential probe and follow the applicable safety procedure. Check its differential, common-mode, and transient voltage ratings, bandwidth, and safety category for the actual circuit. Never remove the oscilloscope’s protective-earth connection or use a cheater plug to make a single-ended probe seem safe. Tektronix explains the risks of probe grounding and the use of differential probing in its power-converter probing guidance.
Choose the probe and measurement point
Pick the point that matches the question
- Across the output capacitor: Often useful for examining ripple generated by a converter.
- At the load terminals: Shows what the load receives, including cable, connector, and return-path effects.
- At a designated test point: Use this location when the manufacturer’s specification defines a particular point or test fixture.
Measurements at regulator pins, output capacitors, and remote loads are not automatically interchangeable. Cable impedance and load current can change the result. Keep the tip and return close together, and use the same location as the requirement you intend to compare against.
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Match the probe to signal level and safety
| Probe approach | Useful when | Trade-offs and checks |
|---|---|---|
| 10× passive probe | Initial voltage verification and ordinary ground-referenced measurements where the voltage range and bandwidth fit. | Its attenuation can make millivolt-level ripple harder to resolve. Confirm the scope’s probe factor matches the probe. |
| 1× or other low-attenuation probe | More vertical sensitivity is needed for small, safely ground-referenced signals. | Often has lower bandwidth, greater circuit loading, and a lower voltage limit. Check the specific probe’s ratings. |
| Power-rail probe | Low-level ripple on a larger DC rail requires low noise, low loading, offset, or a suitable 50-Ω signal path. | Its capabilities and connection requirements vary. Keysight describes these techniques for rail-ripple work in its power-rail measurement overview. |
| Differential probe | The measurement is floating, high-side, or not safely referenced to earth. | Choose for common-mode and transient ratings as well as differential voltage and bandwidth; a differential probe is not automatically more accurate in every setup. |
Probe characteristics are model-specific. Tektronix gives an example in which a 3-mV ripple signal is difficult to resolve with a 10× probe; it also notes that many 1× probes have bandwidth around 15 MHz while 10× probes commonly offer more bandwidth and voltage range. Those are example characteristics, not universal specifications. See its probe selection and measurement guidance.
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Use a short return connection
Replace the long alligator ground lead with a probe ground spring or another short, low-inductance connection where possible. The loop formed by the probe tip and a long lead can pick up magnetic fields; the lead’s inductance can also interact with probe capacitance. Fast switching edges can excite ringing in that loop. Overshoot, ringing, or spikes that shrink or disappear when the connection is shortened may be measurement artifacts rather than voltage present across the output. Tektronix discusses this effect in its power-converter probing note.
Consider loading before using a 50-Ω input
A 50-Ω input or coaxial path can reduce susceptibility to pickup in an appropriate rail-measurement setup, but it can heavily load a supply that cannot drive 50 Ω. Before enabling 50 Ω, confirm the supply can safely drive it, check the scope input’s maximum voltage, and use any intended probe or attenuator. Do not connect a supply directly to a 50-Ω input without calculating the resulting current. Keysight describes 50-Ω paths as one technique for low-level rail measurements, not as a universal connection method: power-rail measurement overview.
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Measure ripple step by step
- Establish a safe reference. Identify whether the output is earth-referenced. Use a single-ended probe only when its ground can be connected safely to the intended return; otherwise use an appropriately rated differential probe.
- Connect at the required point. Probe directly across the output capacitor, load terminals, or specified test point. Use a ground spring or other short return.
- Verify the probe factor. Start with a 10× passive probe for initial voltage verification when appropriate, and check that the scope channel is set to the actual probe attenuation. A mismatch—for example, scope set to 1× with a 10× probe—makes displayed voltage wrong by a factor of ten.
- Start in DC coupling. Check the complete output level and confirm that the probe is connected as expected. DC coupling also lets you observe startup or other slow behavior that may be hidden later.
- Switch to AC coupling for a closer ripple view. AC coupling blocks or attenuates the DC component so the small AC variation can be displayed at greater vertical sensitivity. It can also attenuate slow changes and hide startup, dropout, load transients, or low-frequency components.
- Set the vertical scale. Begin conservatively and reduce volts per division until ripple uses a useful part of the display without clipping. A low-attenuation probe can help with a small signal only if its voltage, bandwidth, and loading limits are suitable.
- Set the time base and trigger. Show several cycles of the ripple. Use a slower, typically millisecond-per-division view for 100/120-Hz line-related ripple; for switching ripple, begin with several switching periods on screen. Trigger on the output for periodic ripple. Trigger on a switching node only with a properly rated probe and safe setup.
- Choose acquisition and bandwidth deliberately. Use full bandwidth first to discover components, then apply any bandwidth limit required by the specification. Adjust trigger level or coupling, reduce noise bandwidth, or use a longer record if a periodic waveform is hard to stabilize.
- Measure the required quantities. On the scope, select a measurement such as Measure → Peak-to-Peak and Measure → AC RMS or RMS, depending on the instrument. Check the waveform source, gates, and time window so the result does not accidentally include startup, unrelated spikes, or probe ringing.
- Repeat under defined operating conditions. Measure at the specified input voltage and load current, and record the setup. If the device’s ripple requirement specifies a bandwidth, coupling method, or fixture, reproduce those conditions for comparison.
Some oscilloscopes provide an automated “Output Ripple” or power-analysis measurement. Its result depends on its bandwidth, coupling, interval, and statistical method; check these settings rather than treating the label as a complete definition. Keysight’s InfiniiVision HD3 Series PWR Users Guide describes output-ripple analysis.
Set bandwidth and acquisition to fit the measurement
Bandwidth should account for the converter’s switching frequency, edge rise and fall times, ringing frequency, harmonics the specification intends to include, and the bandwidth of both probe and oscilloscope. It should also match the supply manufacturer’s stated test method where applicable. Tektronix gives a rule of thumb of roughly five times the fastest signal speed and emphasizes that the fastest edge—not simply the switching frequency—sets the requirement: probing techniques for power converters.
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- Too much bandwidth with a poor connection can fill the display with pickup and probe-induced ringing.
- A defined bandwidth can improve repeatability when the specification sets one. A 20-MHz limit is a common diagnostic option, not a universal requirement; it changes the result if it filters components the requirement intends to count.
Tektronix’s power-supply analysis note discusses ripple and bandwidth limiting, including the risk of removing needed harmonics: Power Supply Measurement and Analysis.
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For stable periodic ripple, averaging can reduce random noise and make the repeating component easier to see. It can also hide intermittent spikes, switching bursts, transients, or unstable behavior. Use persistence, segmented memory, or single-shot acquisition when investigating events that may occur only occasionally, and state whether averaging was enabled.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Interpret the waveform without assuming its shape
Frequency and behavior are more informative than appearance alone. Line-related ripple may repeat near twice the mains frequency; switching ripple may track the converter’s switching rate and its harmonics. Ringing can follow fast switching edges, while a slower sustained oscillation may point to control-loop behavior. Random noise and occasional bursts may coexist with all of these.
Triangular, sinusoidal, or parabolic shapes are not universal signatures. Waveform shape depends on converter topology, inductor current, output-capacitor ESR and ESL, switching transitions, control mode, load, and measurement bandwidth. Check frequency, operating conditions, and whether the trace changes when you improve the probe connection before diagnosing the supply from shape alone.
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Troubleshoot a noisy, missing, or implausible reading
- Reading looks too large: Shorten the ground connection, probe directly across the output capacitor, move away from transformers and switching nodes, and check whether the spikes change. Excessive bandwidth, loop pickup, probe ringing, poor ground return, or calibration error can inflate the display.
- Reading looks too small: Check for a 10× probe on a very small signal, excessive volts-per-division, bandwidth filtering, scope noise-floor limits, and probe loading. Confirm that AC coupling is not attenuating behavior at the frequency of interest.
- Trace changes when the probe moves: Compare locations deliberately. The change may reflect pickup, circuit loading, or a real ground-return or cable effect. Repeat with a short spring connection and identify the exact point.
- Trigger will not stabilize: Trigger on the output or an appropriate related signal, adjust level and coupling, reduce noise bandwidth, or use a longer record for low-frequency ripple.
- There are occasional spikes: Compare a single acquisition with averaging and use persistence or segmented capture. Averaging may make periodic ripple clearer while concealing infrequent events.
- Output-capacitor and load readings differ: Record both locations if useful; cables, connectors, load current, and return paths can make them genuinely different. Compare against the location specified by the requirement.
- Waveform clips or seems inconsistent: Check probe voltage rating, scope input range, probe factor, attenuation, bandwidth, and calibration. Do not continue probing if the connection or voltage rating is uncertain.
If the setup checks out, examine the output capacitor, its ESR, layout, ground return, and load wiring, then repeat at several input voltages and load currents. A second probe or scope channel can help separate setup effects from behavior at the test point.
Worked setup: a nominal 5-V regulator
Suppose the goal is to measure a nominal 5-V regulator under a defined 1-A load. First verify approximately 5 V in DC coupling. Then use the lowest safe probe attenuation that does not exceed probe or circuit limits, connect with a short ground spring at the required output point, and switch to AC coupling to inspect the small ripple. Set the vertical scale so a suspected 20-mVpp signal spans multiple divisions, and measure Vpp and AC RMS over a stable time window.
If the requirement calls for a 20-MHz bandwidth limit, repeat with that limit and report it; do not silently compare the filtered result with a full-bandwidth measurement. A properly qualified report might read: “8.6 mVpp, 2.1 mVrms, measured at the output capacitor, 5-V output, 1-A load, 10× passive probe, 20-MHz bandwidth limit.” Those figures are an example of reporting format, not a claimed test result for a particular regulator.
Report enough detail for the result to be repeatable
A ripple number is not portable without the conditions that produced it. State the output voltage, input voltage, load current, measurement location, probe type and attenuation, scope bandwidth or limit, coupling, measurement window, and whether averaging was used. Include Vpp or RMS explicitly; “12 mV ripple” does not say which quantity was measured.
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Reusable format: Output ripple: ___ mVpp, ___ mVrms, measured at ___ under ___ V input, ___ V output, and ___ A load, using a ___ probe at ___ attenuation, ___ coupling, ___ MHz bandwidth (or full bandwidth), over ___, with averaging ___.
When another measurement tool is appropriate
A short-ground single-ended probe is often enough for a safely grounded low-voltage output. A differential probe is appropriate for a floating or high-side measurement; a power-rail probe or low-noise rail-measurement setup may help when the ripple is so small that ordinary probing cannot separate it from instrument noise. Spectrum analysis can help identify frequency-dependent components, while power-analysis software can automate repeatable measurements. For compliance or a datasheet comparison, follow the applicable specification and its test fixture rather than substituting a visually cleaner scope trace.
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