HOW-TO GUIDE
How to Find Signal Clipping on an Oscilloscope
By the OhmSavvy Research Team ·
Safety: amplifiers, speakers and the scope ground
- The ground clip of a bench scope probe is connected to mains earth. Clip it only to the circuit's ground. Bridged (BTL) amplifier outputs have no grounded speaker terminal, so measure each side to ground or use a differential probe.
- Drive power amplifiers into a resistive dummy load when you push them into clipping, and keep the level low for as short a time as possible. Clipped output can damage speakers.
What clipping looks like
A clean sine has rounded peaks. When a stage clips, the peaks stop at a fixed voltage and run flat until the signal comes back down, leaving sharp corners where the flat part starts and ends. Hard clipping, typical of op-amps and transistor stages hitting their supply rails, gives crisp corners. Soft clipping, from stages that saturate gradually, rounds the corners but still squashes the peaks. The flat part always sits at the same voltage however hard you drive the input, and that voltage is the clip level.
The giveaway
Real circuit clipping vs the scope's own clipping
On most digital scopes the analogue-to-digital converter covers only the visible screen height plus a small margin. If the V/div setting is too sensitive, the waveform runs past the top and bottom of that range and the captured data is clipped by the scope, even though the circuit is fine. Automatic measurements such as Vpp and RMS are then wrong; many scopes flag an over-range reading, but how they show it varies, so check your manual.
1Set up both channels on DC coupling
Put the stage's input on CH1 and its output on CH2, both grounds on the circuit ground, and trigger on CH1. The input is your reference for what the output should look like. Use DC coupling so you can see where the flat tops sit relative to the supply rails; AC coupling hides the absolute level. Use 10x probes with the channel attenuation set to match. For the full two-channel setup, see how to compare input and output signals.
2Fit the whole waveform on screen
Adjust V/div until the output fills roughly six of the eight vertical divisions, with space above and below. Use vertical position if a DC level pushes it off center, as explained in why a waveform shifts vertically. Never judge clipping from a trace that touches the top or bottom edge.
3Raise the input level until the peaks flatten
With a sine from a signal generator, increase the amplitude in small steps while watching CH2. The first sign is a slight flattening at the very tips, often on one side first. Note the output voltage where it begins: that is the clip level for this supply and load. If the input on CH1 flattens too, the clipping is happening before the stage, in the generator or an earlier stage.
4Change V/div to confirm it is real
Turn the output channel to the next larger V/div setting so the waveform shrinks. Real clipping keeps its flat tops at the same voltage at any scale. Display clipping disappears and the peaks come back rounded. Then measure the flat level with a cursor and compare it to the supply rails.
01
Fit on screen
About 6 of 8 divisions tall
02
Larger V/div
Do the flat tops survive?
03
Check CH1
Is the input already flat?
04
Measure level
Cursor on the flat, compare to rails
Worked example: headroom in an op-amp stage
Take a non-inverting op-amp stage with a gain of 10 on ±12 V rails. Many op-amps cannot swing all the way to their rails; say the datasheet gives an output swing of about ±10.5 V into your load. That is the clip level, so the largest clean input is 10.5 ÷ 10 = 1.05 V peak. Rail-to-rail output op-amps get closer to the rails, so always check the datasheet.
| Input (V peak) | Output needed | Output on the scope | Result |
|---|---|---|---|
| 0.3 | 3 V peak | 3 V peak | Clean, 10.9 dB headroom |
| 0.5 | 5 V peak | 5 V peak | Clean, 6.4 dB headroom |
| 1.05 | 10.5 V peak | 10.5 V peak | At the clip level |
| 1.5 | 15 V peak | Flat at 10.5 V | Clipped |
| 2.0 | 20 V peak | Flat at 10.5 V | Clipped, flat about 65% of each cycle |
Headroom in dB is 20 × log10(clip level ÷ normal level). With a normal output of 3 V peak, that is 20 × log10(10.5 ÷ 3) ≈ 10.9 dB before clipping.
Other places clipping hides
Amplifier stages are the classic case, but the same flat-topped shape turns up elsewhere, and the same checks find it.
- Analogue-to-digital converters. An ADC input driven beyond its full-scale range returns its maximum code, so the reconstructed signal is flat at full scale. Probe the analogue input to the ADC and compare its peaks with the reference or full-scale voltage in the datasheet.
- Protection clamps. Diodes on an input clamp the signal about one diode drop beyond the supply rails. The flat tops then sit slightly above the positive rail and slightly below ground or the negative rail, rather than inside them.
- Supply sag. If the flat level moves down as the signal gets bigger, the supply itself may be drooping under load. Put a second channel on the supply rail and watch whether it dips at each peak.
- Deliberate clipping. Guitar distortion pedals and some limiter circuits clip on purpose, often softly through diodes. There the question is whether the clip level matches the design, not whether clipping exists.
In every case, the V/div check from step 4 still applies first: confirm the flat tops are in the signal before you go looking for a cause.
How to fix clipping
| What you see | Likely cause | Fix |
|---|---|---|
| Both peaks flat at the same level | Input too large for the gain and rails | Lower the input or the gain |
| Only one peak flat | Bias point off center, or unequal rails | Check the no-signal DC level against mid-supply |
| Flat tops at the screen edge only | V/div too sensitive | Use a larger V/div setting |
| Input on CH1 already flat | Clipping in the source or an earlier stage | Move the probe back stage by stage |
| Clips well below the rails | Load too heavy for the output | Check the load against the datasheet |
Other fixes depend on the design: higher supply rails within the parts' ratings, a rail-to-rail op-amp, or attenuating the signal before the stage. When clipping is gone, you can measure frequency response with a clean signal using oscilloscope bandwidth measurement for an amplifier, and read exact levels with how to measure voltage with an oscilloscope.
Which scope you need to find clipping
Two channels let you compare input and output directly, and a built-in generator supplies the test sine. Bandwidth is rarely the limit for audio work. Compare options in our best beginner oscilloscopes roundup.
Bench scopes in our reviews
100MHz, 2 channels, 1 GSa/s; an oscilloscope and waveform generator in one unit.
50MHz, 4 channels, 1 GSa/s; the default beginner recommendation for years.
100MHz, 4 channels, 1 GSa/s; for projects that will get more demanding.
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Frequently asked questions
Can a signal clip on only one side?
Yes, and it is a useful clue. One-sided clipping usually means the stage is not biased at the middle of its available swing, so one peak reaches its limit first. Single-supply amplifiers with a drifting bias point and stages with unequal rails do this. Check the DC level at the output with no signal and compare it to the midpoint between the rails.
Is clipping the same as distortion?
Clipping is one kind of distortion, and usually the most severe. Other kinds, such as crossover distortion in a class B output stage or the gentle bending from a transistor's curve, change the shape without flattening it. Clipping is easy to see on a scope; low-level distortion often needs a spectrum view or a distortion analyzer.
Can clipping damage speakers?
It can. A clipped output carries much more high-frequency energy than a clean sine, and a heavily clipped waveform delivers more average power than a clean sine with the same peak level. Tweeters are especially exposed. That is why amplifier testing is usually done into a resistive dummy load, with the level turned down as soon as the tops flatten.
Why does my scope show a flat line at the top of the screen?
Usually the vertical scale is too sensitive and the waveform runs past the top of the display, so the scope captures only the part inside its range. Turn the V/div knob to a larger value or use AUTO, and check the channel's probe setting matches the probe. If the flat top is still there with the whole waveform on screen, the circuit is clipping.
Can I use the oscilloscope's FFT to detect clipping?
Yes, and it often shows clipping before the flat tops are obvious. A clean sine puts nearly all its energy at one frequency. As clipping starts, harmonics climb quickly: symmetrical clipping adds mostly odd harmonics such as the 3rd and 5th, while one-sided clipping adds even harmonics too. Feed a sine, open the FFT math function and watch the harmonics as you raise the level.
