HOW-TO GUIDE
How to Compensate an Oscilloscope Probe
By the OhmSavvy Research Team ·
The calibration output is safe; your next measurement may not be
Compensating against the scope's own calibration terminal is a low-voltage job with no special risk. The danger comes afterwards, when you move the freshly compensated probe to a real circuit. On a bench scope the probe's ground clip is normally connected to mains earth through the power cord, so clipping it to a point that isn't at earth potential, such as anywhere on a mains-connected, non-isolated circuit, creates a short through the ground lead that can cause damage, fire or injury. Never remove or defeat the scope's earth to get around this; use an isolation transformer on the device under test, a differential probe, or an isolated-channel or handheld scope rated for the job. Check the CAT rating and maximum input voltage printed on the probe as well as the scope: standard 1x/10x probes are not for mains work unless their rating explicitly covers it.
Ground leads, ground springs and 1x/10x matching are covered in how to connect an oscilloscope probe correctly.
What probe compensation actually fixes
A 10x passive probe is a voltage divider. A resistor in the probe works against the scope's input resistance (commonly 1 MΩ on general-purpose scopes) to divide the signal by ten. That alone would be fine at DC, but the scope input and the probe cable also have capacitance, and capacitance forms a second divider that behaves differently as frequency rises. The probe therefore includes a small adjustable capacitor, the compensation trimmer, so the capacitive divider can be set to the same ten-to-one ratio as the resistive one. When the two ratios match, the probe divides low and high frequencies equally and a square wave arrives at the screen with its shape intact.
In the diagram, the resistive divider is fixed. The capacitive divider is formed by the trimmer on one side and the scope's input capacitance plus the cable on the other, and only the trimmer can be changed. That is why the adjustment exists: it is the one part you can turn to make the high-frequency ratio equal the DC ratio.
Input capacitance differs from one scope to another and can differ slightly between channels on the same scope, which is why compensation is not set once at the factory and forgotten. Each time you move a probe to a different channel or a different scope, check it again.
Rule of thumb
1x vs 10x: which setting and why it matters
Many probes have a slide switch for 1x and 10x. Compensation applies to the 10x setting, which is the one you'll use for most work.
| 1x setting | 10x setting | |
|---|---|---|
| Signal reaching the scope | Full signal, no attenuation | Signal divided by 10 |
| Loading on your circuit | Higher: the circuit sees the scope input directly plus the cable | Lower: roughly ten times the input resistance and much less capacitance |
| Bandwidth | Typically far lower than the same probe at 10x; check the probe's rating | Full rated probe bandwidth |
| Compensation needed | No (the trimmer network is not used) | Yes, adjust with the trimmer |
| Good for | Very small, slow signals where dividing by 10 would bury them in noise | General work, faster signals, higher voltages within the probe's rating |
Whatever you choose on the probe, the channel's probe or attenuation setting on the scope must match it, otherwise every voltage reading will be off by a factor of ten. Some scopes detect the probe ratio automatically through a sense pin on the connector; many budget scopes don't, so set it by hand in the channel menu.
The three shapes: over, under, and correct
Connected to the calibration output, a probe can show one of three shapes. The flat parts of the wave are set by the resistive divider, so they sit at the same height in all three; only the edges and corners change.
In these diagrams one cycle spans 5 divisions at 200 µs/div, which is 1 ms, matching a 1 kHz calibration signal. Your scope's output frequency and amplitude may differ.
The procedure at a glance
The whole job is five short steps, shown below and then explained one at a time. Steps 1 to 3 are setup; step 4 is the only one that changes the probe itself.
01
Set probe to 10x
Probe switch and channel menu
02
Connect to cal out
Tip to signal, clip to ground
03
Get a stable view
Two to four cycles, a few div tall
04
Adjust the trimmer
Small turns until corners are square
05
Repeat per channel
Each probe on the channel it lives on
1Set the probe and channel to 10x
Slide the probe switch to 10x (if it has one), plug the probe into the channel you'll actually use, and set that channel's probe ratio to 10x in its menu. Set the channel to DC coupling so the scope shows the full square wave rather than a version with its DC level stripped off.
2Connect to the calibration output
Find the probe-compensation terminal on the front panel. It's often a small metal tab or hook labeled with a square-wave symbol, with a separate ground terminal next to it. Hook the probe tip to the signal terminal and the ground clip to the ground terminal. Many scopes output a square wave of around 1 kHz here, but the exact frequency and voltage vary by model, so check your manual or datasheet for the values to expect.
3Get a clear, stable picture
Press Autoset, or set the controls by hand: a vertical scale that makes the wave a few divisions tall, and a time base that shows roughly two to four cycles across the screen. You want the edges and corners large enough to judge. If the trace won't hold still, see how to trigger an oscilloscope; a rising-edge trigger at mid-signal works well here. If you're unsure what the divisions mean, our guide to reading an oscilloscope screen covers it.
4Adjust the trimmer until the corners are square
Locate the compensation trimmer. Depending on the probe it is on the probe body or on the connector end that plugs into the scope. Turn it with the small plastic adjustment tool supplied with many probes, or another non-metallic screwdriver, in small steps while watching the screen. If you see overshoot spikes, turn one way; if the result gets worse or the corners start to round off, turn the other way. Stop when the tops and bottoms are flat and the corners are square. A metal screwdriver can disturb the reading while it's touching the trimmer, so judge the final shape with the tool removed.
5Repeat for each probe and channel
On a multi-channel scope, compensate each probe on the channel it will live on. If you label probes to channels and keep them there, you'll rarely need to touch the trimmers again; swap them around and you should recheck.
Why a miscompensated probe gives wrong answers
Because the error is frequency-dependent, it shows up exactly where it's hardest to spot. A DC voltage or a slow signal can read correctly while a fast edge is wrong. An overcompensated probe exaggerates high-frequency content, so peak-to-peak readings on fast signals come out too high and you may see overshoot that isn't really in the circuit. An undercompensated probe does the opposite: edges look slower than they are and high-frequency amplitudes read low. Either way, rise-time and voltage measurements on fast signals become unreliable, and you can waste time chasing ringing or slow edges that are artefacts of the probe. This also interacts with oscilloscope bandwidth: a scope's rated bandwidth assumes a probe that is properly matched and compensated.
Troubleshooting compensation
| What you see | Likely cause | What to try |
|---|---|---|
| Amplitude is 10x too big or too small | Probe switch and channel probe setting don't match | Set both to 10x (or both to 1x) |
| Trimmer makes no difference | Probe is switched to 1x | Switch the probe to 10x |
| Can't get flat tops at either end of the trimmer's range | Probe's compensation range may not suit this scope's input capacitance | Check the probe's spec against the scope; try a probe made for that scope |
| Flat line or no signal | Tip or ground clip not on the calibration terminals, or channel turned off | Reconnect and check the channel is enabled |
| Small ringing on the edges after adjustment | Long ground lead picking up and resonating | Keep the ground lead short; use a short ground spring if your probe has one |
Where compensation fits
Compensating your probes is the first thing to do with a new scope, before any real measurement. If you're just getting started, read what an oscilloscope is and then try a frequency measurement on the calibration signal itself as a quick self-check. Choosing a first scope? The best beginner oscilloscopes list and the buying guide explain the trade-offs. Note that pocket scopes such as the FNIRSI DSO152 differ from bench scopes in their inputs and accessories, so follow that model's own manual for probe setup.
Bench scopes in our reviews
50MHz, 4 channels, 1 GSa/s, 7-inch display and strong community documentation.
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Frequently asked questions
Why is probe compensation necessary?
A 10x probe forms a voltage divider with the scope's input, and that divider only holds its ratio flat across frequency if the probe's trimmer capacitor matches the scope input's own capacitance. Every probe-and-scope pairing has slightly different capacitance, so an uncompensated probe over- or under-shoots square edges and distorts fast signals, even though it reads DC and slow signals correctly.
How often should you compensate an oscilloscope probe?
Check it the first time you use a 10x probe, every time you move it to a different channel or a different scope, and whenever a square wave looks wrong. If each probe stays on the same channel of the same scope, the setting holds and you rarely need to touch it. A quick look at the calibration output before an important measurement costs only a few seconds.
Do you need to compensate a probe set to 1x?
No. In the 1x position the probe passes the signal straight through and the compensation trimmer is not part of the circuit, so turning it changes nothing on screen. Compensation only matters on 10x. If you switch a probe from 1x back to 10x, check the calibration square wave again before trusting fast edges or amplitudes.
What tool do you use to adjust the probe compensation trimmer?
Use the small plastic adjustment tool that comes with many probes, or another non-metallic screwdriver that fits the trimmer slot. A metal blade can change the capacitance it is adjusting while it touches the trimmer, so the shape you see can shift when you pull it away. Make small turns and judge the final square wave with the tool removed.
Can you compensate a probe without the scope's calibration output?
Yes, if you have another clean square wave of a known, fairly low frequency, such as around 1 kHz from a function generator. The method is the same: set 10x, show a few cycles and adjust until the tops are flat and the corners square. The source needs clean, fast edges itself, or you will be compensating against its flaws rather than the probe's.
Can you use a probe from one oscilloscope on another?
Often yes, as long as the probe's compensation range covers the new scope's input capacitance and its ratings suit the job. Recompensate it on the new scope's channel before use. If the trimmer cannot reach flat tops at either end of its range, the probe is a poor match for that input, and a probe intended for that scope is the better choice.
