HOW-TO GUIDE
How to Connect an Oscilloscope Probe Correctly
By the OhmSavvy Research Team ·
The ground clip is not a second probe
- On a mains-powered bench scope the ground clip is connected to earth through the power lead. Clip it only to the circuit's ground or 0 V.
- Never clip it to mains live, neutral, or the primary side of a mains power supply. Use a differential probe or isolated method for those.
- Never remove the scope's earth to make it "float". Its metal parts can then sit at a lethal voltage.
The parts of a passive probe
BNC connector
Bayonet plug: push on, then twist to lock onto the channel input.
1x / 10x switch
Selects direct or divide-by-ten. Many probes have it on the body.
Hook tip
Spring-loaded hook that grips a lead or test pin hands-free.
Ground clip
Short crocodile-clip lead to circuit ground.
Ground spring
Tiny spring that grounds right beside the tip, for fast edges.
Trimmer
Compensation adjuster, on the probe body or BNC end.
1Attach the BNC
Line up the two slots on the probe's BNC with the pins on the channel input, push it home and turn it clockwise until it clicks. A half-seated BNC gives intermittent traces that come and go as the cable moves. If you have a four-channel scope, note which channel each probe is on; the colored rings that come with most probes help.
2Match 1x/10x on the probe and the scope
A 10x probe divides the signal by ten before it reaches the scope. The scope cannot see the switch, so you tell it in the channel menu (often labeled Probe or Attenuation). If the two disagree, every voltage reading is out by a factor of ten.
| Probe switch | Channel setting | A 5 V signal reads as |
|---|---|---|
| 10x | 10x | 5 V (correct) |
| 1x | 1x | 5 V (correct) |
| 10x | 1x | 0.5 V (ten times too low) |
| 1x | 10x | 50 V (ten times too high) |
3Connect the ground clip
Find the circuit's 0 V point: a GND pin, the negative terminal of a battery-powered board or the ground plane near the part you are probing. Clip the ground lead there before the tip touches anything. Grounding first means the probe never sits at an unknown voltage, and it stops the tip from picking up stray hum while you position it.
4Place the tip
Extend the hook and grip a component lead, test loop or header pin. Avoid holding the tip against a fine-pitch IC pin by hand: a slip can short two pins. Solder a short wire or use a fine probe tip instead.
01
BNC on
Push and twist to lock
02
Match 10x
Probe switch and channel menu
03
Ground
Clip to circuit 0 V first
04
Tip
Hook onto the test point
05
Compensate
Once per probe and channel
Why ground lead length matters
The standard ground clip lead is a loop of wire, and a loop has inductance. Together with the probe's capacitance it forms a resonant circuit that rings on every fast edge. On slow signals you will not notice, but on a logic clock or switching circuit the ringing can look like a real fault.
Rule of thumb
Earthed scopes and ground mistakes
- All channel grounds are joined. Clipping CH1 ground and CH2 ground to two different voltages shorts those points together through the scope.
- Earthed circuits create a loop. If the device under test is also earthed, the ground clip and the mains earth form a loop that can add 50 Hz hum. Keep leads short and route them together.
- Floating signals need another method. To measure between two points neither of which is ground, use two channels and the scope's math subtract function, or a differential probe.
Quick check before you measure
Touch the tip to the scope's probe compensation terminal and its ground to the terminal's ground. You should see a square wave of the amplitude printed beside the terminal. Correct amplitude confirms the 1x/10x match; square corners confirm compensation. If the corners are rounded or spiked, follow our guide to compensating an oscilloscope probe.
Scopes and probes
The scopes we review come with passive probes suited to their bandwidth, and the connection steps above are the same on all of them. For first-session settings after the probe is on, see how to use an oscilloscope.
Scopes in our reviews
100MHz, 4 channels, well-regarded interface.
100MHz, 2 channels, built-in waveform generator.
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Frequently asked questions
How do you ground an oscilloscope probe?
Clip the probe's ground lead directly to the circuit's ground or 0 V reference, as close to the test point as the lead reaches — a GND pin or the negative supply rail on most circuits. Keep the lead short, since a long ground return picks up noise and causes ringing on fast edges. Never clip it to a point that sits at a voltage relative to earth; the scope's chassis ground is typically tied to earth through the mains plug, and doing so creates a short.
Where should I connect the oscilloscope ground clip?
To the ground or 0 V reference of the circuit you are measuring, as close as practical to the test point. On a microcontroller board that is a GND pin or the negative supply rail. Never clip it to a point that sits at a voltage relative to earth, because the scope's ground is earthed and you will create a short.
Why is my oscilloscope reading 10 times too high or too low?
The probe's 1x/10x switch and the channel's probe attenuation setting disagree. If the probe is on 10x but the channel thinks it is 1x, every voltage reads ten times too small; the reverse reads ten times too large. Set both to the same value and the numbers will be right.
Do I need a ground clip on every probe?
Every channel shares the same ground inside a bench scope, so electrically one ground connection serves them all. For clean high-speed measurements, though, each probe should still have its own short ground near its tip, because a long shared return path picks up noise and causes ringing.
Should I use 1x or 10x?
Use 10x for most work. It loads the circuit far less and gives the full bandwidth of the probe. Switch to 1x only for very small signals, a few millivolts, at low frequency, where the extra sensitivity matters more than bandwidth.
Do I need to compensate the probe after connecting it?
Yes, whenever you move a 10x probe to a different channel or scope. Compensation matches the probe to that input so square edges look square. It takes a minute using the scope's calibration output, and it is covered step by step in our separate probe compensation guide.
