Oscilloscope voltage probe selection guide: types, probe loading and how to choose
The probe you choose directly affects the accuracy, signal fidelity and safety of every oscilloscope measurement. This guide explains the main oscilloscope voltage probe types, passive, active, differential and specialised, their strengths and trade-offs, and how to choose the right one for your circuit.
Probe loading in briefPassive probesActive probesDifferential probesSpecialised probesHow to select the right probeFAQ
Fig 1: A range of oscilloscope voltage probes. Each type is built for a different kind of measurement.
Probe loading in brief
A probe does not just observe your circuit. Once connected, it becomes part of it. The probe’s resistance, its capacitance and the inductance of its ground lead all load the test point, changing the signal in your circuit and the waveform you see on screen. This is known as probe loading, and it comes from three sources:
Pulls down high-impedance nodes
A 10x passive probe presents about 10 MΩ to the circuit: a 9 MΩ tip resistor in series with the oscilloscope’s 1 MΩ input. Usually negligible, but it can shift voltages on sensor outputs and other high-impedance nodes.
Slows edges
Standard passive probes add about 8 to 20 pF, active probes often below 1 pF. Together with the source impedance (τ = R × C), this slows edges and attenuates high frequencies.
Causes overshoot and ringing
A long ground lead adds inductance that resonates with the probe’s capacitance. Fast edges make it ring, showing overshoot and ringing that are not in your circuit.
Part 1: Beware of probe loading artifacts with passive probes →Part 2: How to reduce probe loading when using passive probes →
High impedance, single-ended passive voltage probes
Fig 2: Single-ended passive voltage probe.
Rugged, reliable, inexpensive and found in almost every lab, passive voltage probes are the default choice for general-purpose work. A 10x passive probe presents about 10 MΩ, so resistive loading is small, and passive probes can handle much higher voltages than active probes, typically a few hundred volts.
The trade-offs are lower bandwidth and higher capacitive loading than active probes, which can distort fast signals. The long ground lead in the accessory kit adds ringing of its own. If you see overshoot and ringing on fast edges, see how to reduce probe loading when using passive probes.
High impedance, single-ended active voltage probes
Fig 3: Single-ended active voltage probes.
Active probes use an amplifier at the probe tip to buffer the signal. This gives them higher bandwidth and much lower input capacitance than passive probes, often below 1 pF, so their input impedance stays high even at high frequencies. That makes them the better choice for fast digital and RF signals.
The trade-offs are a much smaller input voltage range and a higher cost. Note that many active probes have a DC input resistance of about 1 MΩ, lower than a 10x passive probe. Their advantage is at high frequencies, not at DC.
Differential oscilloscope voltage probes
Fig 4: Differential voltage probe.
Differential probes measure the voltage difference between two points, rather than between one point and ground. This lets you measure floating signals, such as a high-side switch node, safely without floating the oscilloscope.
Advantages: Safe floating measurements without floating the oscilloscope, and rejection of unwanted common-mode signals.
Disadvantages: Higher cost, and the need to consider parameters such as:
- Common mode voltage (Vcm) range
- Differential mode voltage (Vdm) range
- Common mode rejection ratio (CMRR)
A standard differential probe is not electrically isolated. It still has a resistive path to the oscilloscope’s ground. When you need true isolation, use an optically isolated probe (see specialised probes below).
Never clip a passive probe’s ground lead to a point that is not ground. The ground lead connects to the oscilloscope chassis, which is connected to mains earth through the power cord. Clipping it to a floating or high-side point creates a short circuit through the oscilloscope, which can damage your circuit, the probe and the oscilloscope, and can be dangerous. Use a differential probe instead.
Low voltage differential probes
Fig 5: Low voltage differential probes.
Low voltage differential probes offer high bandwidth and excellent signal integrity, making them ideal for small, high-speed differential signals. However, their limited Vcm and Vdm ratings restrict them to lower-voltage circuits.
High voltage differential probes
Fig 6: High voltage differential probe.
High voltage differential probes are built for very high Vcm and Vdm, making them common in power electronics. They trade bandwidth for voltage capability, and their higher attenuation can add more noise to the measurement.
Specialised oscilloscope voltage probes
- Optically isolated probes, e.g. Teledyne LeCroy HVFO108 and DL-ISO: true galvanic isolation for fast, high-voltage floating signals, with minimal probe loading.
- Rail probes, e.g. Teledyne LeCroy Active Voltage Rail Probe: designed for low-voltage DC power rails, with very low noise and high sensitivity.
- High common mode differential probes, e.g. Teledyne LeCroy DL-HCM: common mode voltage up to 60 V with wide bandwidth, well suited to measuring many serial buses.
How to select the right oscilloscope voltage probe
1. Know your DC bus voltage
- 60 Vdc (nominal)
- 170 to 1000 Vdc
- 1500 Vdc
- 6000 Vdc
2. Know your application
- Power semiconductor device testing
- Floating sensor measurements
- Discrete component measurements
- System inputs/outputs
3. Know your sub-application
- Gate drive measurements (Vgs or Vge)
- Switching outputs (Vds or Vce)
4. Consider semiconductor material and bandwidth
- Silicon (Si): about 100 MHz bandwidth
- Silicon carbide (SiC): about 350 MHz bandwidth
- Gallium nitride (GaN): about 1 GHz bandwidth
- Lower bandwidth may be sufficient for floating sensor or system I/O measurements
Why the difference? SiC and GaN devices switch much faster than silicon, and faster edges need more bandwidth to capture accurately. A useful rule of thumb: bandwidth ≈ 0.35 ÷ rise time. For example, a 1 ns rise time needs at least 350 MHz.
Frequently asked questions
What are the different types of oscilloscope probes?
The main oscilloscope voltage probe types are passive probes, active probes and differential probes, plus specialised probes such as optically isolated probes, rail probes and high common mode probes. Current probes are a separate family used to measure current rather than voltage.
What is the difference between passive and active oscilloscope probes?
Passive probes use resistors and capacitors to scale the signal and are the standard choice for general-purpose measurements. Active probes use a built-in amplifier, which gives them much lower input capacitance and higher input impedance at high frequencies, making them better suited to high-speed digital, RF and sensitive analogue measurements.
1x vs 10x probe: which should I use?
Use 10x for most measurements. A 10x probe presents about 10 MΩ and around 10 pF, with bandwidth into the hundreds of MHz. A 1x probe loads the circuit more, at 1 MΩ and much higher capacitance, and its bandwidth is typically limited to a few MHz. Use 1x only for very small, low-frequency signals, where the 10x attenuation would bury the signal in noise.
When should a differential probe be used?
Use a differential probe when the signal is not referenced to ground, for example floating measurements, high-side switch nodes, power electronics and motor drives. It lets you measure these safely without floating the oscilloscope. Note that a standard differential probe is not electrically isolated. For true isolation, use an optically isolated probe.
Can I use a passive probe on a floating circuit?
No. A passive probe’s ground lead is connected to the oscilloscope chassis and mains earth. Clipping it to a floating or high-side point creates a short circuit through the oscilloscope, which can damage your equipment and can be dangerous. Use a differential probe or an optically isolated probe instead.
How do I choose the right oscilloscope voltage probe type?
Consider the signal’s speed (rise time and bandwidth), its voltage level, whether it is ground-referenced or floating, and the source impedance of the test point. Passive probes cover most routine measurements, while active, differential or specialised probes are needed for high-speed, high-voltage, floating or precision measurements.
Can the wrong oscilloscope probe affect measurement accuracy?
Yes. An unsuitable probe can add excessive loading, limit bandwidth, distort the waveform or give inaccurate voltage readings. See Part 1: Beware of probe loading artifacts with passive probes for a real example.
Conclusion
Choosing the right oscilloscope probe is the first step to an accurate measurement. Bandwidth, voltage range, input impedance and probe loading all affect how a signal is captured and interpreted.
Different applications need different probes: passive, active, differential, specialised, or a current probe for analysing current flow and power behaviour. Browse our oscilloscope probe range, or speak to us about matching a probe to your circuit.
Speak to a Genetron T&M specialist about your measurement requirements.
