Measurement and Noise
Model class: Standard approximation
Oscilloscope probe loading
Estimate probe loading from source resistance, probe input resistance and capacitance, and the frequency being observed.
Interactive engine
Start with the stated conditions.
Values stay in this browser. Choose a representative scenario, then calculate deliberately.
Example ready
Calculate to inspect the result.
The result will identify the direct answer, assumptions, and any warning that changes the next decision.
Assumptions to check
- Source can be represented by a Thevenin resistance.
- Probe input is represented by one resistance and capacitance.
- Lead inductance, compensation, scope bandwidth, and connection geometry are separate.
A probe changes the node
An oscilloscope probe is part of the circuit under test. Its input resistance can attenuate a high-impedance source, and its capacitance combines with source resistance to form a loading pole that changes amplitude and edge shape.
This tool makes the RC time constant and pole explicit. It does not dismiss the measurement just because the scope has a high bandwidth. The source and probe together determine what the probe can faithfully observe.
Choose the measurement method deliberately
At a high-impedance or fast node, a lower-capacitance probe, active probe, buffer, or a different test point may be necessary. Include the actual probe model, compensation state, ground connection, and scope bandwidth in the final measurement plan.
Plan the connection as part of the experiment
Start with the node impedance and the signal bandwidth or edge-rate that matters, then compare the source resistance with the probe capacitance. The resulting pole indicates whether a passive probe could attenuate or reshape the waveform before the scope displays it. Probe resistance also matters for DC and low-frequency loading, particularly at bias nodes, divider outputs, and high-value sensor networks.
Use the physical connection deliberately. A long ground clip adds inductance and can create ringing that is not present at the circuit node. Confirm compensation, voltage rating, common-mode range, bandwidth, and the selected scope input mode. When an observation drives a design conclusion, compare it with a lower-loading probe, buffer, differential method, or another independently justified measurement approach. Keep the probe model and connection method in the test record.
Common mistakes
- Using only probe resistance for a fast signal.
- Ignoring ground-lead inductance.
- Assuming a high-bandwidth scope removes probe loading.
Model limit and handoff
Compare with the exact probe specifications and validate the measurement method against source impedance and edge-rate requirements.
FAQs
Why does a 10x probe still load a node?
Its capacitance forms an RC network with the source resistance even when its resistance is high.