Effect of the Measuring Probe on the Circuit (Probe Loading)
It is the measuring instrument changing the point it is connected to by some amount. The probe joins the circuit with its own resistance, capacitance and the inductance of its ground lead; especially at high-impedance analogue points and on fast switching edges, the waveform you see is not the truth itself but the joint result of the probe and the circuit.
Three effects make a difference in practice:
- Resistive loading: A probe with 1 MΩ input resistance measurably lowers the voltage when connected to a 100 kΩ divider point. At low-impedance power points the effect is negligible.
- Capacitance: The probe and its cable typically add 10–100 pF; this rounds fast edges and makes the rise time look longer than it is. Using the 10x position reduces this effect considerably.
- Ground lead: The inductance of a long coiled ground lead forms a resonant circuit together with the probe capacitance. Most of the high-frequency ringing you see on the screen arises from this arrangement.
When watching the driver output during switching, use a short spring ground tip and connect the ground to the chassis point nearest the point you are measuring. In the same way, when putting a multimeter in series with the circuit to measure current, do not forget that the instrument's own shunt resistance also adds a small drop to the circuit.
Context where this term is used: DC motor and driver fault diagnosis matrix