Explore: Principles and types of oscilloscope probes

First, the probe itself has input resistance . Just like the principle of measuring voltage with a multimeter , in order to minimize the impact on the measured circuit , the input resistance Rprobe of the probe itself is required to be as large as possible. However, since Rprobe cannot be infinite, it will produce voltage division with the measured circuit, and the actual measured voltage may not be the real voltage before the probe point, which is often encountered in the test of some power supplies or amplifier circuits. In order to avoid the influence of the probe resistance load, it is generally required that Rprobe is more than 10 times greater than Rsource and Rload. The input impedance of most probes is between tens of k ohms and tens of megohms.

Secondly, the probe itself has input capacitance . This capacitance is not deliberately added, but is the parasitic capacitance of the probe. This parasitic capacitance is also the most important factor affecting the bandwidth of the probe, because this capacitance will attenuate the high-frequency component and slow down the rising edge of the signal. Usually, the parasitic capacitance of high-bandwidth probes is relatively small. Ideally, Cprobe should be 0, but it is not possible in reality. Generally, the input capacitance of a passive probe is between 10pf and several hundred pf, and the input capacitance of an active probe with a higher bandwidth is generally between 0.2pf and several pf.

Secondly, the probe input will also be affected by inductance. The input resistance and capacitance of the probe are relatively easy to understand, but the inductance of the probe input is often overlooked, especially when measuring at high frequencies. Where does the inductance come from? We know that where there is a wire, there will be inductance. There must be a wire connecting the probe and the circuit under test, and the signal return must pass through the probe ground wire. Usually, the ground wire of a 1mm probe will have an inductance of about 1nH. The longer the signal and ground wires, the greater the inductance value. The parasitic inductance and parasitic capacitance of the probe form a resonant circuit. When the inductance value is too large, high-frequency resonance may occur under the excitation of the input signal, causing signal distortion. Therefore, the length of the signal and ground wires needs to be strictly controlled during high-frequency testing, otherwise ringing is likely to occur.