Ground Inductance Effects

Probe grounding techniques are an important factor in making accurate high frequency measurements. The main limitation, probe resonance, is a function of the input capacitance of the probe and the inductance of the ground return. These two parameters in series form an LC resonant circuit that, when connected to the circuit under test, becomes part of the circuit's response.

The probe resonance can cause overshoot and ringing on pulse edges that contain energy in the same frequency band as the resonance. The true response is masked, the false response gets transferred to the instrument, and the instrument display shows an incorrect result. If overshoot and ringing added by a probe during troubleshooting changes how the circuit functions, it can lead to an incorrect judgment about circuit operation.

To minimize the problem of ground ringing, use the shortest possible ground with a probe that has the lowest possible input capacitance. The following equation can be used to calculate the frequency where a certain probe and grounding technique resonates.

where

C is the probe input capacitance. It is usually found in the probe data sheet.

L is the inductance of the ground return. It can be approximated using the constant of 25 nH per inch.