The Realities of Oscilloscope Probes

Figure 1: Teledyne LeCroy's ZS2500

high-impedance active probe

Looking for a good, albeit very expensive, paperweight? Try using an oscilloscope without probes! Probes are often taken for granted but they are one of the most critical elements of the signal chain in any test scenario.

Ideally, an oscilloscope probe would make contact with the circuit under test and transmit its signal from the tip of the probe to the instrument's input with perfect fidelity. We would like to see the probe exhibit zero attenuation, infinite bandwidth, and linear phase characteristics at all frequencies.

The circuit under test has its particular electrical characteristics for a given signal, which are what we want to measure. Alas, the probe itself is a circuit with its own electrical characteristics. When the probe tip meets the circuit under test, their characteristics combine in a way that can affect the measurement results. The probe's input impedance will be introduced into the circuit. All probes present some amount of resistive, capacitive, and inductive loading that must be accounted for.

Figure 2: Probe input equivalent circuit

For one thing, the probe's lead length will present some amount of inductive loading to the input ground leads, as shown in an equivalent circuit diagram of a probe input (Figure 2). The ground lead is the primary return path for current that results from the input voltage acting with the probe's input impedance. The ground lead and input lead's inductances combine with the probe's input capacitance to form a series LC network. That network's impedance drops substantially at its resonant frequency, and this effect, known as ground lead corruption, is the cause of ringing often seen after the leading edge of pulses.

How can ground lead corruption be alleviated? One way is to raise the resonant frequency of the LC network by decreasing the inductance, the capacitance, or both. Realistically, because the input capacitance is already very low, the only option is to reduce the input inductance. This is achieved by using input and ground leads that are as short as possible.

Capacitive loading can be a difficult nut to crack, as it can affect rise-time, bandwidth, and delay measurements. At high frequencies, capacitive loading can affect the amplitude and waveshape of measured signals.