Input Capacitance

When selecting differential probes, the input capacitance value is typically listed as a parameter. The following factors should be considered:

Characteristics of the measured signal

Such as signal bandwidth/rise time. Typically, measuring high-frequency signals requires extremely low input capacitance. Since the input capacitance forms a low-pass filter with the impedance of the signal under test, higher capacitance results in a lower cutoff frequency, allowing fewer high-frequency signals to pass through. This consequently reduces the bandwidth of the test system.

Impedance of the measured signal

Similarly, even a very small input capacitance (such as 1pF) can form a low-pass filter with a cutoff frequency as low as approximately 160MHz at a high-impedance node (such as 1kΩ), severely limiting the usable bandwidth.

Resonance effect

The inductance formed between the probe input capacitance and the probe ground wire, combined with any inductance present in the circuit under test, creates a resonant circuit. A low input capacitance helps shift the resonance point above the measurement bandwidth, preventing resonance peaks within the frequency range of interest that could cause waveform distortion.

From the three points above, low output capacitance is critical for measurement instruments. Below is a comparison of recommended input capacitance ranges and probe types for different testing environments:

Therefore, users can select the appropriate probe based on the characteristics of the actual signal being measured and the application scenario before testing.