Effect of Oscilloscope Probe Input Capacitance on High Frequency Testing

Why do experienced engineers adjust the probe position before testing? In addition to the different input impedance, bandwidth, rise time, etc., there is another very important parameter, which is the input. How much influence does it have on the measured signal?

The input capacitance of the probe has a great influence on high-frequency signals. The higher it is, the greater the influence. What are the specific effects?

1. Probe Loading Effect

Simply put, the load effect of the probe means that when the probe is used to measure the waveform of two points in the circuit, a load is connected between the two test points. The size of this load will directly affect the state of the circuit and cause inaccuracy in the measurement results.

Each probe has its input impedance, which is the characteristic impedance. It is not just resistance, but also capacitance and inductance. Due to the additional load introduced by the probe, when the probe is connected to the circuit under test, it will draw energy from the signal, which will actually affect the circuit under test. The worst consequence is that the circuit was originally working normally, but it is abnormal after the oscilloscope probe is introduced. At this time, it is easy to draw a conclusion contrary to the facts. Therefore, we must consider the load characteristics of the probe and the impedance matching of the test circuit when conducting analysis and measurement.

When the probe is at the ×1 position, the signal enters the oscilloscope directly. This type of probe connects its own capacitance (including the capacitance of the cable) to the input impedance of the oscilloscope at the test point. This is the load effect of the probe.

Figure 1 x1 file structure model

当信号升高时，探头的容性负载效应就变得更加显著。x1档位输入电容通常为55±10pF，此时等同于在被测电路上加了一个低阻抗负载，在输入电容为50pF时，若测试10MHz的信号，根据容抗计算公式：Xc(Cp) = 1/（2×π×f×C），此时容抗约为318Ω，且x1档时带宽较低，测试出的结果是不准确的。

2. Reasons for adjusting the probe position

The figure below is the schematic diagram of the passive voltage probe x10, where Rp (9 MΩ) and C1 are located inside the probe tip. The compensation capacitor C3 is adjusted to match the RC product of the probe and the oscilloscope channel. This ensures that the displayed waveform is normal without over-compensation or under-compensation.

Figure 2 x10 gear structure model

At this time, the oscilloscope input signal is attenuated to 1/10 of the measured input signal. For higher frequency input signals, the effect of capacitive reactance on the signal will be greater than that of impedance. For example, when the probe is at x10, the input impedance is 10MΩ, the input capacitance is 10pF, and the frequency of the input signal is 100MHz. At this time, the probe input capacitive reactance is Xc(Cp) = 1/(2×π×f×C) = 159Ω. At this time, the capacitive reactance is much smaller than the probe impedance. The signal current will pass more through the low-resistance loop provided by the input capacitor, and the high-resistance loop is equivalent to a bypass.

The probe is the first step in testing. Whether it can transmit the signal to the oscilloscope with high fidelity is the key to accurate testing and analysis. Therefore, when testing higher frequency signals, it is necessary to pay attention to whether the bandwidth and input capacitance of the probe are appropriate. The following table shows the parameters of the standard probe of the ZDS2000 series oscilloscope.

Table 1 ZP1025S probe parameters