How to achieve the best match for oscilloscope probes?

                                   Figure 1  Probe classification

1.11.1  High-impedance passive probes
       From the perspective of actual needs, high-impedance passive voltage probes with compensation are used in the largest proportion and can meet the needs of most low-speed digital signals, power supplies and other typical oscilloscopes.
       This type of probe has a higher input resistance (generally above 1MΩ) and an adjustable compensation capacitor. When connected to an oscilloscope for the first time, it is generally necessary to adjust the capacitance value with an adjustment rod to match the oscilloscope input capacitance and eliminate low-frequency or high-frequency gain. The left side of Figure 2 shows an under-compensated waveform, the middle shows a normal waveform, and the right side shows an over-compensated waveform.

                                     Figure 2  Passive probe compensation

For example, the ZDS2024PLUS comes standard with the ZP1025S high-impedance passive voltage probe, and its specific parameters are as follows:

                             Table 1  ZP1025S specifications and models

 

1.21.2  High-voltage differential probe
       First, let's introduce the concept of differential: differential signals are signals that reference each other, not the ground. The high-voltage differential probe is essentially composed of two symmetrical voltage probes, each with good insulation and high impedance to the ground. It can provide a high common-mode rejection ratio in a wider frequency range and can convert any two-point floating signals into ground signals. It is mainly used for testing industries such as switching power supplies. The schematic diagram is shown in Figure 3.

                             Figure 3  High-voltage differential probe principle 

The following table shows the specific parameters of common high-voltage differential probes:

                     Table 2  Specifications of common high voltage differential probes

1.31.3  Current probe
       The current probe uses a Hall sensor and an induction coil to measure DC and AC currents. Its working principle is to convert the current signal into a voltage signal. The oscilloscope collects the voltage signal and then displays it as a current signal.

         Figure 4  Current probe image

The advantage of the current probe is that the current can be measured without disconnecting the power supply line. Typical applications are system power measurement, power factor measurement, and switch impact current waveform measurement. The main disadvantage is that its small current measurement capability is limited by the bottom noise of the oscilloscope, so the small current measurement capability is limited.
       2. How to choose a suitable probe?
       The above briefly introduces some common probes. So how should we choose a suitable probe? What parameters of the probe should we pay attention to?
       1. Impedance matching: The input impedance of the probe should match the input impedance of the oscilloscope used to reduce the load on the circuit under test. For oscilloscopes with low input impedance, active probes or probes with 50Ω input impedance should be selected; for oscilloscopes with high input impedance, ×10 probes should be selected. For example, if the input impedance of the oscilloscope is 1MΩ/10pF, the probe input impedance is preferably 10MΩ/1pF. Such a probe has 10 times the signal attenuation, a small load on the measured signal, and can match the input impedance of the oscilloscope.
        2. Bandwidth: The bandwidth of the probe should be equal to or greater than the bandwidth of the oscilloscope. If a pure sinusoidal signal is observed, the probe bandwidth can be equal to the highest value of the measured signal frequency; if a non-sinusoidal signal is observed, the probe bandwidth should accommodate the fundamental wave and the most important harmonic components of the measured signal.
       3. Rise time: In order to accurately measure the rise time and fall time of the pulse, the rise time of the system (that is, the sum of the oscilloscope and probe rise time) should be 3-5 times faster than the rise time of the measured signal.
       3. Summary
       To ensure the accuracy of the test results, the probe is required to have minimal impact on the circuit under test and maintain the maximum signal fidelity transmitted to the oscilloscope. If the probe changes the signal or the operation mode of the circuit under test in any way, the oscilloscope will see the distortion of the actual signal, which may lead to incorrect measurement results, or misleading measurement results. Only probes that are well matched to both the oscilloscope and the circuit under test are your best choice!