Oscilloscope voltage probe circuit principle

With the development of wide bandgap semiconductor devices, the switching speed of power electronic devices is getting faster and faster, and the operating voltage is gradually increasing, which also increases the influence of the performance of voltage probes on the transient voltage measurement results of power electronic devices. Below PRBTEK shares with you the circuit principle of a typical oscilloscope voltage probe:

High impedance passive probe

Passive probes are widely used in general testing situations because of their low price, strong mechanical structure, wide dynamic range, and high input resistance. Commonly used passive probes are high-impedance passive probes with 10x attenuation, which mainly include the probe front end, lossy transmission line, and compensator. Its typical circuit model is shown in Figure 1.

Figure 1 Typical circuit model of a high impedance passive probe with 10x attenuation

The front end of the probe is used to connect the detection point, where the signal end provides a high-resistance resistor Rt to reduce the load effect, and exists in the parasitic capacitance Ct; the ground wire end is generally a trailing alligator clip with a parasitic inductance Lg.

The transmission line is used to provide the measurement distance, and its length is generally 1 to 2 meters. The transmission line can be equivalent to a lumped element model with distributed parameters such as RLGC. When its terminal impedance is not matched, the high-frequency signal will resonate. In order to better suppress the resonance problem, the transmission line can be designed as a lossy type, that is, it contains a certain distributed resistance.

The compensator is used to match the impedance of the probe and oscilloscope, and is represented by an RC series network in Figure 1. According to the theory of the compensated attenuator, in order to maintain the linear attenuation of the signal in a larger frequency domain, the variable capacitor Cc can be adjusted to make the time constants of the input network and the output network equal, that is,

RtCt=Rs(Clt+Cc+Cs)

Where Clt is the total capacitance of the lossy transmission line. According to the working principle of the transmission line, in order to improve the high-frequency gain of the probe, the variable resistor Rc can be adjusted to make the load impedance close to the characteristic impedance of the transmission line, that is,

Where Z0 is the characteristic impedance of the transmission line; fb is the bandwidth of the probe.

Active single-ended probes

The front end of the active single-ended probe is equipped with a field effect transistor, which makes it have very small input capacitance, but at the same time leads to a small linear dynamic input range. In addition, the active single-ended probe is expensive and has a fragile mechanical structure, which limits its application range. Figure 2 shows a circuit model of an active single-ended probe with 10 times attenuation, which mainly includes an attenuator, a buffer and a lossless transmission line.

Figure 2. Circuit model of an active single-ended probe with 10x attenuation.

The signal is first attenuated 5 times by the attenuator, then voltage-followed by the buffer, and finally transmitted to the oscilloscope by the lossless transmission line. The buffer has high input impedance and strong output drive capability, which isolates the attenuator and the lossless transmission line. On the one hand, it is convenient to match the impedance between the input and output ends and improve the signal transmission capability; on the other hand, it can make the attenuator as close to the test point as possible to reduce uncontrollable parasitic parameters. The characteristic impedance of the lossless transmission line is generally 50Ω. R3 and Rs respectively match the impedance of its source and load ends to improve the fidelity of signal transmission, and at the same time produce a 2-fold attenuation of the signal at the output end of the buffer.

Active High Voltage Differential Probes

Active differential probes are mainly used to measure differential signals and can be divided into low-voltage and high-voltage types. High-voltage differential probes with better versatility are usually selected to measure switching power supply signals. Figure 3 shows a classic active high-voltage differential probe circuit model, which mainly includes an attenuator, a buffer, a differential amplifier, and a lossless transmission line. In the figure, Lp+ and Lp- are the parasitic inductances of the two signal ends.

Figure 3. Active high-voltage differential probe circuit model

First, the differential signal passes through two theoretically identical attenuators and buffers to achieve high attenuation and voltage following; then it is converted into a single-ended signal to ground through a differential amplifier; and finally it is transmitted to the oscilloscope through a lossless transmission line. Common-mode rejection ratio is an important indicator of differential probes. The common-mode gain of active differential probes mainly comes from two sources: ① the incomplete symmetry of the resistance, capacitance, buffer and parasitic parameters of the two differential signal transmission paths; ② the common-mode gain inherent in the differential amplifier.

Optical isolation probe

The optical isolation probe is also used to measure differential signals. Its principle block diagram is shown in Figure 4. The probe mainly includes an attenuator, an electrical-optical-electrical conversion network, a lossless transmission line, and an oscilloscope connector. The electrical-optical-electrical conversion network is the core of the optical isolation probe. It realizes the electrical isolation between the device under test and the oscilloscope through the electrical-optical converter, optical fiber, optical-electrical converter, and controller, shortening the transmission path of the differential signal. This greatly improves the common-mode rejection ratio of the probe, allowing the optical isolation probe to measure differential signals with high bandwidth and high common-mode voltage.

Figure 4. Block diagram of an optically isolated probe.