Voltage Probe Types and Their Advantages

passive voltage probe

Passive probes are made of wires and connectors, and when compensation or attenuation is needed, resistors and capacitors are also included. There are no active components (transistors or amplifiers) in the probe, so no power supply is needed for the probe

Because they are relatively simple, passive probes are generally the most robust and economical. They are easy to use and are the most widely used probes. However, don't be fooled by their simplicity or structure—high-quality passive probes rarely have simple designs!

Passive voltage probes offer various attenuation coefficients for different voltage ranges—1X, 10X, and 100X. Among these passive probes, the 10X passive voltage probe is the most commonly used and is the type typically offered as a standard accessory for oscilloscopes

For applications where the signal amplitude is 1V peak-to-peak or lower, 1X probes may be more suitable, or even essential. In applications where low and medium amplitude signals are mixed (tens of millivolts to tens of volts), switching 1X/10X probes is much more convenient. But remember, switchable 1X/10X probes are essentially two different probes in one product, not only with different attenuation coefficients, but also with different bandwidth, rise time, and impedance (R and C) characteristics. As a result, these probes cannot fully match the oscilloscope input and cannot deliver the optimal performance achieved by standard 10X probes

Most passive probes are designed for general-purpose oscilloscopes, so their bandwidth range is generally between less than 100 MHz and 500 MHz or higher

However, there is a specific type of passive probe that offers much higher bandwidth, known as 50-ohm probes, Zo probes, and divider probes. These probes are designed for 50-ohm environments, typically high-speed device calibration, microwave communications, and time-domain reflectometers (TDR). Typical 50-ohm probes used in these applications have bandwidths of several kilohertz and rise times of 100 picoseconds or faster

active voltage probe

Active probes include or rely on active device operations, such as transistors. The most common active device is a field-effect transistor (FET).

The advantage of FET input is that it offers very low input capacitance, generally a few picofarads, with a minimum of less than 1 picofarad. This ultra-low capacitance can achieve a variety of effects desired by users

First, a low capacitance value C corresponds to a high capacitance reactance value Xc. This can be seen from the following Xc formula:

Since capacitive reactance is the main input impedance element of a probe, low capacitance leads to high input impedance over a wider frequency band. As a result, the specified bandwidth for active FET probes is generally between 500 MHz and several GHz

Besides higher bandwidth, the high input impedance of active FET probes allows measurements at test points with unknown impedance while reducing the risk of load effects. Additionally, due to low capacitance, ground interference is reduced, allowing for longer ground lines. Most importantly, FET probes provide very low loads, so they can be used in high-impedance circuits that place heavy loads on passive probes

Because of these positive advantages, including bandwidth of several GHz DC, you might ask: why use passive probes?

However, the high bandwidth of FET probes is a major advantage, with their linear voltage range covering many typical semiconductor voltages. Therefore, active FET probes are typically used for low-level applications, including fast logic series such as ECL, GaAs, and so on

differential probe

differential signals are signals that reference each other, not to ground. Figure 2 illustrates multiple examples of these signals, including signals formed in collector load resistors, disk drive read channel signals, multiphase power systems, and various other cases where signals essentially "float" above ground.

can detect and measure differential signals in two basic ways. Figure 3 illustrates these two methods.

As shown in Figure 3, using two probes for two single-ended measurements is a common method. Usually, it is also the least desirable method for differential measurements. However, this method is so commonly used because dual-channel oscilloscopes have two probes. Measuring the signal to ground (single-ended) and using the oscilloscope's mathematical function to subtract one channel from the other channel (channel A signal minus channel B) seems to be an excellent solution for obtaining differential signals. This solution can be used when the signal is low-frequency, has sufficient amplitude, and can exceed any concerning noise

Combining two single-ended measurements has multiple potential issues. One of these issues is that there are two separate long signal paths along each probe until each oscilloscope channel. Any delay difference between these two paths will cause a time shift between the two signals. On high-speed signals, this offset can cause significant amplitude and timing errors in the computed differential signal. To minimize this error, matched probes should be used

Another problem with single-ended measurement is that they cannot provide sufficient common-mode noise suppression. Many low-level signals, such as disk read channel signals, are transmitted and processed differentially to take advantage of common-mode noise suppression. Common-mode noise is noise caused by nearby clock lines on two signal lines, or noise from external sources such as fluorescence. In differential systems, this common-mode noise is generally removed from the differential signal. Successfully achieving this function is called the common-mode rejection ratio (CMRR).

Due to channel differences, as frequency increases, single-ended CMRR performance quickly drops to disappointing levels. If the signal source maintains the common-mode rejection ratio, this will cause the signal to exceed actual noise levels

On the other hand, differential probes use a differential amplifier to subtract two signals, allowing one oscilloscope channel to measure a single differential signal (Figure 3b). This can deliver significantly higher CMRR performance over a wider frequency range. Additionally, advances in circuit micronization technology allow differential amplifiers to be moved to the actual probe head. In the latest differential probes, such as the Tektek P6247, this achieves CMRR performance from 60 dB (1000:1) at 1 MHz to 30 dB (32:1) at 1 GHz. As disk drive read/write speeds reach and exceed the 100 MHz mark, this kind of bandwidth/CMRR performance is becoming increasingly essential

high-voltage probe

"High voltage" is a relative concept. What is considered high voltage in the semiconductor industry actually has no meaning in the power supply industry. But from the probe perspective, we can define high voltage as any voltage that can be safely handled by a typical general-purpose 10X passive probe

Generally, the maximum voltage of general-purpose passive probes is around 400 - 500 V (DC peak AC). On the other hand, high-voltage probes can have a maximum rated voltage of up to 20,000 V. An example of this probe is shown in Figure 4

Safety is especially important for high-voltage probes and measurements. To accommodate this, many high-voltage probe cables are longer than standard cables. A typical cable length is 10 feet. Placing the oscilloscope outside the safety enclosure or behind the safety cover is usually sufficient. Additionally, if you need to further remove the oscilloscope from the high-voltage power supply, a 25-foot cable can be chosen

reprinted on Tektech