Types of oscilloscope probes

Before making measurements with a passive probe, it’s important to compensate. Probe compensation is a calibration process that fine-tunes the ratio of capacitances within both the probe and the oscilloscope input. Not compensating probes can result in measurement inaccuracies, impacting parameters such as amplitude and pulse shape. Therefore, it is crucial to perform probe compensation when using a probe-oscilloscope pair for the first time or when conducting critical measurements.

The compensation procedure is straightforward:

• Connect the probe to the scope's built-in compensation signal and ground.
• Adjust the compensation capacitor until the signal achieves a rectangular waveform.

This ensures that the probe accurately represents the input signal characteristics, allowing for precise and reliable measurements.

Want to learn more? Then check out our dedicated articles on Understanding passive oscilloscope probes and Understanding probe compensation.

Active probes - single-ended FET probes

An active probe, as the name suggests, incorporates active or powered components in its probe tip. The active component is typically a field-effect transistor (FET). A standout advantage of active probes is minimal loading over a wide frequency range - thanks to the low input capacitance that translates into a high input impedance. This ensures accurate measurements without unduly affecting the observed circuit.

In addition, active probes offer the benefit of a high input offset. In other words, an active probe can handle signals that aren't centered around zero volts. This feature is handy when you're dealing with signals that might have a DC (direct current) component or a non-zero baseline.

An active probe usually has a proprietary connector, which allows an oscilloscope to automatically detect and calibrate the probe. Power can be provided through this specialized interface or supplied externally. Note that most active probes require a 50-ohm termination setting on the oscilloscope channel.

Differential scope probes

Differential probes are designed to measure the voltage difference between two points in a circuit. They feature two inputs that can be connected to various points in the circuit, with no requirement for a ground reference at either location. Using an internal differential amplifier, these probes generate an output voltage that reflects the difference between the selected measurement points, often scaled by a user-defined attenuation factor.

An important characteristic of differential probes is their immunity to "common mode" signals, which are signals that are simultaneously present at both measurement points. This makes them great for measuring low-level signals in noisy environments. They can also be used for single-ended measurements, which are accomplished by simply grounding one of the leads.

Current probes

All the probes we’ve been talking about so far - passive, active and differential - produce a voltage at the oscilloscope input. This is because oscilloscopes measure voltage as a function of time. But what if you want to measure current? Well, you need a way to a way to create a voltage that corresponds to a current in a consistent and predictable way. In other words, you need to “convert” a measured voltage into a current value. For example, 1 V at the scope input could be used to indicate that the measured current is 1 A.

Current probes are a way to achieve this conversion. They work by capturing the electromagnetic field generated by the current flowing through the conductor and converting it into voltage using a known ratio of volts per amp. These probes are positioned or "clamped" around the current-carrying conductor, with an arrow marker indicating the direction of current flow.

Most current probes are active devices, which means that they need an external power source for operation. While all current probes can detect and measure AC, some are also capable of measuring DC currents. AC current measurements rely on a current transformer, while DC or very low frequency AC measurements use a Hall effect sensor.

Normal current probes typically cannot handle large currents. This is where high current probes come in. These probes are characterized by a special structure that allows them to measure high current with low resistance. They often use specialized sensors to measure the magnetic field generated by current flow. This enables non-contact measurement, which is essential when dealing with high currents. Additionally, high current probes usually offer higher accuracy and resolution compared to normal current probes. This is necessary for applications such as power electronics and energy systems, where small changes in current can have significant implications.

Current probes are often used for power measurements involving both voltage and current. In some cases, issues such as time offsets or "skew" can arise due to variations in propagation times within the probe leads. This skew can potentially lead to inaccurate power results.

Deskew fixtures are used to tackle this problem. They serve as specialized tools to identify and compensate for skew. These fixtures generate time-aligned voltage and current pulses, measured simultaneously by attached current and voltage probes. If the test waveforms exhibit skew, the appropriate deskew value can be entered on the oscilloscope. This correction ensures that the current and voltage waveforms are brought back into phase, mitigating the impact of skew on subsequent measurements and maintaining the accuracy of power calculations.