Without a high-quality probe, the highest resolution of the oscilloscope ADC is meaningless

There is a common misconception that measurement accuracy is determined only by instrument specifications, such as the number of waveforms displayed on the screen. However, the factors that affect actual accuracy are much more complex. Accuracy is closely related to the measurement setup and how well it maintains the integrity of the signal being measured. The validity of any measurement ultimately depends on how well the signal integrity is maintained throughout the measurement process.

To make accurate signal measurements, an oscilloscope must be connected to the electronic circuit being measured via probes. Probes play an important role in ensuring that the signal reaching the oscilloscope is pure, undistorted, and as close as possible to the original signal flowing through the circuit . Without proper signal conditioning, even a high-resolution oscilloscope can produce misleading results, reducing the usefulness of the oscilloscope in actual measurement scenarios.

Oscilloscope Accuracy: ADC Resolution Isn't the Only Factor

There is no single parameter that affects the accuracy of an oscilloscope's signal measurements. The most common mistake people make is to confuse the accuracy of an oscilloscope with the resolution of the oscilloscope's ADC (analog-to-digital converter). While higher ADC resolution means that a signal can be represented with more discrete levels, higher resolution does not account for real-world issues such as noise, distortion, or the quality of the signal path feeding the ADC.

To achieve higher measurement accuracy, the effective number of bits (ENOB) is often used. ENOB takes into account noise, distortion, and other imperfections that affect the oscilloscope's front-end measurement chain. But the measurement chain does not start at the oscilloscope connector, but at the probing system. Achieving high ENOB requires not only a good oscilloscope, but also excellent signal conditioning throughout the probing system. In some cases, a low-resolution oscilloscope with high-quality probes and proper signal conditioning can produce more accurate measurements than an oscilloscope with a higher nominal ADC resolution but poor signal conditioning.

Essential Probe Characteristics for Accurate Measurements

To ensure that the signal reaches the oscilloscope with the least possible attenuation, two key factors must be considered: the probe specifications and the skill of the engineer using the probe. Different types of probes have different characteristics that are important for proper signal conditioning in various applications.

For example, the importance of passive probes is often underestimated. Passive probes are used to attenuate the signal before it reaches the oscilloscope, thus preventing the front end from exceeding the input voltage limit. Passive probes with an impedance of 1 MΩ are ideal for high impedance circuits because they measure the signal with minimal interference. However, passive probes also need to provide minimal capacitive loading and wide bandwidth to match the input bandwidth of the oscilloscope. For example, the Tektronix TPP1000 passive probe offers 1 GHz bandwidth with less than 4 pF of capacitive loading, helping to maintain signal integrity.

Figure 1: The additional capacitance of a probe increases the rise time of the measurement by adding both resistance and capacitance.

In high-speed digital signal applications, passive probes are not an ideal choice because they can overload the circuit and limit the signal bandwidth. For this reason, we offer active probes with accurate built-in amplifiers that also present a low impedance load to the circuit and maintain signal fidelity. In some cases, the application requires the maximum possible bandwidth and voltage capability, such as fast switching power electronics components supported by wide bandgap technology.

This case study clearly explains why the “oscilloscope + probe” chain is more important than the oscilloscope ADC resolution alone. In fact, in these scenarios, in noisy environments or when dealing with small differential signals buried in large common-mode noise, the probe itself is relied upon to suppress noise and interference. Tektronix active probes have low input capacitance and high input impedance, which means that the probe has minimal loading effect on the circuit under test. This is very important to help maintain the integrity of the signal and ensure that the measurement itself does not change the behavior of the circuit. In some cases, galvanic isolation is required to achieve common-mode rejection performance over a wide bandwidth, just as with new wide-bandgap materials that enable testing of fast-switching power electronics components.

Signal Path and Measurement Chain

The importance of the measurement chain extends beyond the probe body. The entire signal path, including cables, adapters, and probe tips, plays a very important role in maintaining signal accuracy. Proper shielding and isolation, as well as avoiding ground loops, are necessary to prevent common-mode noise from distorting the signal.

Each specific measurement scenario requires a well-designed probing system, including appropriate probe tips and accessories optimized for different types of test points, such as fine-pitch leads or test pads. Using the right probe tip ensures quality signal capture and avoids introducing additional resistance, capacitance, or inductance that can distort measurements.

Figure 2: The complete signal path from the test equipment to the oscilloscope input.

Figure 3: IsoVu probes reject common-mode interference and provide a true view of the signal.

Conclusion: Signal Conditioning Facilitates Accurate Measurements

While oscilloscope resolution is an important factor affecting signal measurements, it is not the only consideration. Even a high-resolution instrument can produce inaccurate results if poor signal conditioning causes the signal to become corrupted before it reaches the oscilloscope. Proper signal conditioning with high-quality probes ensures that the integrity of the signal (its amplitude, frequency, and phase characteristics) is faithfully transmitted from the circuit under test to the oscilloscope. This avoids the introduction of noise, distortion, or loading effects, resulting in more reliable and accurate measurements.