Not all oscilloscopes are created equal: Why ADCs and low noise floor matter

By Michelle Tate, Product Marketing Manager, Keysight Technologies

In the field of engineering, accuracy is the core element. Whether it is performing quality and performance testing on advanced electronic equipment or debugging complex systems, the level of measurement accuracy is directly related to the success of the project. At this time, the concept of vertical accuracy in the oscilloscope is particularly important. It measures the consistency between the voltage and the actual measured signal voltage. To achieve high vertical accuracy, the key lies in two factors: one is the number of bits of the analog-to-digital converter (ADC), and the other is the background noise of the oscilloscope.

The role of ADC bits

The horizontal axis of an oscilloscope represents the time base, usually expressed in s/div, while the vertical axis represents voltage, expressed in V/div. Vertical accuracy is related to the accuracy of the signal voltage displayed by the oscilloscope, which is crucial for intuitively displaying the waveform and characteristics of electrical signals and achieving accurate measurements. In short, the closer the voltage value read on the oscilloscope screen is to the actual signal voltage, the higher its vertical accuracy.

To get the best readings, engineers rely on oscilloscopes with the highest ADC bits and lowest noise floor. Higher ADC bits provide higher vertical resolution, allowing for more accurate display of signals. Lower noise floor minimizes interference with the signal by the oscilloscope. This combination ensures that the oscilloscope can accurately capture and display the details of the signal, minimizing any distortion or noise that could affect the measurement results.

Specifically, an oscilloscope with an 8-bit ADC can encode the analog input signal into 256 different levels (28 = 256). Every time the ADC bit number increases by one bit, the number of signal levels it can convert doubles. Therefore, the conversion accuracy of a 9-bit ADC is 512 signal levels (29 = 512), the conversion accuracy of a 10-bit ADC is 1,024 signal levels (210 = 1,024), and so on.

An oscilloscope equipped with a 14-bit ADC can encode the analog input signal into 16,384 levels (214 = 16,384). This resolution is 4 times that of a conventional 12-bit ADC oscilloscope and 64 times that of an 8-bit ADC oscilloscope. Thanks to this ultra-high resolution, the oscilloscope can capture finer details in the signal, thereby providing a more accurate waveform display.

When this principle is applied to an oscilloscope set to a vertical scale of 100 mV per division and with 8 vertical divisions, the full-screen display range of the oscilloscope reaches 800 mV (100 mV/div * 8). When an 8-bit ADC is used, this 800 mV full-screen range is divided into 256 levels, which means that the resolution of each level is about 3.125 mV. In contrast, when a 14-bit ADC is used, the same 800 mV display range can be finely divided into 16,384 levels, making the resolution of each level as high as 48.8 µV. As shown in Figure 1, such a significant increase in resolution allows engineers to capture more subtle changes in the signal and make precise measurements.

Figure 1: As the number of ADC bits increases, the number of signal levels converted also increases. This results in higher vertical resolution, allowing engineers to measure smaller signal changes.

The Importance of a Low Noise Floor

While higher ADC bits are critical to vertical accuracy, they are not the only consideration. The oscilloscope’s noise floor also plays a pivotal role. The noise floor here refers to the inherent noise generated by the oscilloscope itself, which may interfere with the signal being measured, causing inaccurate readings.

All electronic devices, including oscilloscopes, generate some level of noise. However, Keysight's goal is to reduce this noise as much as possible. A lower noise floor means the oscilloscope will have less impact on the signal, resulting in more accurate measurements. Also, it is important to note that signal details smaller than this noise floor will not be visible to the user. This is especially important when measuring tiny voltages, where even a small amount of noise can cause significant deviations in the readings.

For example, Figure 2 shows an oscilloscope measuring a 53 µV signal. When set to 2mV/div, the oscilloscope's noise floor is less than 50 µV. Thanks to such a low noise floor, the oscilloscope is able to successfully capture the tiny 53 µV signal. However, if other general-purpose oscilloscopes are used, since their noise floors often exceed 100 µV, this signal is likely to be obscured by the noise and cannot be observed.

Figure 2: An oscilloscope with a noise floor of less than 50 µV can capture a weak signal of 53 µV, which would be buried in the noise floor of other general-purpose oscilloscopes and not be observed.

Combining high ADC bit count and low noise floor

Combining a high-bit ADC with a low noise floor enables extremely high vertical accuracy. This allows the oscilloscope to capture and display signals with the greatest accuracy, allowing engineers to make precise measurements and effectively avoid errors that could lead to costly errors.

For example, if an oscilloscope has a 14-bit ADC and can maintain a noise floor of less than 50µV for a 50 Ohm input at 2 mV/div, 1 GHz bandwidth, it will demonstrate excellent vertical accuracy, allowing engineers to capture the tiniest changes in a signal. This advantage is critical for engineers to gain insight, understand, debug, and characterize their designs. In addition, inaccurate oscilloscope results may extend development cycles, affect product quality, and even lead to increased risk in component selection. Therefore, engineers need to rely on tools and technologies that can provide them with the best insights and the highest accuracy.

Summarize

It must be understood that not all oscilloscopes are created equal. Engineers should select oscilloscopes with the highest number of ADC bits and the lowest noise floor to ensure the highest vertical accuracy. This combination ensures that the oscilloscope can accurately capture and display the signal, minimizing any distortion or noise interference that may affect the measurement results. High vertical accuracy is critical for precise measurements, reducing errors, and saving time and resources. Therefore, if engineers invest in and use oscilloscopes with high vertical accuracy, they can have confidence in their measurement results and improve debugging efficiency.