Oscilloscope Probe Grounding: What to Watch For

You may encounter detection-related issues when using an oscilloscope. These issues manifest in two categories: probe load and probe grounding. Probe load issues usually affect the device under test, and oscilloscopes probe grounded issues can affect the accuracy of the measuring instrument's data

> probe design minimizes the first probe load issue, while the second problem can be solved through accumulated detection experience

> first ask a question: Is grounding the oscilloscope probe considered grounding?

This statement is self-evident, but for oscilloscope probes, this may not inaccurate. The probe's grounding method will be incorrect. The probe's grounding lead has inductive properties, and its impedance increases with frequency. The longer the probe ground lead, ground lead, the larger the loop caused by the lead, which also becomes a larger antenna that picks up stray noise. It is best to always use the shortest possible ground connection

the oscilloscope probe input impedance

logic probes are passive probes that provide high input impedance and high bandwidth. They often provide some signal attenuation to the oscilloscope, usually 20 dB

The input impedance of a passive probe is usually specified based on parallel capacitance and impedance. Impedance is the sum of the terminal resistance value and the input impedance of the testing instrument (see the figure below).

capacitance is a series of combinations of end compensation capacitors and cables plus instrument capacitors grounded parallel to stray end capacitors. When this results in the correct input impedance specification for DC and low frequencies, the high-frequency model of probe input is more useful. This high-frequency model considers pure end grounding capacitance and series end impedance, as well as the cable's unique impedance(Zo)

DC and low-frequency probe equivalent circuits

high-frequency probe equivalent circuit

The impedance diagrams for both models are shown in these diagrams. By comparing these two graphs, you can see that both the series end resistance and the cable's unique impedance clearly expand the input impedance. The stray end capacitance is usually smaller (1pF), set the final breakpoint on the impedance chart

impedance and frequency of the two probe circuit models

logic probes are represented by the high-frequency circuit model shown above. They are designed to provide as many series of end impedances as possible. With proper mechanical design of the probe end assemblies, stray end ground capacitance is minimized. This provides the maximum input impedance for high frequencies

oscilloscope probe grounded

The oscilloscope probe grounding is a low-impedance path through which current returns from the probe to the source. Increasing the length of this path will create a large common-mode voltage input to the probe at high frequencies. According to the following equation, the behavior that generates voltage acts as if the path is a sensor:

Increase grounding induction (L), Increase current (di) or decrease transition time (dt) will cause voltage to increase (V). When this voltage exceeds the threshold voltage defined by the oscilloscope, incorrect data measurement will occur

grounding a probe and sharing it with many probes will force all the current to flow back to each probe as it passes through the probe (whose ground return is shared). The result is that in the above equation, the current increases (di), and depending on the transition time (dt), the common-mode voltage may increase to a level that produces erroneous data

common-mode input voltage model

Besides common-mode voltage, a long ground loop also reduces the pulse fidelity of the probe system. Rise time increases, ringing increases due to dry LC at the probe input LC circuit. Because digital channel displays reconstruct the waveform, they do not show ringing or disturbances. By examining the waveform display, you won't find grounding issues. In fact, problems may be found through cluttered burrs or contradictory data measurements. Use analog channels to view ringing and disturbances

the the best detection habit

due to variable L, di and dt, you may not be able to determine how much margin there is in the measurement settings. Below is a guide to develop good oscilloscope probe detection habits:

• If capturing data using any channel in a group, the ground wires of each digital channel group (D15–D8 and D7–D0) should be connected to the ground of the device under test.

• When capturing data in a noisy environment, in addition to grounding the channel groups, a ground should be used for every three digital channel probes.

• For high-speed timing measurements (rise time

When designing high-speed digital systems, you should consider designing dedicated test ports that directly face the instrument probe system. This will make the measurement setup easier and allow repeated use for acquiring test data. The design of the 01650-61607 16-channel logic probe and 01650-63203 terminal adapter can more easily connect to industrial standard 20-pin PCB connectors. The cable is a 2m logic analyzer probe cable, and the terminal adapter can provide the correct RC network through a highly convenient software package.

Reprinted from Rohde & Schwarz "What are the Precautions for Oscilloscope Probe Grounding?"