High Frequency Measurements Web Page
Douglas C. Smith

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Technical Tidbit - April 2012
Single Point Grounding - Not Achievable at High Frequencies
(greater than a few MHz)

An example of "single point ground noise current"

Figure 1.
Voltage Drop Across a PCB's Very Short "Single Point Ground" Connection
(Vertical scale ~ 3V/div, Horizontal Scale = 5 ns/div)

Abstract: Single point grounding is used successfully at power line frequencies and at DC to keep currents from flowing in unintended paths within electronic systems as well as in a few other applications. But, above a few MHz and sometimes even at lower frequencies, single point grounding is not achievable and significant currents and voltages will couple through parasitic capacitance and inductance in the physical system. After a discussion of some of the issues, a list of articles on this website that address some of the problems that can result when single point grounding is attempted in the presence of high frequency sources like ESD and EMI are given.

Discussion: Figure 1 shows the voltage induced across a short 1.5 cm long single point ground connection to a circuit board when the underlying "chassis" is hit with an ESD pulse. One would think that since the board is grounded in only one point there should be no current in the ground connection. But even a single picofarad of capacitance can allow Amperes of current to flow that will produce significant voltage drop in circuits. Figure 1 appears in the May 2002 Technical Tidbit on this website.

Consider a 2000 Volt change in potential across one pF of capacitance:

I = C dv/dt = 10-12F * 2000V/10-9s = 2 Amperes!

A 2000 Volt ESD event is below the threshold that most humans can perceive!

The vertical scale in Figure 1 is about 3 Volts/div so the peak is about 10 Volts. And this across a short, thick wire about 1.5 cm long! So we see that "single point ground" is not necessarily effective at high frequencies. In my experience, ESD and EMC problems often result from single point grounding systems unless very special attention is paid to parasitic loops formed by inductance of conductors and capacitance between conductors.

A list is included below the summary to four other Technical Tidbits on this website that discuss this topic.

Summary: Single point grounding systems can cause significant problems at high frequencies where many form of electrical stress, such as EFT, ESD, and RF sources have significant energy. Only with very close attention to the design can these high frequency problems be avoided in single point grounding systems. Most of these systems I have seen have not been successful in avoiding ESD and EMC problems.

I am writing this from my new office in Boulder City, NV!!! Still more work to do on the move so the next few weeks will be very busy and the May Technical Tidbit may be a little late as well. In addition, once the move is complete, I expect to do morning 5 to 10 minute podcasts on technical topics every morning I am in the office. These podcasts will appear on the home page of http://CircuitAdvisor.com by late morning each day except for days when I am not in the office.

Additional articles on this website related to this topic are:

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If you like the information in this article and others on this website, much more information is available in my courses. Click here to see a listing of upcoming courses on design, measurement, and troubleshooting of chips, circuits, and systems. Click here to see upcoming seminars in Boulder City, NV.

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Copyright 2012 Douglas C. Smith