Ground Bounce Questions Answered

During a November 2021 webinar, Don’t Let Ground Bounce RuinYour Day (Part 7 of an 8-part Power Integrity webinar series), Dr. Eric Bogatin was asked several questions regarding his topic of presentation. Here are his answers.

Q: From what frequency should we consider ground bounce to be a problem?

A: Ground bounce is really due to a dI/dt. Generally, it becomes a problem with rise times shorter than 100 ns. The bandwidth of this is about 3.5 MHz. This means ground bounce can be an issue at relatively low frequency.

Q: How do you properly choose the best place to put your shorting via to accurately measure ground bounce?

A: Placement of the shorting via is based on where you want to measure the cross talk. If you care about crosstalk at a certain location, placing a shorting via and routing a quiet line to it is a good idea. This way, you are sensitive to the cross talk pick up everywhere along the signal-return path.

Q: On 10- or 12-layer boards, is there a problem with vias cutting into the ground plane and disturbing the return path?

A: As long as you keep the clearance holes to a minimum, with no non-functional capture pads, you will not have large cuts in the other planes. However, sometimes when vias are clustered together, their clearance holes can overlap, and you get inadvertent gaps in the return planes. It is always a good idea to check the planes to make sure there are no gaps.

Q: How does setting a line to quiet low tell anything about the way different nets are acting? There are a lot of possible “victim loops” in a design.

A: A quiet line acts as a sense line, picking up any noise occurring in the “victim loop” next to it. It is up to you to decide which are the aggressor nets likely to cause noise on the victim loops. You can toggle on and off nets to see which ones might be most sensitive at generating switching noise. If you choose the victim I/O pins well, you can see the switching noise on shared package leads.

Q: Can any line in a parallel bus be used as a "quiet low out" during the measurement?

A: Yes, exactly correct.

Q: Since a quiet low acts like a pass gate, and the I/O devices are usually large and can act as low pass filters, the noise you see will be different from the actual ground bounce, won’t it? Why does a rising edge create a ground bounce?

A: Yes, the return path discontinuity will act as a low pass filter, and the bandwidth of the signal getting through the path will be a little lower. It will also cause the rising edge to be a little longer. We will see ground bounce on both the rising and falling edge—it is any dI/dt: an increasing or a decreasing current.

Q: You showed the effect of ground bounce on device outputs, but can it also affect the input voltage to an I/O?

A: Correct. If the input voltage of the I/O is referenced to local ground or the internal ground rail of the IC, it will see the noise on the ground rail as part of its input voltage.

Q: Can we identify a screwed up return path using a TDR measurement?

A: Yes, but sometimes it is difficult to tell if the problem is due to the signal conductor or the return conductor. A discontinuity in either one will cause a reflection, which we will see in TDR.

Q: When measuring ground bounce, should we use only differential probes to isolate Probe GND and DUT GND?

A: If the ground of your system is not the same as the ground of the oscilloscope, absolutely use differential probes. When the grounds are the same, you can get away with single-ended probes. You are always measuring the local voltage difference between the ground near the signal where the probe is located and the signal pin.

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