PCB – part 1 of X… Ground bounce

Hi,

I’m really happy by the great interest in printed circuit board tips and tricks… This is the first post in a series of posts in this series. Other series might come in the future. I will cover both the digital domain and some analog (no pure RF designs as this is not my area of knowledge). However, your comments and ideas will guide me through this series of posts. Hope you will find some parts that you can make use of in your designs.

So, today will be about ground bounce. For some of you this might sound really weird, but in digital designs and especially high-speed designs, this effect really matters. As with any kind of analog or digital design it’s important to understand how things work in a bit more detail level than just input high or low, frequencies, static voltages, static currents and so forth. Ground bounce may cause our designs to malfunction or being unreliable if we don’t understand this parameter. Ground bounce may cause a high level on a digital input, for a short period of time, be seen as a zero by the chip inside the IC. How can this be possible when I have double checked input signal levels for input high and input low? It might be due to this ground bounce effect.

First. A chip is encapsulated in a plastic package with pins or balls that is connected to the PCB. Between the pin or in some cases the ball and the chip there is a thin wire called bonding wire. This includes all connections between the chip and the pins or balls. Including power supply and ground as well. Any wire has an impedance. And the longer the wire the higher impedance. From Wikipedia ” impedance is the measure of the opposition that a circuit presents to a current when a voltage is applied.” A wire acts like an inductor and this means that as in this case when the chip dynamically changes its current consumption there a voltage over the bonding wire. This means there is a temporary difference in the potential between the chip ground and the ground of the PCB (there is a voltage between the grounds). When digital outputs of the chip switches there are spikes in the current consumption of the chip due to the output switching transistors. These spikes might increase if the output drives highly capacitive loads. And if the ground signal from the PCB ground plane to the ground pin or ball of the package is not very short – this just add to this ground bounce voltage (a temporary difference in the potential between the chip ground and the ground plane of the PCB).

So what should we do about this in our PCB layouts?

  • Keep the signals from ground plane to ground pins/balls as short as possible.
  • Avoid highly capacitive loads on switching outputs.
  • SMD capacitors are great due to low impedance at high frequencies (no legs).

That’s all for this post… My intension is to write 1-2 PCB-posts per week for some time. See you soon and take care!

/Anders

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