PCB trace termination techniques to ensure signal integrity

Post your question about our webinar on PCB trace termination techniques to ensure signal integrity with EMA Design Automation here.

See below the Q&A recap from the webinar.

Question: Is Sierra Circuits compatible, import, and export with Orcad and KiCad?

Amit Bahl: Yes. We also accept IPC-2581.

Question: Are PCB termination required for low switching frequency? Or is it mainly used for high-speed circuits?

Ernie Frohring: It’s got to do with low frequency versus high frequency. It usually has to do with the wavelength of the signal.

The way that I like to think of it is if the wavelength of the signal is small compared to the structures on the board, then what you need is considered high frequency, and you have to take into account the system. That means that smaller circuit boards would require termination at lower frequencies.

One one way to think of it is the fact that, on a certain circuit board, the speed of light is about half. It’s a nanosecond.

Gigahertz is nanosecond. So, if and the rule of thumb is using a six. So in other words, if you have a if you have a structure which is a sixth of the side of the structure you’re dealing with, then you do have to do termination because the thing follows the transmission line rules.

If it’s low enough frequency so that the voltage on the board effectively change, you don’t change that much on one side of the board or one side of the trace to the other, then you don’t need termination.

And by the way, our termination checks are included in many of the tools that both OrCad and Cadence have for layout.

Question: Which termination is the least power consuming?

Ernie Frohring: The least power consuming would be the series of instance terminations because they only dissipate power when the signal is changing because there’s no current flowing through the resistor if it’s going up or down. The second would be the AC termination because the AC termination, again, the resistor has current flowing through it only when the level of the of the signal is changing.

Question: Why would AC trace termination permit wider frequency range?

Ernie Frohring: One thing with AC termination is because of the choice of the capacitor and the resistor can be different, you can take into account the frequency range of the termination. Now there’s a straight resistor termination that’s going to be only dependent on the frequency ranges at the transmission line and the length of the transmission line.

Question: Can the receiver impedance be modeled as an open high impedance termination?

Ernie Frohring: Yes. In fact, that’s pretty close to what it’s modeled as. The receivers generally have high impedance. And so the result is if I put an open circuit there, the submitted model would not be too much different than what you saw before.

Question: This is why when the signal reaches the receiver it reflects back and effectively doubles the voltage?

Ernie Frohring: That’s correct. And so, otherwise, the output impedance of the driver plus the termination resistance is matching the impedance of the transmission line. Therefore, the reflection coefficient is what is half. So it exactly doubles the voltage on the receiver. And when that signal goes back, it now matches the termination resistor so that all the energy is dissipated in the termination resistor on the bounce back. Therefore, you don’t get any bouncing.

Question: Are power planes considered reference layers?

Ernie Frohring: Yes. For the power plane, it is the reference layer. That’s another discussion we could have in terms of return current.

One of the things that happen in the pictures that we do, we’re always looking at the signal going out through the driver, through the transmission line of the receiver. But in reality, the current has to come back from the receiver to the driver somewhere. And that current comes back from a reference plane. It’s very important to look at your return path.

In other words, with high speed, the return path should be a reference plane right below the trace so there isn’t any impedance added by the return current.

Question: Why should the series resistor be close to the transmitter? What is the reason in terms of signal integrity?

Ernie Frohring: It’s more likely to cause kind of resonance problems. So in general practices we put it close to the transmitter, And it turns out that when we use a die transmission like that, it’s very close.

If it isn’t close, then you can have other issues happening before you get to your termination in the transmission lines.

Question: If the receiver input impedance already matches the transmission line impedance and driver output impedance, then why is termination still needed?

Ernie Frohring: If the receiver impedance matches the transmission line impedance, then effectively you’re doing what is the equivalent of parallel termination.