Understand the basics of capacitive coupling.
Learn the pros and cons of capacitive coupling.
Get tips on how to reduce capacitive coupling.
Ask any teacher how many kids it takes to turn a classroom into a loud circus and the answer will be, “Just two”. This is especially true when the two kids in question are talkers and seated next to each other. With little separating them, it doesn’t take much for a few side remarks to turn into a full-blown conversation. Soon, the entire lesson must be halted to address the chatty pals in the corner. As the parent of a talkative son, I’m all too familiar with the frustration this causes a teacher.
Of course, one of the first lessons a teacher learns is that seat assignments matter, and once you identify your “talkers”, you place them as far apart as possible if you want to keep the classroom running efficiently.
This same principle applies to noisy traces in a PCB. The good news is the answer to, “How to reduce capacitive coupling?” might be more straightforward to solve than unwanted classroom conversations. In this article, we’ll explore the basics of capacitive coupling and learn how to effectively reduce it to avoid crosstalk.
What Is Capacitive Coupling?
Capacitive coupling is the transfer of electric energy between two insulator-separated conductors.
If you’re familiar with how a capacitor works, you’ll find it easy to understand capacitive coupling. Otherwise, here’s a quick refresher:
Capacitors are made from two conductive terminals which are separated by an insulator. When one of the terminals is brought to a higher voltage potential than the other, electric charges build up between the terminals. When the voltage is removed from the terminal, the capacitor releases the charges in the form of a current.
The behavior of the capacitor makes it reject a direct current (DC), but it becomes a low impedance path for an alternating current (AC). In a PCB, there are various elements that could form a virtual capacitor and allow energy to transfer between them.
Capacitive coupling, also known as electrostatic coupling, is how energy moves between conductive elements that are separated by insulators. One example is if you place two copper traces close to each other, the capacitive coupling will cause the energy on one trace to be transferred to the other.
In the next section, we’ll take a closer look at how this energy transfer could be a positive or negative thing, depending on your goals.
The Good and Bad of Capacitive Coupling
Capacitive coupling often causes crosstalk in electronics.
Looking on the bright side, a talkative child can be a sign of a future leader, debater, or lawyer. In this same way, capacitive coupling can be good or bad, depending on the circumstances.
When intentionally applied in electronics, the principle of capacitive coupling gives rise to the miracle of modern devices like touch panels and capacitive sensing buttons. These devices detect and convert the changes in measured capacitance into user inputs.
However, capacitive coupling that results from unintended capacitance or parasitic capacitance is disruptive to a circuit’s performance. Capacitive coupling is what causes crosstalk, where the signal from one conductor is coupled into an adjacent one.
For example, if SPI (serial peripheral interface) data runs close to an analog signal, the latter will pick up faint pulses of the SPI transmission. Mutual capacitive coupling can also occur between two traces carrying high-speed signals, compromising the integrity of both signals. As a result, you’ll get high error rates at the receiving end of both signals.
How to Reduce Capacitive Coupling
Shorten traces to reduce capacitive coupling
The effect of capacitive coupling will rear its ugly head when the PCB is in operation. By then, there’s nothing you can do to mitigate its effect. To reduce capacitive coupling, you must start during the design stage.
Here are three proven ways to reduce capacitive coupling in your PCB layout.
Increase Distance Between Traces
Understandably, space is a luxury in a classroom or on a PCB. Still, if it’s possible, try to increase the distance between your “talkers” or adjacent traces, particularly on those carrying high-speed signals. As capacitance is inversely proportional to the distance between terminals, keeping traces further apart will reduce capacitive coupling.
If there isn’t much space to work on, consider shielding high-frequency traces on both sides with a ground terminated copper strip. The ground strip acts as a shield and prevents the charge from coupling to other traces.
Keep Traces Short
Long traces increase the area of the conductor, which increases capacitive coupling. Therefore, keep high-frequency traces as short as possible.
Luckily, how to reduce capacitive coupling isn’t as hard as you think when you have advanced PCB design software. With OrCAD’s range of tools, editing a PCB layout is a breeze and helps you avoid unintended capacitive coupling. You can also use InspectAR to interactively assess and improve your PCB design process using augmented reality. Inspecting, debugging, reworking, and assembling PCBs has never been simpler or more accurate.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.