Understand stray capacitance in a circuit.
Learn how stray capacitance affects electronic circuits.
Explore strategies for reducing stray capacitance in a circuit.
I love pets, and yet I would never own one because of the great responsibility involved. I’ve seen how domestic dogs and cats are abandoned by their owners because of the necessary care they require. In fact, I find myself making friends with strays that I’ve come across in the city in which I live.
What do stray pets have to do with capacitance? These abandoned pets are left to wander through streets and alleyways, much like stray capacitance, which you will find lurking about in circuits. In this article, we’ll take a look at how stray capacitance occurs in an electronic circuit, how it can affect that circuit’s performance, and how to reduce stray capacitance in your next design.
What Is Stray Capacitance?
Stray capacitance is like having invisible capacitors on the circuit.
Basic electronics describes capacitance as a measurement of charge that builds up across two terminals of different voltage potential. It is how capacitors are produced. Stray capacitance is the unintentional manifestation of electric charge in a circuit or non-capacitive components.
Much like a stray dog or cat, stray capacitance just happens to be where it is due to the circumstances. It is quite easy for stray capacitance to materialize on a circuit. All you need is two conductive elements that are closed enough over an insulator to behave like a capacitor.
Stray capacitance can be present on components like an inductor, transistor, or a diode. Even for two parallel conductors on a PCB, there will always be some stray capacitance that exists. You’ll also find stray capacitance in any conductors with respect to the ground.
On paper, stray capacitance is expressed by the formula C = Q/V. It is the measurement of the charge that builds up over a differential potential. In PCB design, you’ll find the formula C= ϵA/D, which implies the relationship of the dielectric constant of the insulator, area, and distance between conductors for the capacitance value.
Does Stray Capacitance Affect Circuit Performance?
Stray capacitance effects are apparent at high-frequency.
I can vouch that most stray pets are usually quite harmless. As for stray capacitance, it depends on the frequency. When you place a capacitor on a circuit, it acts as an open circuit in low frequency. As the frequency increases, the same capacitor starts allowing current to pass through it.
Therefore, if you’re working on a low-frequency design, you’re unlikely to end up with serious issues of stray capacitance. For example, a simple LED blinker will not be affected by stray capacitance. You’ll find issues creeping up when the design involves high-frequency that could significantly reduce the reactance of the stray capacitance.
At high-frequency, elements with stray capacitance tend to act like a short circuit or a virtual resistor. The effects of stray capacitance are a notorious issue with op-amp designs, particularly between the input and output of the amplifier. It creates a feedback path that will inaccurately increase the gain and change the peak frequency.
In transmission lines, stray capacitance between conductors can lead to transmission loss. This means degradation in transmission power and signal quality. Stray capacitance can also form between a conductor and an adjacent ground plane, which causes signal integrity issues at high frequency. Cross talks and EMI noise are some of the concerns of unwanted capacitance.
How To Reduce Stray Capacitance
Spreading out copper traces minimizes stray capacitance.
It isn’t all bad news when discussing stray capacitance. While it can’t be totally eliminated, there are some ways to minimize stray capacitance in a circuit.
1. Place conductors further apart
Whether it’s the traces on the PCB or cables running alongside each other, you can reduce stray capacitance by increasing the distances among them. Capacitance is inversely proportional to distance and that’s an important rule in mitigating stray capacitance.
2. Shield the conductor
If you’re worried that stray capacitance builds up from adjacent traces, you can add a strip of ground copper between them. The copper strip acts as a shield in preventing the charges from building up.
3. Decrease trace width
Capacitance increases when the cross-section area of the conductor increases. Therefore, try to minimize the trace width, particularly for those conducting high-frequency signals.
4. Remove inner-layer ground plane
A huge inner-layer ground plane may be great for thermal management and EMI control, but it’s a bad idea for mitigating stray capacitance. Make this a point of consideration when pouring the ground plane for your design.
With the right PCB design software, you can ensure that your circuit design is free from the effects of stray capacitance. OrCAD PCB designer has the required design and analysis tools to ensure uncompromised signal integrity. You can also use InspectAR to interactively assess and improve PCBs 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.
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