Stray capacitance is unintended and unwanted capacitance in a circuit.
Capacitance doesn't exist only within capacitors. In fact, any two surfaces at different electric potential, and that are close enough together to generate an electric field have capacitance, and thus act like a capacitor. Such effects are often present within circuits (for example between conductive runs or component leads), even though they are not intended. This unintended capacitance is referred to as stray capacitance, and it can result in a disruption of normal current flow within a circuit.
Stray capacitance is the unwanted capacitive effect that occurs when two metallic parts are placed very close to each other on a PCB or any other circuit board.
Designers of circuits try to minimize stray capacitance as much as possible. They do this by keeping the leads of electronic components very short and grouping components in such a way to eliminate capacitive coupling.
Fig. 1: Stray capacitance concept.
A good example of this is an inductor. When buying an inductor fresh from a manufacturer, the leads of the inductor are usually quite long.
Fig. 2: Inductor.
The leads can extend for several inches on inductors, as can be seen from the illustration above.
However, plugging an inductor with the leads this long into a circuit can create some issues in the circuit.
When you have inductors that have very long leads close to each other, these leads are, essentially, wires. When you have wires close to each other in circuits, they can produce a capacitive effect. Even a small amount of wire can have considerable capacitance. When you have this, the capacitance can serve to impede or block low-frequency signals. This is because capacitance or a device that acts as an capacitor has high impedance to low-frequency signals. Thus, it is difficult for low-frequency signals to pass through a circuit that exhibits capacitive qualities.
When you add unwanted capacitance into a circuit, the circuit can now act (again unwanted) to block low-frequency signals. Thus, an inductor, which is a device that has low reactance, or impedance, at low frequencies, may now impede low-frequency signals from passing through. If this is radio circuit or an audio circuit, entire ranges of frequencies may be blocked, which of course can cause poor transmission of numerous frequencies. This is why it is desired that no stray capacitance exist in a circuit.
Therefore, this is the reason inductor leads must be kept short, less than 1.5mm in length ideally, to effectively stop capacitive effects, which can limit an inductor's ability to pass low-frequency signals.
Surface mount inductors are even better to use in circuits because their lead-less terminals being placed directly on the power plane of a circuit stops almost all capacitance. This is because surface mount components have no leads, but just terminals. This will limit the inductance that inductors can emit and will allow them to pass low frequency signals better in a circuit.
How to Reduce Stray Capacitance
In many applications, the stray capacitance between multiple signal lines could deplete or impact the design. It is worth mentioning that at lower frequencies, the stray capacitance can often be ignored. However, it could be the main problem in electric circuits at high frequencies. We can control stray capacitances at the layout level.
Stray capacitances often arise due to an electrical coupling generated between a signal line and the other signal line or the substrate and a signal line. It is worth mentioning that it can become essential to lower the stray capacitance of a specific net regarding other signals in many designs.
These are a few of the most effective methods for decreasing stray capacitance:
- Increasing the spacing between the various nets from the specific set is crucial (stray capacitance is quite important).
- I am using higher metals for those nets where the stray capacitance is vital.
- Avoiding excess parallel routing of metals
- Place another reference signal between networks that require low (where the stray capacitance is not essential). It is called shielding.
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