Minimize Crosstalk With Capacitive Coupling Noise Reduction Methods

Key Takeaways

• Capacitive coupling noise is dependent on voltage variations in a circuit and the value of coupling capacitance. The coupling capacitance is influenced by the distance between two circuits.

• The most prominent method of capacitive coupling noise reduction is shielding.  A Faraday cage is a type of shielding used to reduce coupled interferences. 

• The presence of stray capacitance is a catalyst to increase the coupling capacitance value. If the stray capacitance is reduced or eliminated, it changes the coupling capacitance value, reducing the capacitive coupling noise in the circuit.

Coupled noise in a circuit is called crosstalk

Sometimes, in circuits, analog and digital signals are mixed. In most cases, this mixing happens from noise coupling—either capacitive noise coupling or inductive noise coupling. In a circuit, this coupled noise is called crosstalk and can be described as a type of common-mode interference. It is essential to prevent the mixing up of signals and one of the methods used to do this is called capacitive coupling noise reduction. 

Capacitive Coupling Noise  

Consider two circuits or two wires operating close to each other. Intrinsic capacitance exists between the circuits or conductors. The capacitance forms a conductive path for the signals to mix up between the lines and introduce noise into the circuits. These capacitive coupling noises can also be called electric field coupling or near-field interferences. 

Capacitive coupling noise is dependent on the voltage variations in the circuits and the value of coupling capacitance. The coupling capacitance is influenced by the distance between the two circuits. When the circuits are closer to each other, the coupling capacitance increases and vice versa. The coupling capacitance can also be called mutual capacitance. 

Capacitive Coupling Noise Reduction Methods

The best method to reduce capacitive coupling noise is to decrease the coupling capacity. As we discussed above, the coupling capacity is inversely proportional to the distance between the circuits, so increasing the distance between two circuits is an effective solution. However, due to the downsizing of circuits, this is a fairly impractical method. Instead, consider the methods suggested below.

Shielding

The most prominent method of capacitive coupling noise reduction is shielding. A Faraday cage is a type of shielding used for reducing coupled interferences. The shielding is placed between the capacitively coupled circuits and is connected to the ground at one point other than the signal source connection. The ground connection routes the noise currents directly into the ground. While using a Faraday cage, it is important to ground the shield. If left open-circuited or floating, it increases the capacitive coupling noise in the system. 

Limit the Length of Parallel Wires

When two wires are running parallel to each other, natural capacitance is created between them. The wires act as the plates of the capacitor and the air is the dielectric. Limiting the length of the wires running parallel can reduce the coupling capacitance. 

High DV/DT Signals

In switching circuits, high dv/dt signals are a source of coupling capacitance. They mostly induce coupling capacitance between the switching points and the ground, which leads to common-mode interference noise. The capacitive coupling noises generated from high dv/dt switching signals can be minimized by reducing the dv/dt of the signals. This is possible by increasing the signal rise time. 

Grounding Methods

The use of proper grounding schemes in circuits is a preferred capacitive coupling noise reduction method. Grounding methods such as single-point, multipoint, or hybrid grounding need to be selected by considering circuit specifications, especially the frequency of operation. 

Trace Length

In circuit boards, keeping the traces short is a possible method to reduce capacitive coupling noise. The equalization of input lead lengths is another coupling capacitance reduction method. 

Stray Capacitance

The presence of stray capacitance is a catalyst to increase the coupling capacitance value. If the stray capacitance is reduced or eliminated, it brings changes in the coupling capacitance value, reducing the capacitive coupling noise in the circuit. 

Other Noise Reduction Methods

Besides the capacitive coupling noise reduction methods discussed above, there are a few other reduction techniques available. Twisting signal wires to reduce capacitance is one alternative method. However, noise reduction techniques should be employed carefully so that they do not cause other circuit problems. 

Capacitive coupling noise reduction is essential when circuits are placed in close proximity. Cadence’s software can help you design circuits with reduced coupled interferences.   

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