1. The high-frequency switching noise generated by PCB components during switching propagates along the power line. The primary role of the decoupling capacitor is to provide a local DC power source to the active device, reducing the spread of switching noise across the board and directing the noise to ground.
2. In practice, both bypass capacitors and decoupling capacitors should be placed as close as possible to the power supply input to effectively filter high-frequency noise. The capacitance of the decoupling capacitor is typically about 1/100 to 1/1000 of the bypass capacitor value.
3. To achieve optimal EMC performance, the decoupling capacitor should be positioned as close as possible to each integrated block (IC), since trace impedance can significantly diminish the effectiveness of the capacitor.
4. Ceramic capacitors are commonly used for decoupling, and their values are selected based on the rise and fall times of the fastest signals. For instance, a 4.7-100nF capacitor is suitable for a 33MHz clock signal, while a 10nF capacitor is appropriate for a 100MHz clock signal.
5. When choosing a decoupling capacitor, it’s important to consider not only the capacitance value but also the ESR (Equivalent Series Resistance), as it can impact the decoupling efficiency. A capacitor with an ESR lower than 1Ω is recommended for decoupling applications.
From a circuit perspective, it can be divided into the source signal that needs to be driven and the load that needs to be driven. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitance to achieve the signal transition. When the rising edge is steep, the current is higher, causing the drive circuit to draw a significant amount of current from the power supply. The inductance and resistance (especially the inductance on the chip pins, which will cause voltage spikes) mean that this current behaves as noise in relation to the normal operating conditions, which can interfere with the proper functioning of the previous stage, leading to coupling. The decoupling capacitor serves as an energy reservoir to accommodate changes in the drive circuit’s current and to prevent mutual coupling interference. To better understand this, we can combine the concepts of bypass and decoupling capacitors. A bypass capacitor is essentially a decoupling capacitor, but it typically refers to high-frequency bypassing—providing a low-impedance path to filter high-frequency switching noise. High-frequency bypass capacitors are typically small, such as 0.1μF or 0.01μF, chosen based on the resonant frequency, while decoupling capacitors are generally larger, often 10μF or more, depending on the circuit’s distributed parameters and the size of current changes in the drive circuit. The purpose of bypassing is to filter out interference from the input signal, while decoupling filters interference from the output signal, preventing it from returning to the power supply. This distinction is key.
In the context of a PCB, the decoupling capacitor serves two main functions between the integrated circuit’s power supply and ground: it acts as an energy storage capacitor for the IC and also bypasses high-frequency noise from the device. A typical decoupling capacitor value in digital circuits is 0.1μF, with a typical distributed inductance of 5μH. The 0.1μF capacitor, with 5μH of inductance, has a parallel resonance frequency of approximately 7MHz. This means it is effective at decoupling noise below 10MHz, but less effective for frequencies above 40MHz. Capacitors of 1μF or 10μF, with higher parallel resonance frequencies above 20MHz, are more effective at filtering high-frequency noise. For every 10 integrated circuits, a charge and discharge capacitor (or energy storage capacitor) of about 10μF is typically added. It’s best to avoid using electrolytic capacitors because of their rolled film structure, which behaves as an inductance at high frequencies. Instead, use tantalum or polycarbonate capacitors. The selection of decoupling capacitors is not overly critical and can generally be based on the formula C = 1/F, where a 0.1μF capacitor is suitable for 10MHz and a 0.01μF capacitor for 100MHz.
The VCC network should be connected to the VCC plane at only one point, meaning that noise within and outside the IC must pass through this PCB via to reach the power plane. The additional impedance introduced by the via prevents the noise from spreading throughout the rest of the system.
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