The need to mitigate system noise in analog and digital devices is paramount. The desire for technology that has speedy interfaces and exhibits lower power consumption has created a generation of devices that are sensitive to disturbances from power and signal lines. A decoupling capacitor or bypass capacitor in a circuit is the solution for such issues as they provide high transient current to an IC and reduce power ripples. That being said, they are often found alongside the power pins of an integrated circuit (IC).
Analog circuits like audio amplifiers produce a hum or crackling noise when operated, while digital circuits such as microcontrollers display erratic and unpredictable behavior. This usually happens because the input voltage is unstable. Devices can perform optimally if glitches, voltage spikes, and AC components remain within their designated tolerances. A good PCB design will ensure smooth input voltages by combatting intra-system power noise with precisely positioned decoupling and bypass capacitors.
Decoupling is necessary because it provides a low impedance path from the power supply to the ground. As such, selecting a low-inductance, high-value capacitor is critical. Beyond such specifications, it is important to note that integrated circuits are sensitive to power supply fluctuations. This sensitivity is outlined by the power supply rejection ratio (PSRR) or PSR (in dB). The PSRR is defined as the ratio of change in output voltage to the change in power supply voltage.
What is a Decoupling Capacitor?
A decoupling capacitor is a passive component that has the ability to store energy in a localized fashion. Additionally, it prevents rapid shifts in voltage by providing the correct DC supply to protect the system or IC. However, due to the nature of its design, it needs ample time to charge and discharge. Decoupling capacitors are usually connected in between a power supply and load/IC in parallel. They must be situated as close to the IC as possible to inhibit the voltage disturbance for each IC. Furthermore, decoupling capacitors control voltage supply drops and ringing, as well as provide stability in the circuit voltage.
What is a Bypass Capacitor?
A bypass capacitor is utilized to prevent noise from making its way into the system by redirecting it to the ground. Generally, it is linked between the supply voltage (Vcc) and ground (GND) pins to reduce power supply noise and voltage spikes on supply lines.
Decoupling Capacitors v. Bypass Capacitors
While decoupling capacitors store energy and dissipate it back into the power rail for the maintenance of smooth current flow, bypass capacitors provide the AC signal return path to switch between the power and ground rail. With their purpose and function in mind, both of these capacitors can be used interchangeably. Nonetheless, there are a few key differences. First, bypass capacitors are utilized to provide a low impedance shunt path to the high-frequency noise signals. With such capacitors, high-frequency noise is mitigated before it flows through the entire circuit, resulting in circuit malfunction and electromagnetic interference problems. Meanwhile, decoupling capacitors are used to stabilize voltage variations.
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