System noise is a common problem facing all digital devices today. The continuous trend towards faster interfaces and lower power consumption has resulted in devices that are increasingly susceptible to disturbances from power and signal lines.
Fortunately, this noise can be abated by using decoupling to isolate localized circuits from other circuits in a system.
What is a decoupling capacitor?
Decoupling capacitors help to isolate, or de-couple, local circuits from noise and power anomalies from other devices on shared power, ground, and other nets. They are typically applied to power sources to provide a localized source of instantaneous current and provide isolation of the local circuit from power noise in other areas of the design.
This localized access is necessary because all power distribution systems have a real impedance and inductance that prevents the truly instantaneous supply of current. When large switched loads occur, the current draw can cause voltage supply dips and ringing, and this can violate the required circuit voltage conditions or result in false signaling.
Bypass capacitor vs. decoupling capacitor
When discussing decoupling capacitors, it is important to understand the differences between decoupling capacitors vs. bypass and coupling capacitors.
Bypass capacitors are used to provide a low impedance shunt for high frequency noise on high impedance paths, and in many instances they are also referred to as decoupling capacitors as they help to ensure higher frequency noise is minimized before it has a chance to spread to other portions of the circuit where it could cause circuit malfunction or problems with containing EMI generated by the design.
Coupling capacitors, on the other hand, provide DC isolation while creating an intentional path for audio, video, RF, and high-speed digital data. Coupling capacitors are often found on high speed interfaces to ensure that any DC potential difference on connected devices does not manifest as ground currents between the devices.
How do decoupling capacitors work?
Decoupling capacitors are used to counter disturbances from many different sources. Synchronously switched logic and data busses can cause large instantaneous current flows that draw significant charge from the local power delivery system (PDS). When these instantaneous loads occur, inductance in the PDS prevents the power supply in the design from instantaneously delivering extra current to the load, and this can cause the local supply voltage to dip or ring.
Decoupling caps help to provide a local instantaneous charge source that prevents the voltage source from dipping and a bypass path that dampens ringing. Noise on the PDS is also locally damped, helping the local circuit remain unaffected by ripple on the power plane that could otherwise disturb the circuit. This effect also extends to noise from other portions of the design when they experience instantaneous current draws. Not only do their own decoupling caps provide local stabilization of the voltage supply, when any residual disturbance reaches other portions of the design, it is further reduced by the local decoupling in that portion of the circuit. Finally, bypass caps used in decoupling roles help to shunt high frequency return paths and prevent them from flowing between circuit areas and potentially causing circuit malfunctions or system level EMI problems. Learn more about bypass capacitance and why it matters.
Decoupling capacitor selection guide
While any decoupling capacitors are arguably better than none, there are several guidelines to consider when implementing a decoupling scheme. Because the capacitors will need to provide current very quickly, the first and most important aspect is to choose capacitors with low equivalent series resistance (ESR), which sums characteristic impedance with any impedance related to inductance. Ceramic capacitors are typically used for decoupling applications due to their wide temperature tolerance, ability to withstand wide voltage ranges, low ESR, stability, and reliability. However, the construction of the capacitor is as important as the size of the package, as inherent benefits from capacitor chemistry can be quickly offset by the added inductance of a larger package size.
The smallest available package that otherwise meets design parameters is often the best choice, although specialized bypass and decoupling capacitor packages that further reduce inductance may be available. Smaller packages also have the benefit of reducing loop size for the capacitor circuit, and this further minimizes the inductance of every decoupling capacitor.
Enhancing capacitors between power and ground
Other ways to optimize the functionality of decoupling caps are to ensure that power and ground planes are continuous and adjacent, by ensuring that capacitors are mounted as closely as possible to the power and ground pins of ICs, by making circuit paths to ground and power planes as short as possible, and by ensuring vias are routed between or beside the pads of the capacitor. Adjacent power and ground planes should be symmetrically placed in the design, and the number of layers between the planes and the decoupling capacitors should be minimized. If possible, capacitors should also be distributed in the area they are decoupling. When this is not possible and a capacitor bank is used, it is best to alternate their orientation to spread their connection points and prevent effective splits in ground or power planed from multiple adjacent vias routed through the plane. The number of capacitors to use depends primarily on the number of power and ground pins present in a localized circuit area or IC, as well as the number of I/O signals present. Designs with analog and digital sections can require that decoupling and bypassing is handled for segments of a circuit or IC.
Today’s digital devices can have significant challenges in maintaining a stable and quiet power supply in the presence of switched loads and other sources of system noise. With the proper use of bulk power capacitors and bypass capacitors in an integrated decoupling scheme, designers can ensure that problems associated with intra-system power noise and other noise sources are mitigated properly and their products will work as designed.
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