2 Essential Capacitor Design Considerations for Better PCBs
Capacitors have several functions in a circuit. This article explains what decoupling capacitors and reservoir capacitors are and explains when to use them.
This air-core parallel plate capacitor model shows a simplified electric field between two differently charged plates.
A capacitor is a type of passive electronic device that stores energy in an electric field. The electric field is established between two metals of different electrical potential that are separated by an insulating dielectric layer.
To increase the amount of energy that is stored, manufacturers increase the area of the conductor, increase the number of layers, change the dielectric material, or decrease the spacing between the layers. All capacitors have an upper limit to the amount of energy they can store because eventually enough energy is present to allow charges to jump directly from one metal layer to another, breaking through the dielectric layer – creating either a temporary or permanent short-circuit.
The atoms and molecules of a dielectric material can polarize – allowing a denser electric field to be established between the plates of a capacitor – thereby storing more energy.
Capacitors are manufactured in a variety of physical sizes and are referred to by their dimensions, mounting style, and construction material.
Decoupling capacitors (also referred to as Bypass Capacitors) are an essential component in any PCB design. They are often the first component laid out on a PCB after the microchips and hardware have been positioned. The only time you won’t see a decoupling capacitor on a modern circuit board is when an engineer has designed them out of bare copper from the underlying layers of a PCB stack-up to save valuable routing space.
Every power pin on every IC in your circuit should have a dedicated decoupling capacitor
Decoupling capacitors are named so because they “decouple” one portion of a circuitboard from another. These capacitors confine electrical disturbances to one area of the board, and they exist on a board specifically because of the phenomena known as Ground Bounce and Vcc Sag.
These capacitors do not need to have large capacitance values, but they must have low Equivalent Series Resistance (ESR) values to do their job correctly. Unless your datasheet specifies otherwise, consider a 0.1 µF SMD ceramic capacitor that uses the X7R dielectric. Use the search terms “0.1 0603 X7R” at your favorite parts supplier where 0.1 is the capacitance value in microfarads, 0603 is the size, and X7R is the type of dielectric. Attach the decoupling capacitor to every power pin in your design (Vcc, 3V0, DVdd, AVdd etc…). If your IC has multiple power pins, use multiple decoupling capacitors.
This schematic drawing shows 0.1uF decoupling capacitors on each power pin of Analog’s LTC2380. Additional 10 uF capacitors are present per datasheet recommendation.
Decoupling capacitors have a small amount of readily available energy that they can transfer quickly to a nearby IC to stabilize the electric potential of the pins they are attached to.
Reservoir capacitors have greater energy storage capabilities than decoupling capacitors, but they also have higher Equivalent Series Resistance (ESR). One or more capacitors are often used at power entry points on a PCB to provide ripple-free power to the rest of the PCB.
This schematic shows two reservoir capacitors used to filter power coming into a board.
Additional Reservoir capacitors can be added to a design to ensure that smooth, ripple free power is delivered to analog or mixed-signal (analog + digital function) integrated circuits. This is especially true of switch-mode power supply circuits.
When used for power-entry – reservoir capacitors should be used near the jack, header, or solder-points where power enters the board.
When used to filter power supplies, capacitors should be used near the output of the switch-mode power supply, and depending on board layout, again before the power reaches the IC. Capacitors used on the output of switch-mode power supplies help to smooth noisy output before it negatively impacts the rest of the PCB. Capacitors used on IC input pins help to further smooth ripple that might negatively impact an analog or mixed-signal IC.
See the datasheet for your device for proper capacitor selection. If you are using a jack, consider searching for a “10 1206 X5R”, a 10 µF, 1206 size ceramic capacitor that uses the X5R dielectric.
This view of two PCB sections shows the combined use of reservoir capacitors and decoupling capacitors for a sensitive 24-bit SAR ADC as well as two reservoir capacitors near a power entry plug. Any noise in the power delivery can negatively affect the accuracy of the ADC.
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