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Choosing and Using Bypass Capacitors (Part 1 of 3)

(Editor's Note: Part 2 and Part 3 will examine common capacitor types, tradeoffs, package types, sizing, bandwidth issues, and application examples.)

Bypass capacitors are found in every working piece of electronic equipment. Most engineers know that systems, circuits, and individual chips need to be bypassed. The methods for choosing bypass capacitors typically follow decisions of tradition instead of optimizing for any particular circuit. This article aims to bring the design aspect back to this seemingly simple component. After discussing the motivation for using bypass capacitors, we form a vocabulary around the basics: equivalent circuit, dielectrics and types of available capacitors.

The next step is identifying the primary function and environment of the bypass capacitor. Circuits that exhibit large current spikes have different bypassing needs than ones that solely operate at high frequencies. A few special options are discussed, like scaling multiple bypass capacitors, as well as the importance of board layout.

Finally, we present four application examples. These represent circuits with high and low currents as well as those with high and low frequencies.

Motivation
It's far too common (and quite distressing) to breadboard a circuit in an ideal configuration, only to find that it doesn't work well or it doesn't work at all (Figure 1 ). Noise may have coupled into the circuit from the power supply, internal IC circuitry, or nearby IC. Wires and board connections act like antennas and power supply levels change with current draw.


Figure 1: Breadboarded non-inverting amplifier configuration (Av=2)
(Click to Enlarge Image)

A look at the power-supply pin on the oscilloscope shows the following result (Figure 2 ):


Figure 2: Oscilloscope view of DC power supply pin on non-inverting amplifier
(Click to Enlarge Image)

As you can see, there is a lot of high frequency noise displacing the DC level, approximately 10 mV peak-to-peak. Then, far more pronounced, there are regular spikes in excess of 50 mV. Since power supplies are assumed to be stable (constant DC voltage), any perturbations will couple directly into the circuit and might cause instability.

The first line of defense against unwanted perturbations on the power supply is the bypass capacitor. A bypass capacitor eliminates voltage droops on the power supply by storing electric charge to be released when a voltage spike occurs. It also provides this service at a wide range of frequencies by creating a low-impedance path to ground for the power supply.

We have three questions to answer before grabbing the closest capacitor:
1. What size bypass capacitor do we need?
2. Where do we place the bypass capacitor for maximum effect?
3. What type of bypass capacitor will work best/adequately in our circuit/system?
4. And a hidden fourth question: what type of package do I need to choose for my bypass capacitor? (which will depend on the size needed, the board area available, and type of capacitor chosen)

The simplest question to answer is placement, #2. A bypass capacitor should be placed as close as possible to the power-supply pin of each chip, Figure 3 . Any extra distance translates into additional series inductance, which lowers the self-resonant frequency (useful bandwidth) of the bypass capacitor.


Figure 3: Breadboard circuit of non-inverting amplifier with bypass capacitors.
(Click to Enlarge Image)

The effect of the bypass capacitor on the stability of the output of the non-inverting amplifier is shown in Figure 4a and Figure 4b .

(Click to Enlarge Image)



Figure 4a and b: Output of non-inverting amplifier shown in Figure 3 without bypassing (a, top image) and with bypassing (b, bottom image).
(Click to Enlarge Image)

Further improvements in dealing with the placement and routing of the bypass capacitor will involve discussion of printed circuit board design, which is the topic of our next in-depth discussion.

The other three questions (about capacitor size, type, and package choice) are the heart of the immediate discussion. Those topics will be discussed in detail as soon as we review capacitor basics.


Figure 5: Capacitor structure and basic equations
(Click to Enlarge Image)

Capacitor Basics
The classic definition of a capacitor is two conductive plates separated by a dielectric material. As charge collects on the plates, an electric field builds across the dielectric. The amount of charge needed to create a certain potential between the plates is referred to as capacitance and is measured in farads (F) . The capacitance can also be measured by the dimensions of the plates and quality of the dielectric (Figure 5, Equation 1 ).

Capacitance increases as the area of the plates increases, since more charge can be stored as the potential is created. The distance between the plates dictates the attraction between charges stored on them. As the distance increases, the interaction is decreased, and therefore so is the capacitance. This discussion also relates the relationship shown in Figure 5, Equation 2 .

The last of the basic equations involves current. By definition, current is the movement of charge (Figure 5, Equation 3 ). Therefore, there can only be movement of charge when the voltage (potential between the plates) is changing. In other words, if the voltage is constant, the charge that is forming must also be constant; so no current is flowing.

In summary, the size of a capacitor has a direct effect on its ability to store charge. The second determining factor of capacitance is the quality of the dielectric.

Dielectrics
The dielectric is the material between the two conductors forming a capacitor. It has a high impedance and does not allow significant current to flow from one plate to the other. Different materials used as a dielectric have varying amounts of temperature stability, breakdown voltages, and loss coefficients. The materials in Table 1 are accompanied by their dielectric constant (???epsilon), the coefficient that directly relates to the capacitance of a structure through Equation 1 in Figure 5.


Table 1: Example dielectric materials and their dielectric constants
(Click to Enlarge Image)

Equivalent Circuit Model
Once the structure is understood, the next logical step is creating an equivalent circuit model to use in simulation. The equivalent circuit model is shown in Figure 6 . The main component, the capacitance, has a leakage resistance in parallel with it to represent any losses through the dielectric.


Figure 6: Equivalent circuit model with component description
(Click to Enlarge Image)

In series with that RC pair is another resistive term, in addition to an inductive term. These two values, equivalent series resistance and equivalent series inductance (ESR and ESL), represent the entire amount of DC- and frequency-dependent losses of the capacitive structure, the connection to the printed circuit board (solder) and the traces that connect the capacitor to the integrated circuit and power supply. Again, capacitor type and structure will dictate the values of these parasitic components.

(Parts 2 and 3 will examine common capacitor types, tradeoffs, package types, sizing, bandwidth issues, and application examples.
To read Part 2, click here.)

About the authors
Mike Wong is the VP of application engineering focusing on high speed analog and mixed signal applications at Intersil Corp. He has previously worked on power supplies at ASTEC. He has a BSEE from the University of California at Davis.

Tamara Schmitz is a principal application engineer for analog applications at Intersil Corp. She is also a full-time professor of Electrical Engineering at San Jose State University. She has a BSEE, MSEE and PhD in RF >CMOS design from Stanford University.

Editor's Note : If you found Part 1 of this article useful or interesting, you may want to check out this other article, also by these authors:

Solve ground loop problems in long-distance video transmission,
http://www.planetanalog.com/document.asp?doc_id=527428&site=planetanalog

– x x x-

2 comments on “Choosing and Using Bypass Capacitors (Part 1 of 3)

  1. amrutah
    February 27, 2013

    The link for Part 2 is broken.

    When clicked it takes to the home page of planetanalog….

  2. Brad Albing
    February 28, 2013

    It should be OK now.

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