Connect with us

×

ARTICLES

DESIGN NEWS

PROJECTS

TECHNICAL ARTICLES

INDUSTRY ARTICLES Coming Soon

BY
CATEGORY

Forum

ALL FORUMS

CIRCUITS & PROJECTS

EMBEDDED & MICRO

MATH & SCIENCE

COMMUNITY

Education

Textbooks

Video Lectures

Worksheets

Webinars Coming
Soon

By
Category

IoT

ARDUINO

Power

Log In/Sign Up

Log
In

Sign
Up

Sign
in with

Member
Benefits

Log
In

Sign Up

Sign in with

Member
Benefits

Home

Technical Articles

Clean Power for Every IC, Part 1: Understanding Bypass
Capacitors

Related Content

Clean Power for Every IC, Part 1:
Understanding Bypass Capacitors

September 21, 2015
by Robert Keim

A thorough understanding of bypass
capacitors will help you to properly incorporate these critical components into
your designs.

Articles That Provide Supporting Information

AC Capacitor Circuits

Capacitor Quirks

Capacitors, Capacitors Everywhere

It is not inconceivable that a dedicated, successful engineering student
would graduate from college knowing almost nothing about one of the most
pervasive and important components found in real circuits: the bypass capacitor.
Even experienced engineers may not fully understand why they include 0.1 µF
ceramic capacitors next to every power pin of every IC in every circuit board
they design. This article provides information that will help you to understand
why bypass capacitors are necessary and how they improve circuit performance,
and a follow-up article will focus on details related to choosing bypass
capacitors and the PCB layout techniques that maximize their efficacy.

The Perils of Transient Current

Any component in which outputs transition rapidly from one state to another
will generate transient currents. When these transient currents are drawn
directly from the power supply, transient voltages are created as a result of
the power supply’s source impedance as well as parasitic inductance associated
with wires and PCB traces. This effect is increasingly problematic when a
component must drive a low-resistance or high-capacitance load: low-resistance
loads create higher-magnitude transients, and high-capacitance loads can lead to
ringing or even severe oscillations in the power line. The end result can be
anything from suboptimal circuit performance to system failure.

Let’s briefly explore this issue of transient current using a very simple
simulation.

The circuit is the famous CMOS inverter, as confirmed by the relationship
between the input and output voltage. Though the eminently clever design of this
inverter requires no steady-state current, we need to remember that significant
transient current flows as the input voltage passes through the region in which
both transistors are conducting. This current creates a disturbance on the
inverter’s voltage supply corresponding to the voltage drop across the source
resistance (this simulation uses 2 Ω, which is about how much internal
resistance you would expect from a 9 V battery):

It is true that the magnitude of this disturbance is very small, but remember
that an integrated circuit could contain hundreds or thousands or millions of
inverters. Without proper bypassing, the cumulative effect of all these
transient currents would be a seriously noisy—if not catastrophically
unstable—voltage supply. Experiments performed by engineers at Texas Instruments
demonstrated that an improperly bypassed line driver IC switching at 33 MHz
resulted in ringing amplitude as high as 2 V peak-to-peak—on a 5 V power
rail!

The following plot shows the supply voltage when the simulation circuit is
expanded to include a mere 8 inverters along with 1 nH of parasitic inductance
in series with the source resistance:

The magnitude of the transients has increased to almost 0.5 mV, and both
disturbances exhibit some oscillatory behavior:

Digital circuits certainly have a special aptitude for degrading power
quality, but analog ICs also need bypassing to compensate for rapid output
transitions and to protect them from power supply noise generated by other
devices. For example, an op-amp’s power supply rejection ratio decreases as the
power supply noise increases in frequency; this means that an improperly
bypassed op-amp could create high-frequency power-line disturbances that would
propagate to the op-amp’s own output signal.

The Solution

It is convenient that such a serious problem can be effectively resolved with
a simple, widely available component. But why the capacitor? A straightforward
explanation is the following: A capacitor stores charge that can be supplied to
the IC with very low series resistance and very low series inductance. Thus,
transient currents can be supplied from the bypass capacitor (through minimal
resistance and inductance) instead of from the power line (through comparatively
large resistance and inductance). To better understand this, we need to review
some basic concepts related to how a capacitor affects a circuit.

First, though, a brief note about terminology: The components discussed in
this article are regularly referred to as both “bypass capacitors” and
“decoupling capacitors.” There is a subtle distinction here—“decoupling” refers
to reducing the degree to which one part of a circuit influences another, and
“bypass” refers to providing a low-impedance path that allows noise to “pass by”
an IC on its way to the ground node. Both terms can be correctly used because a
bypass/decoupling capacitor accomplishes both tasks. In this article, however,
“bypass capacitor” is favored in order to avoid confusion with a series
decoupling capacitor used to block the DC component of a signal.

Charging and Discharging

The fundamental action of a capacitor is storing charge and releasing charge
in such a way that it opposes changes in voltage: If the voltage suddenly
decreases, the capacitor supplies current from its charged plates in an attempt
to maintain the previous voltage. If the voltage suddenly increases, the
capacitor’s plates store charge from the current generated by the increased
voltage. The following simple simulation can help you to visualize this:

Note that the current is positive (i.e., flowing from the source through
R1 to C1) when the capacitor is charging and negative
(i.e., flowing from C1 through R1 to the source)
when the capacitor is discharging.

This fundamental charging and discharging behavior does not change depending
on whether the capacitor is exposed to low-frequency or high-frequency signals.
However, in a discussion of power supply bypassing, it is helpful to analyze a
capacitor’s influence in two different ways—one for low-frequency situations and
one for high-frequency situations. In a low-frequency or DC context, a bypass
capacitor opposes changes in the voltage line by charging or discharging. The
capacitor functions like a low-impedance battery that can supply small amounts
of transient current. In a high-frequency context, the capacitor is a
low-impedance path to ground that protects the IC from high-frequency noise on
the power line.

A Standard Approach

The foregoing analysis helps us to understand a classic bypassing scheme: a
10 µF capacitor within an inch or two of the IC, and a 0.1 µF ceramic capacitor
as close to the power pin as possible:

The larger capacitor smooths out lower-frequency variations in the supply
voltage, and the smaller capacitor more effectively filters out high-frequency
noise on the power line.

If we incorporate these bypass capacitors into the 8-inverter simulation
discussed above, the ringing is eliminated and the magnitude of the voltage
disturbance is reduced from 1 mV to 20 µV:

Ideal vs. Reality

At this point you may be wondering why we need a 0.1 µF capacitor in addition
to a 10 µF capacitor. What is the difference between 10 µF and 10.1 µF? This is
where the bypass-cap discussion becomes more complicated. The efficacy of a
particular bypassing scheme is closely related to two of the chosen capacitor’s
nonideal characteristics: equivalent series resistance (ESR) and
equivalent series inductance (ESL). In the simulation just mentioned, the
parallel 10 µF and 0.1 µF ideal capacitors become nothing more than a 10.1 µF
ideal capacitor. To make the simulation anything approaching realistic, we need
to include reasonable values of ESR and ESL. With this modification, we have the
following:

Though still an improvement compared to the case with no bypass capacitors,
these results are significantly worse than what we saw with the ideal caps.

This simple simulation cannot possibly account for all the parasitic
impedances and other subtle influences present with real integrated circuits on
a real PCB (especially one that includes high-speed digital signals). The point
here is to demonstrate that designing a bypass network involves careful
consideration of a capacitor’s ESR and ESL. Equally important are proper
component placement and PCB layout techniques. We will explore all these details
in the next article.

Next Article in Series: Clean
Power for Every IC, Part 2: Choosing and Using Your Bypass
Capacitors

Tags:

types
of capacitor System
Noise decoupling
bypass
capacitor Signal
and Power Integrity

Share

Share

Share

Share

Recommended Articles

Five Places to Find Part Info Online

These online sources not only help you find part data
quickly, but can also help you determine availability and manage bills of
materials.

Dan
Romanchik

June 25, 2015

Superposition and What it Means For You

This article describes superposition and its simple
applications.

Tristan
Cornell

May 11, 2015

Resistance in Parallel Networks

Parallel circuits are one of two fundamental circuit
configurations we come across every day. Like a parallel universe, parallel
circuits and networks can give us unexpected results. Adding more...

Marie
Christiano

June 30, 2015

Beyond the DMM: Components and Circuits for Measuring
Current and Voltage

Learn about devices and techniques used for in-circuit
monitoring of currents and voltages.

Robert
Keim

June 27, 2015

Voltage and Current Dividers: What They Are and What
They Do

This article describes what voltage and current dividers
are and how they can be used.

Tristan
Cornell

June 26, 2015

Comments

3 CommentsLogin

Please login before you can comment.

HighVoltage! 2015-09-22

Great Article! Are there formulas for choosing what capacitor sizes are
needed (Ex: AC Filter design)?

Reply

hariharanm 2015-09-25

Good Work! I’m waiting for the next part :D

Reply

cuyler1
2015-09-27

Thanks for this article as just this past wednesday I was showing a friend
a circuit of reverse voltage he thought was untrue.I would like to know the
reason for utilizing a ceramic 10uf cap and the reason for not using a 10uf
electrolytic or axial lead one. I know of a young fellow whom lives out in bfe
and the parts are not available for months. He gets discouraged.Maybe delve
into this for a sentence or two.

Reply

Please login before you can reply to this comment.

Close

Other Content

Intro to Magnetic Devices and the Science Behind
Them
Technical Article

High Temperature Sensors Take the Heat
News

Factors Affecting Capacitance
Textbook

Passive Integrator and Differentiator
Circuits
Worksheet

← Articles Index

WHO WE ARE

More
about us

Content

News

Projects

Technical
Articles

Textbook

Downloads

Video
Lectures

Worksheets

Forum

Categories

Latest

Arduino

Power

Embedded

IoT

Sensors

Digital
ICs

Test &
Measurement

Connect with Us

Facebook

Twitter

Contact Us

YouTube

LinkedIn

Sign Up

Register

© EETech Media, LLC. All
rights reserved

Privacy
Policy · Terms of
Service · User
Agreement