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Texas Instruments Incorporated
High-Performance Analog Products
Analog Applications
Journal
Fourth Quarter, 2008
© Copyright 2008 Texas Instruments
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Texas Instruments Incorporated
IMPORTANT NOTICE
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High-Performance Analog Products
4Q 2008
Analog Applications Journal
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Texas Instruments Incorporated
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Data Acquisition
Stop-band limitations of the Sallen-Key low-pass filter ........................... 5
An ideal low-pass filter attenuates all frequencies of the input signal that are above the cutoff frequency.
Under the right conditions, the gain of a Sallen-Key low-pass filter can initially decrease as expected but
will then start to increase as the frequency continues to increase. This article characterizes this pitfall of
the Sallen-Key low-pass filter with three different op amps. As a possible alternative, a multiple-
feedback low-pass filter is presented that does not have gain-reversal characteristics at higher
frequencies.
Power Management
Getting the most battery life from portable systems .............................. 8
Most portable systems have a common requirement to maximize run time between charges without
sacrificing battery service life. Depending on usage and cell aging, previous-generation fuel gauges have
required battery capacity to be underestimated by up to 15% to ensure adequate shutdown warning.
This article describes TI’s Impedance Track ™ technology, which can provide up to 99% fuel-gauge
accuracy for the entire lifetime of the battery pack. Also covered are the available Impedance Track
evaluation modules that support SMBus interface options, and extensive software analysis tools to
control, log, and display real-time battery conditions.
Compensating and measuring the control loop of a high-power LED driver . . . . . . . . 14
Compensating the loop of a WLED current-regulating boost converter is a bit different than
compensating the same converter configured to regulate voltage. This article describes how to modify a
simplified, small-signal control-loop model to fit a WLED current-regulating boost converter. A method
to measure the boost converter’s control loop is also provided.
Designing DC/DC converters based on SEPIC topology.......................... 18
The single-ended primary-inductance converter (SEPIC) is handy when the designer uses voltages from
an unregulated power supply. The standard SEPIC topology requires two inductors that greatly increase
the supply footprint. This article explains how to design a SEPIC with a coupled inductor that permits a
smaller footprint and requires only half the inductance of a standard SEPIC.
Amplifiers: Op Amps
Input impedance matching with fully differential amplifiers ...................... 24
This article shows how to analyze the input impedance of a fully differential amplifier (FDA). Circuit
analysis is performed to aid understanding of the key design points, and a methodology is presented to
illustrate how to approach the design variables and calculate component values. A spreadsheet and
TINA-TI™ SPICE models are available as design aids.
A dual-polarity, bidirectional current-shunt monitor............................. 29
In-circuit current monitoring can be as simple as monitoring the voltage across a shunt resistor.
However, current monitoring becomes more complicated when the measured current is bidirectional
and the supply-voltage and common-mode-voltage (CMV) ranges are independent of each other. This
article explores the application of three TI current-shunt-monitor ICs that support bidirectional current
monitoring and various CMV ranges.
Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
To view past issues of the
Analog Applications Journal , visit the Web site
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Analog Applications Journal
4Q 2008
High-Performance Analog Products
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Texas Instruments Incorporated
Introduction
Analog Applications Journal is a collection of analog application articles
designed to give readers a basic understanding of TI products and to provide
simple but practical examples for typical applications. Written not only for
design engineers but also for engineering managers, technicians, system
designers and marketing and sales personnel, the book emphasizes general
application concepts over lengthy mathematical analyses.
These applications are not intended as “how-to” instructions for specific
circuits but as examples of how devices could be used to solve specific design
requirements. Readers will find tutorial information as well as practical
engineering solutions on components from the following categories:
•DataAcquisition
•PowerManagement
•Amplifiers:OpAmps
Where applicable, readers will also find software routines and program
structures. Finally, Analog Applications Journal includes helpful hints and
rules of thumb to guide readers in preparing for their design.
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High-Performance Analog Products
4Q 2008
Analog Applications Journal
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Texas Instruments Incorporated
Data Acquisition
Stop-band limitations of the Sallen-Key
low-pass filter
By Bonnie C. Baker
Senior Applications Engineer, Data Acquisition Products
We might expect the gain
amplitude of an analog, low-
pass anti-aliasing filter to
continually decrease past
the filter’s cutoff frequency.
This is a safe assumption
for most filter topologies,
but not necessarily for a
Sallen-Key low-pass filter
(Figure 1). The Sallen-Key
filter attenuates any input
signal in the frequency
range above the cutoff fre-
quency to a point, but then
the response turns around
and starts to increase in
gain with frequency.
Figure 1 shows circuit diagrams for a second-order,
Sallen-Key low-pass filter and a second-order, multiple-
feedback (MFB) low-pass filter. In terms of the sign orien-
tation of these two filters, the Sallen-Key filter produces a
positive voltage from input to output without changing the
sign. An MFB filter changes a positive input voltage into a
negative voltage at the output of the filter. This
difference provides the system designer added
flexibility.
The relationships between the resistors and
capacitors in both of these filters establish the
filters’ corner frequencies and response charac-
teristics. The frequency responses of the two
filters in Figure 1 are fundamentally the same.
Theoretically, an input signal from DC to the
filter’s corner frequency passes to the output of
the filter (V OUT ) without change. These two fil-
ters attenuate higher-frequency input signals that
are above the cutoff frequency of the filter at a
rate of 40 dB per frequency decade. Figure 2
illustrates the ideal transfer function of these
two filters in the frequency domain. This figure
shows a Butterworth, or maximally flat, response.
Chebyshev and Bessel responses will be different.
Figure 1. Second-order, active low-pass analog filters
Sallen-Key
C 1
R 2
V IN
+
Multiple Feedback
V OUT
R 1
C 2
R 5
–
C 5
R 3
R 4
V IN
–
V OUT
C 4
+
The filter-response DC gain in Figure 2 is equal to 0 dB.
The corner frequency of this low-pass filter occurs at 1 kHz,
and the gain magnitude at 1 kHz is equal to –3 dB. Follow-
ing this corner frequency, the filter response falls off at a
rate of –40 dB/decade. Theoretically, the attenuation con-
tinues to occur as the frequency increases.
Figure 2. Ideal transfer function of low-pass filter with
1-kHz corner frequency
0
–20
Corner Frequency, 1 kHz
–40
–60
–80
10
100
10 k
100 k
1 k
Frequency (Hz)
5
Analog Applications Journal
4Q 2008
High-Performance Analog Products
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