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Texas Instruments Incorporated
High-Performance Analog Products
Analog Applications
Journal
Fourth Quarter, 2011
© Copyright 2011 Texas Instruments
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Texas Instruments Incorporated
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Copyright © 2011, Texas Instruments Incorporated
SSZZ022B
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High-Performance Analog Products
4Q 2011
Analog Applications Journal
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Texas Instruments Incorporated
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Data Acquisition
How delta-sigma ADCs work, Part 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
This article continues the exploration of the topology and functionality of delta-sigma ADCs that was
begun in Part 1. The modulator described in Part 1 requires a digital/decimation filter to minimize high-
frequency noise and reduce the output-data rate. Operation of this filter is discussed, including the
conversion of the modulator’s 1-bit data stream to 24-bit words and adjusting the decimation ratio by
changing the master-clock and output-data-rate ratio.
Power Management
Solar charging solution provides narrow-voltage DC/DC system bus for
multicell-battery applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Systems powered directly by a solar panel typically do not allow the solar panel to operate at maximum
efficiency. This article presents a solution that efficiently charges multicell batteries and provides
system bus voltage by operating the solar panel at its maximum power point (MPP). This solution also
intelligently connects and disconnects the battery from the system.
Solar lantern with dimming achieves 92% efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Solar lanterns with LEDs have become very popular, and switching regulators are typically used to
efficiently drive LEDs from a variety of battery voltages. Analog and PWM dimming techniques provide
an easy method of controlling light output and extending operating time between charges. This article
presents two 2.8-W solar-lantern solutions, one configured with analog dimming and the other with
PWM dimming. The linearity and efficiency of the two techniques are evaluated to determine how
various operating conditions affect dimming performance.
Interface (Data Transmission)
Extending the SPI bus for long-distance communication . . . . . . . . . . . . . . . . . . . . . . . . 16
Communications between integrated circuits typically use a single-ended interface designed for short
distances. As the distance increases, interface designs that work in the lab may fail on the factory floor
because of added propagation delay and other problems prevalent in the harsh environment. This article
describes typical problems and solutions related to clock synchronization, noise immunity, large ground-
potential differences, unterminated data lines, and electrical transients.
General Interest
Analog linearization of resistance temperature detectors . . . . . . . . . . . . . . . . . . . . . . .21
Precision temperature measurements often use resistance temperature detectors (RTDs) because they
are very stable and useful for temperatures ranging from cryogenic to over 800°C. RTDs have second-
order nonlinearity of approximately 0.38% per 100°C measurement range. In applications where digital
compensation is not available, an analog technique for RTD linearization can be used. This article
describes this technique, which can also be used with bridge sensors and other ratiometric devices.
Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
To view past issues of the
Analog Applications Journal , visit the Web site
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Analog Applications Journal
4Q 2011
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:
• Data Acquisition
• Power Management
• Interface (Data Transmission)
• Amplifiers: Audio
• Amplifiers: Op Amps
• Low-Power RF
• General Interest
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 2011
Analog Applications Journal
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Texas Instruments Incorporated
Data Acquisition
How delta-sigma ADCs work, Part 2
By Bonnie Baker
Signal Integrity Engineer
A strong addition to the process-control design environ-
ment is the delta-sigma (DS) analog-to-digital converter
(ADC). This device’s claim to fame is its high 24-bit reso-
lution, which provides 2 24 or about 16 million output
codes. Granted, not all of the lower bits are noise-free,
but it is not unusual for a DS ADC to have 20 noise-free
bits, or about 1 million noise-free output codes. This is at
least four times better than the performance of 16-bit
converters.
Figure 1 shows a block diagram of a DS ADC. As
explained in Part 1 of this article series (see Reference 1),
the modulator of a DS converter shapes the data in such a
way as to allow high resolution by reducing low-frequency
noise. Part 1 also pointed out that the undesirable charac-
teristics of the modulator output are high-frequency noise
and a high-speed, 1-bit output rate. Once the signal resides
in the digital domain, a low-pass digital-filter function can
be used to attenuate the high-frequency noise, and a
decimator-filter function can be used to slow down the
output-data rate. This article, Part 2, will consider each
function independently, although real-world designs inter-
twine them in the same silicon.
The digital-filter function
The digital-filter function implements a low-pass filter by
first sampling the modulator stream of the 1-bit code.
Figure 2 shows a first-order, low-pass averaging filter. An
averaging filter is the most common filter technique used
in DS converters. As can be seen, the digital filter in
Figure 2 is a weighted averaging filter. Almost all DS ADCs
incorporate a class of averaging filters called sinc filters,
named for their frequency response. Many DS devices,
especially audio devices, use other filters in conjunction
with sinc filters as part of a process called two-stage deci-
mation. Low-speed industrial DS ADCs usually use only
the sinc filter.
Figure 1. Block diagram of DS ADC
Sample Rate (
)
f S
Analog
Input
Modulator
f
S D
/
=
Decimation Ratio (DR)
Data Rate (
f D
)
Digital
Output
Digital
Filter
Decimator
Filter
Digital/Decimation Filter
Figure 2. First-order, low-pass averaging filter
Input
Delay
Delay
Delay
b 1
b 2
b 3
b i
Output
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Analog Applications Journal
4Q 2011
High-Performance Analog Products
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