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Texas Instruments Incorporated
High-Performance Analog Products
Analog Applications
Journal
Third Quarter, 2011
© Copyright 2011 Texas Instruments
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Texas Instruments Incorporated
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Copyright © 2011, Texas Instruments Incorporated
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High-Performance Analog Products
3Q 2011
Analog Applications Journal
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Texas Instruments Incorporated
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Data Acquisition
Clock jitter analyzed in the time domain, Part 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
This article continues clock-jitter analysis by showing how to increase the SNR of an ADC by improving
the ADC’s aperture jitter. One of the methods evaluated to increase SNR was to use a low-noise ampli-
fier for active gain. Another method was to use a step-up transformer for passive gain. The results of the
analysis show that improving the slew rate of the clock signal makes the ADC’s SNR match the predicted
SNR for a given amount of clock jitter.
How delta-sigma ADCs work, Part 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Signal-processing techniques are beginning to shift from analog to digital. The design of delta-sigma
ADCs is approximately three-quarters digital and one-quarter analog. Delta-sigma ADCs are now ideal
for converting analog signals over a wide range of frequencies. This article, the first of a two-part series,
explores the basic topology and function of the delta-sigma modulator.
Power Management
A boost-topology battery charger powered from a solar panel . . . . . . . . . . . . . . . . . . . 17
The growing popularity of charging batteries with solar panels has increased the need to charge multicell
batteries with a solar-panel voltage that is lower than the battery voltage. This situation calls for a boost-
topology charger. This article describes how it is possible to modify a buck battery charger into a boost
or step-up battery charger.
Interface (Data Transmission)
Isolated RS-485 transceivers support DMX512 stage lighting and
special-effects applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Data-transmission networks often reach distances of up to 1200 meters. This article provides an overview
of the DMX512-A standard that specifies EIA-485 as the network’s physical layer. Included is a design
example that shows how to connect an isolated responder node to a DMX512-A network.
Industrial data-acquisition interfaces with digital isolators . . . . . . . . . . . . . . . . . . . . . .24
Galvanic isolation has become the mantra for industrial applications to protect personnel and equipment.
While analog systems use single-channel isolation amplifiers, power-saving digital isolators offer multi-
channel equipment interfaces with smaller form factors. This article explains both types of isolators and
their operation principles.
Amplifiers: Op Amps
Converting single-ended video to differential video in single-supply systems . . . . . .29
Video signals are commonly processed as single-ended, but it is often desirable to use differential
techniques for transmission through cables. This article shows how to use a fully differential amplifier to
convert single-ended video signals to differential to drive a Cat 5 cable with double termination in a
single-supply system. Included is a TINA-TI™ software file for viewing the example circuit simulation
with TI’s free software tool.
Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
To view past issues of the
Analog Applications Journal , visit the Web site
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Analog Applications Journal
3Q 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
3Q 2011
Analog Applications Journal
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Texas Instruments Incorporated
Data Acquisition
Clock jitter analyzed in the time
domain, Part 3
By Thomas Neu
Systems and Applications Engineer
Introduction
Part 1 of this three-part article series focused
on how to accurately estimate jitter from a
clock source and combine it with the aperture
jitter of an ADC. 1 In Part 2, that combined jitter
was used to calculate the ADC’s signal-to-noise
ratio (SNR), which was then compared against
actual measurements. 2 This article, Part 3,
shows how to further increase the SNR of the
ADC by improving the ADC’s aperture jitter,
with a focus on optimizing the slew rate of the
clock signal.
As shown in Parts 1 and 2, a bandpass filter
on the clock signal is a key component for
achieving an ADC’s data-sheet SNR values. The
far-end phase noise of the clock signal adds a
substantial amount to the total jitter of the clock
signal, causing the SNR to degrade even faster
at higher input frequencies.
Unfortunately, there are two major disadvan-
tages associated with the bandpass filter. The
first is that it not only removes the clock signal’s
far-end phase noise, it also eliminates the higher-
order odd harmonics of the fundamental clock
frequency, turning a square wave into a sine wave. These
odd harmonics (third, fifth, etc.) are essential for achieving
a fast slew rate to minimize the ADC’s aperture jitter. The
second disadvantage of the bandpass filter, depending on
topology and order, is that it has some loss associated with
it that can typically range anywhere from 1 to 9 dB. This
Figure 20. SNR versus clock amplitude versus input
frequency (from ADS54RF63 data sheet)
66.0
f = 100 MHz
IN
65.5
65.0
f = 300 MHz
IN
64.5
64.0
63.5
63.0
f = 500 MSPS
S
62.5
0
1
2
3
4
5
Clock Amplitude (V )
PP
loss is equivalent to attenuating the clock amplitude and
thus reducing the slew rate of the clock signal even further.
The slew rate’s impact on an ADC’s SNR performance is
often shown in the ADC’s data sheet as SNR plotted versus
clock amplitude, as in Figure 20. This figure, taken from
the Texas Instruments (TI) ADS54RF63 data sheet, 3 shows
that the larger the clock’s amplitude is, the larger its slew
rate will be. Figure 20 also demonstrates that, as expected,
the SNR sensitivity to the clock’s slew rate increases as
the input frequency, f IN , increases. However, the plot also
indicates that overdriving the clock input too much may
actually cause clipping or damage inside the ADC, nega-
tively impacting the SNR.
In an effort to lower the intrinsic noise and reduce the
power consumption, manufacturers produce clock-
distribution ICs with smaller process nodes and conse-
quently lower power-supply rails. For example, it is much
more difficult to generate a fast-slew-rate clock signal
from a 1.8-V device than from a 3.3-V device; and the loss
from the bandpass filter only makes this deficiency worse
(see Figure 21).
The remainder of this article focuses on two practical
ways to maximize the slew rate of the filtered clock signal
in real applications by trying to “restore” the removed
clock harmonics. Essentially, the clock edges need to be
Figure 21. Bandpass-filter input and output
with 1.8-V and 3.3-V logic
3.3-V
Signal
3.3-V
Sine
Wave
1.8-V
Sine
Wave
1.8-V
Signal
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Analog Applications Journal
3Q 2011
High-Performance Analog Products
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