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Texas Instruments Incorporated
High-Performance Analog Products
Analog Applications
Journal
Third Quarter, 2008
© Copyright 2008 Texas Instruments
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Texas Instruments Incorporated
IMPORTANT NOTICE
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enhancements, improvements, and other changes to its products and services at any time and to discontinue
any product or service without notice. Customers should obtain the latest relevant information before placing
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and conditions of sale supplied at the time of order acknowledgment.
TI warrants performance of its hardware products to the specifications applicable at the time of sale in
accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI
deems necessary to support this warranty. Except where mandated by government requirements, testing of all
parameters of each product is not necessarily performed.
TI assumes no liability for applications assistance or customer product design. Customers are responsible for
their products and applications using TI components. To minimize the risks associated with customer products
and applications, customers should provide adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right,
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Dallas, Texas 75265
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Analog Applications Journal
High-Performance Analog Products
3Q 2008
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Texas Instruments Incorporated
Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Data Acquisition
A DAC for all precision occasions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Applications such as automatic test equipment, instrumentation, portable equipment, and digitally
controlled calibration need precision digital-to-analog converters (DACs) to convert digital data back
into the analog domain. This article describes three of the DAC topologies that achieve this task: the R-
2R MDAC, R-2R back-DAC, and the string DAC.
Power Management
New current-mode PWM controllers support boost, flyback, SEPIC, and
LED-driver applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
This article provides an overview on how the TPS40210/1 PWM controllers support isolated and non-
isolated power converters used in industrial, automotive, and battery-powered applications. Advanced
features such as programmable soft start, adjustable/synchronizable oscillator frequency, internal slope
compensation, and the flexibility to provide different power levels by simply changing the power stage
gives these controllers universal appeal.
Interface (Data Transmission)
When good grounds turn bad—isolate!. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Industrial communication systems often require long transmission lines. This article will help designers
be more aware of how large ground-potential differences (GPDs) between remote bus locations can
compromise the integrity of the transmission signal, which can lead to system lockup and, at worst,
destroy the bus transceivers.
Cascading of input serializers boosts channel density for digital inputs . . . . . . . . . . . 16
Traditional parallel interface to analog I/O devices typically required bulky components, many isolation
channels, and a large-footprint host controller which limited channel density and subject to significant
power dissipation. TI’s SN65HVS88x devices serializes inputs into a single SPI data stream and reduces
the number of isolators by 50% and power dissipation by more than 75%.
Amplifiers: Op Amps
A new filter topology for analog high-pass filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Nearly all common high-pass filter topologies include capacitors in the forward signal path. The
venerable “servo” feedback technique instead uses an integrator in the feedback path of a gain block to
create a first-order high-pass filter. A new topology is described which extends this technique to
second-order filters and beyond. Both techniques allow the designer to add circuitry “wrapped around”
another circuit to add a high-pass function without changing the circuit in the forward path.
Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
To view past issues of the
Analog Applications Journal, visit the Web site
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Analog Applications Journal
3Q 2008
High-Performance Analog Products
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Introduction
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: Op Amps
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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Analog Applications Journal
High-Performance Analog Products
3Q 2008
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Texas Instruments Incorporated
Data Acquisition
A DAC for all precision occasions
By Bonnie C. Baker, Senior Applications Engineer
Introduction
Analog-to-digital converters (ADC) routinely
convert analog signals such as temperature,
pressure, sound, or images to a precise digital
representation. Microcontrollers and micro-
processors store, massage, and transmit this
digital information throughout a system. There
are also times when precision digital-to-analog
converters (DAC) convert the digital representa-
tion of these real-world events back into the
analog domain. Three of the DAC topologies that
achieve this feat are the R-2R MDAC, R-2R back-
DAC, and the string DAC. These three topolo-
gies service applications such as automatic test
equipment, instrumentation, portable equip-
ment, and digitally controlled calibration.
The R-2R MDAC
Automatic test equipment or instrumentation typically
uses the R-2R multiplying DAC (MDAC, Figure 1). The
external operational amplifier augments the DAC function
by providing the opportunity for differing supply voltages
and high output currents. MDAC manufacturers are able
to design high resolution devices (16 bit) with ±1 LSB
integral non-linearity (INL) and differential non-linearity
(DNL) specifications. With an appropriate external ampli-
fier, the MDAC exhibits fast settling time (< 0.3 ms) with
a multiplying bandwidth that can be greater than 10 MHz.
The MDAC generates a current that is proportional to
an input digital code. The external amplifier, along with
R FB (internal in the MDAC), converts the DAC’s current-
output signal to a usable voltage level. It would seem that
a simple current-to-voltage conversion is easy to imple-
ment with a DAC, an amplifier, and a resistor. However,
this application circuit has a set of stability issues.
Figure 1. An R-2R multiplying DAC
R
R
R
V REF
2R
2R
2R
2R
2R
R FB
MSB
LSB
External
Op Amp
The output model of the MDAC contains a current
source, variable resistor, and variable capacitor (Figure 2a).
The output resistance and capacitance of the MDAC is
dependent on the input code to the DAC. Programming
the MDAC to zero causes the output resistance (R D ) to be
near infinite. If you program the MDAC to full scale or all
ones, R D is equal to R FB . The output capacitance (C D )
changes according to the number of internal gate-source
junctions across the MDAC output. At full scale, the
MDAC output capacitance is equal to the data sheet
specification. When programmed to zero scale, the MDAC
output capacitance is equal to approximately half the
full-scale value. As we calculate the worst-case stability
condition, we will use the full-scale output values of R D
and C D .
To maintain precision, most MDACs have a feedback
resistor (R FB ) on-chip. The feedback capacitor, C F , is
external and discrete. The unity gain bandwidth (f U ) of
Figure 2a. MDAC model
I DAC
+5 V
C F
R FB
+5 V
C CM
R D
C D
OPA
V OUT
C DIF
C CM
MDAC
–5 V
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Analog Applications Journal
3Q 2008
High-Performance Analog Products
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