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Texas Instruments Incorporated

High-Performance Analog Products

Analog Applications

Journal

Second Quarter, 2006

Texas Instruments Incorporated

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Analog Applications Journal

High-Performance Analog Products

www.ti.com/aaj

2Q 2006

Texas Instruments Incorporated

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Data Acquisition

Matching the noise performance of the operational amplifier to the ADC . . . . . . . . . . 5

Selecting an op amp to drive a SAR ADC requires evaluation of many parameters. Device noise is easy to

overlook but can have a dramatic effect on system performance. This article describes how device noise

specifications can be used to predict system SNR so that the effects of amplifier noise have minimal

impact on ADC operation.

Power Management

TLC5940 PWM dimming provides superior color quality in LED video displays . . . . . 10

High-quality LED video displays require dot correction to equalize the brightness variation of all LEDs

in the display. This article describes PWM-dimming techniques that permit three primary-color LEDs to

produce millions of colors.

Wide-input dc/dc modules offer maximum design flexibility . . . . . . . . . . . . . . . . . . . . . 13

Wide-input nonisolated power modules like the TI PTN78 series offer cost-effective dc/dc power supply

solutions. With typical input voltage ranges between 7 and 36 V, these modules are compact and easy to

use; and they can simplify inventory requirements because they can be used in a variety of system and

portable applications.

Powering today’s multi-rail FPGAs and DSPs, Part 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

This article provides recommendations and guidance for developing a power solution for multi-rail

applications. It focuses on how to choose between the types of point-of-load dc/dc converters and how

to design them to meet dc accuracy, start-up, and transient requirements.

Interface (Data Transmission)

Device spacing on RS-485 buses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

RS-485 bus distributed inductance and capacitance are determined by PCB traces, interconnecting

cable(s), connectors, terminators, and RS-485 devices added along the bus. Analysis in this article

derives a guideline for spacing RS-485 nodes along a bus segment.

Amplifiers: Op Amps

Low-cost current-shunt monitor IC revives moving-coil meter design . . . . . . . . . . . . 27

Moving-coil meters are still preferred in many applications. Using meters in applications with very low

signal current is easily accomplished with modern current-shunt monitor ICs like those in the TI INA19x

family. This article describes the typical operation of INA19x current-shunt monitors when they are

used in meter control circuitry.

Index of Articles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

TI Worldwide Technical Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

To view past issues of the

Analog Applications Journal, visit the Web site

www.ti.com/aaj

Analog Applications Journal

2Q 2006

www.ti.com/aaj

High-Performance Analog Products

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.

Analog Applications Journal

High-Performance Analog Products

www.ti.com/aaj

2Q 2006

Texas Instruments Incorporated

Data Acquisition

Matching the noise performance of

the operational amplifier to the ADC

By Bonnie C. Baker (Email: bonnie@ti.com)

Senior Applications Engineer

Proper selection of the operational amplifier that drives an

analog-to-digital converter (ADC) in a mixed-signal applica-

tion is critical. The designer must compare issues such as

amplifier noise, bandwidth, settling time, and slew rate to

the ADC’s signal-to-noise ratio (SNR), spurious-free

dynamic range (SFDR), input impedance, and sampling

time. This article specifically addresses the matching of

the noise specifications and performance of an op amp

and a successive approximation register (SAR) ADC in a

single-supply environment.

The noise that the amplifier generates originates from

the input differential stage. The input stage of every

amplifier generates transistor-device noise, which spot-

noise graphs describe as referred-to-input (RTI) noise.

With this graphical information we can determine how

much noise reaches the input terminal of the ADC by

calculating the referred-to-output (RTO) amplifier noise.

This discussion begins with a description of the amplifier’s

device noise. The amplifier noise sources are then tied

together into one figure of merit, and the units are

converted from volts to an SNR in decibels. Finally, the

impact of the op amp in this mixed-signal circuit (Figure 1)

is determined by calculating the combination of the op

amp SNR value with the ADC’s SNR performance.

Characteristics of the amplifier noise

It is important to understand the noise that the opera-

tional amplifier generates in this application. The typical

performance of the amplifier given in its product datasheet

shows that the op amp noise behavior over frequency has

a signature that is unmistakable (see Figure 2). In this

article, since we will consider the effects of using a single-

supply CMOS amplifier, the input current noise is low

enough that we can ignore it. Here we will consider only

the effects of the amplifier’s voltage noise.

The amplifier noise specification in the typical amplifier

datasheet is an RTI specification. We can model the

amplifier noise as a voltage source at the non-inverting

input of the amplifier. The electrical characteristics table

of an operational amplifier gives the input voltage noise

Figure 1. Typical driver circuit for SAR ADC

V DD

0.1 µF

V REF

0.1 µF

SDO

SCLK

16-Bit ADC

ADS8325

OPA363

V IN

100-kHz

Filter

Analog Applications Journal

2Q 2006

www.ti.com/aaj

High-Performance Analog Products

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