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Source: Electronic Instrument Handbook

Introduction to Electronic Instruments

and Measurements

Chapter

Bonnie Stahlin*

Agilent Technologies

Loveland, Colorado

1.1 Introduction

This chapter provides an overview of both the software and hardware components

of instruments and instrument systems. It introduces the principles of electronic

instrumentation, the basic building blocks of instruments, and the way that

software ties these blocks together to create a solution. This chapter introduces

practical aspects of the design and the implementation of instruments and

systems.

Instruments and systems participate in environments and topologies that

range from the very simple to the extremely complex. These include applications

as diverse as:

Design verification at an engineer’s workbench

Testing components in the expanding semiconductor industry

Monitoring and testing of multinational telecom networks

1.2 Instrument Software

Hardware and software work in concert to meet these diverse applications.

Instrument software includes the firmware or embedded software in instruments

* Additional material adapted from “Introduction to Electronic Instruments” by Randy

Coverstone, Electronic Instrument Handbook 2nd edition, Chapter 4, McGraw-Hill, 1995, and Joe

Mueller, Hewlett-Packard Co., Loveland.

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1.1

Introduction to Electronic Instruments and Measurements

1.2 Chapter One

Figure 1.1 Instrument embedded soft-

ware.

that integrates the internal hardware blocks into a subystem that performs a

useful measurement. Instrument software also includes system software that

integrates multiple instruments into a single system. These systems are able to

perform more extensive analysis than an individual instrument or combine

several instruments to perform a task that requires capabilities not included in

a single instrument. For example, a particular application might require both a

source and a measuring instrument.

1.2.1 Instrument embedded software

Figure 1.1 shows a block diagram of the embedded software layers of an

instrument. The I/O hardware provides the physical interface between the

computer and the instrument. The I/O software delivers the messages to and

from the computer to the instrument interface software. The measurement

interface software translates requests from the computer or the human into the

fundamental capabilities implemented by the instrument. The measurement

algorithms work in conjunction with the instrument hardware to actually sense

physical parameters or generate signals.

The embedded software simplifies the instrument design by:

Orchestrating the discrete hardware components to perform a complete

measurement or sourcing function.

Providing the computer interaction. This includes the I/O protocols, parsing

the input, and formatting responses.

Providing a friendly human interface that allows the user to enter numeric

values in whatever units are convenient and generally interface to the

instrument in a way that the user naturally expects.

Performing instrument calibration.

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Introduction to Electronic Instruments and Measurements

Introduction to Electronic Instruments and Measurements 1.3

Figure 1.2 Software layers on the host

side for instrument to computer connec-

tion.

1.2.2 System software

Figure 1.2 shows the key software layers required on the host side for instrument

systems. Systems typically take instruments with generic capabilities and

provide some specific function. For instance, an oscilloscope and a function

generator can be put together in a system to measure transistor gain. The exact

same system with different software could be used to test the fuel injector from

a diesel engine.

Generally, the system itself:

Automates a task that would be either complex or repetitive if performed

manually.

Can perform more complex analysis or capture trends that would be

impractical with a single instrument.

Is specific to a particular application.

Can integrate the results of the test into a broader application. For instance,

the system test could run in a manufacturing setting where the system is

also responsible for handling the devices being tested as they come off the

production line.

Please refer to Part 11 of this handbook for an in-depth discussion of instrument

software.

1.3 Instruments

In test and measurement applications, it is commonplace to refer to the part of

the real or physical world that is of interest as the device under test (DUT). A

measurement instrument is used to determine the value or magnitude of a

physical variable of the DUT. A source instrument generates some sort of

stimulus that is used to stimulate the DUT. Although a tremendous variety of

instruments exist, all share some basic principles. This section introduces these

basic principles of the function and design of electronic instruments.

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Introduction to Electronic Instruments and Measurements

1.4 Chapter One

1.3.1 Performance attributes of measurements

The essential purpose of instruments is to sense or source things in the physical

world. The performance of an instrument can thus be understood and

characterized by the following concepts:

Connection to the variable of interest. The inability to make a suitable

connection could stem from physical requirements, difficulty of probing a

silicon wafer, or from safety considerations (the object of interest or its

environment might be hazardous).

Sensitivity refers to the smallest value of the physical property that is

detectable. For example, humans can smell sulfur if its concentration in air

is a few parts per million. However, even a few parts per billion are sufficient

to corrode electronic circuits. Gas chromatographs are sensitive enough to

detect such weak concentrations.

Resolution specifies the smallest change in a physical property that causes

a change in the measurement or sourced quantity. For example, humans

can detect loudness variations of about 1 dB, but a sound level meter may

detect changes as small as 0.001 dB.

Dynamic Range refers to the span from the smallest to the largest value of

detectable stimuli. For instance, a voltmeter can be capable of registering

input from 10 microvolts to 1 kilovolt.

Linearity specifies how the output changes with respect to the input. The

output of perfectly linear device will always increase in direct proportion to

an increase in its input. For instance, a perfectly linear source would increase

its output by exactly 1 millivolt if it were adjusted from 2 to 3 millivolts.

Also, its output would increase by exactly 1 millivolt if it were adjusted

from 10.000 to 10.001 volts.

Accuracy refers to the degree to which a measurement corresponds to the

true value of the physical input.

Lag and Settling Time refer to the amount of time that lapses between

requesting measurement or output and the result being achieved.

Sample Rate is the time between successive measurements. The sample

rate can be limited by either the acquisition time (the time it takes to

determine the magnitude of the physical variable of interest) or the output

rate (the amount of time required to report the result).

1.3.2 Ideal instruments

As shown in Fig. 1.3, the role of an instrument is as a transducer, relating

properties of the physical world to information. The transducer has two

primary interfaces; the input is connected to the physical world (DUT) and the

output is information communicated to the operator. (For stimulus

instruments, the roles of input and output are reversed—that is, the input is

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Introduction to Electronic Instruments and Measurements

Introduction to Electronic Instruments and Measurements 1.5

Figure 1.3 Ideal instruments.

the information and the output is the physical stimulus of the DUT.) The

behavior of the instrument as a transducer can be characterized in terms of its

transfer function—the ratio of the output to the input. Ideally, the transfer

function of the instrument would be simply a unit conversion. For example, a

voltmeter’s transfer function could be “X degrees of movement in the display

meter per electrical volt at the DUT.”

A simple instrument example. A common example of an instrument is the

mercury-bulb thermometer (Fig. 1.4). Since materials expand with increasing

temperature, a thermometer can be constructed by bringing a reservoir of

mercury into thermal contact with the device under test. The resultant volume

of mercury is thus related to the temperature of the DUT. When a small capillary

is connected to the mercury reservoir, the volume of mercury can be detected by

the height that the mercury rises in the capillary column. Ideally, the length of

the mercury in the capillary is directly proportional to the temperature of the

reservoir. (The transfer function would be X inches of mercury in the column

per degree.) Markings along the length of the column can be calibrated to indicate

the temperature of the DUT.

Figure 1.4 A mercury-bulb thermometer.

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