Keithley Instruments, Inc.

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Understanding New Developments in Data

Acquisition, Measurement, and Control

A Practical Guide to High Performance Test and Measurement

1 st

Edition

Specifications are subject to change without notice.

All Keithley trademarks and trade names are the property of Keithley Instruments, Inc. All other trademarks and trade names are the property of their respective companies.

Keithley Instruments, Inc.

Corporate Headquarters • 28775 Aurora Road • Cleveland, Ohio 44139 • 440-248-0400 • Fax: 440-248-6168 •• 11--888888--KKEEIITTHHLLEEYY ((553344--88445533)) wwwwww..kkeeiitthhlleeyy..ccoomm

www.keithley.com U n d e rs ta n d in g N e w D e ve lo p m e n ts in D a ta A cq u is iti o n , M e a su re m e n t a n d C o n tro l

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Understanding New Developments

in Data Acquisition,

Measurement, and Control

A Practical Guide to

High Performance Test and Measurement

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iii

S e c t i o n 1

Data Acquisition and Measurement Overview

1.1 Definition . . . .1-2 1.2 Data Acquisition and Control Hardware . . . .1-3

S e c t i o n 2

Computer Buses, Protocols, and Hardware

2.1 Computer Hardware Overview . . . .2-2 2.2 Processors. . . .2-2 2.3 Bus Architecture . . . 2-4 2.4 Industrial Computer Expansion. . . 2-8 2.5 Connectivity/Data Communications . . . .2-16 2.6 Networked Instrument Basics. . . 2-26

S e c t i o n 3

Software Overview

3.1 Introduction . . . .3-2 3.2 Development Environments . . . .3-2 3.3 Source Code and Source Code Management. . . .3-7 3.4 Reusable Components . . . .3-7 3.5 System Architecture . . . .3-11 3.6 General Program Design . . . .3-13 3.7 Making Measurements . . . .3-15

S e c t i o n 4

Basic Component Theory

4.1 Introduction . . . 4-2 4.2 Passive Components . . . 4-2 4.3 Op Amp Theory . . . .4-11 4.4 Filters . . . .4-19 4.5 Digital I/O . . . 4-20 S e c t i o n 5

Basic Analog and Digital I/O

5.1 A/D Conversion . . . 5-2 5.2 D/A Conversion . . . .5-11 5.3 Interfacing Digital I/O to Applications. . . .5-12 5.4 Isolation. . . .5-17 5.5 Ground Loops. . . .5-19 S e c t i o n 6 Temperature Measurement 6.1 Temperature . . . 6-2 6.2 Thermocouples . . . 6-2

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6.3 Resistive Temperature Detectors . . . .6-12 6.4 Thermistors. . . .6-19 6.5 Semiconductor Linear Temperature Sensors. . . 6-21 6.6 Thermal Shunting. . . 6-22 S e c t i o n 7 Strain Measurement 7.1 Strain . . . .7-2 7.2 Poisson’s Strain. . . .7-2 7.3 Strain Gauges . . . .7-2 7.4 Gauge Factor. . . 7-6 7.5 Sources of Error . . . 7-6 7.6 Operation . . . .7-7 7.7 Strain Gauge Signal Conditioning . . . .7-10 7.8 Shunt Calibration . . . 7-11 7.9 Load Cells, Pressure Sensors, and Flow Sensors . . . 7-11 7.10 Acceleration, Shock, and Vibration . . . .7-12

S e c t i o n 8

Data Acquisition and

Measurement Overview

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1 Data Acquisition and Measurement Overview

1.1 definition

Although concepts like data acquisition, test, and measurement can be

sur-prisingly difficult to define completely, most computer users, engineers, and scientists agree there are several common elements in systems used today for these functions:

• A personal computer (PC) is used to program and control the test and

measurement equipment, and to store or manipulate data subsequently.

The term “PC” is used in a general sense to include any computer run

-ning any operating system and software that supports the desired result.

The PC may also be used for supporting functions, such as real-time

graphing or report generation. The PC may not necessarily be in con

-stant control of the data acquisition and measurement equipment, or even remain connected to some pieces of equipment at all times.

• A test or measurement system can consist of data acquisition plug-in

boards for PCs, external board chassis, discrete instruments, or a com

-bination of all these. External chassis and discrete instruments typically can be connected to a PC using either standard communication ports or a proprietary interface board in the PC.

• The system can perform one or more measurement and control pro

-cesses using various combinations of analog input, analog output,

digi-tal I/O, or other specialized functions. When external instruments are

used, most or all of these functions could reside within the instruments.

Therefore, measurement and control can be distributed between PCs, stand-alone instruments, and other external data acquisition systems.

The difficulty involved in differentiating between terms such as “data

acquisi-tion,” “test and measurement,” and “measurement and control” stems from

the blurred boundaries that separate the different types of instrumentation in

terms of operation, features, and performance. For example, some stand-alone

instruments now contain card slots and embedded microprocessors, use oper-ating system software, and operate more like computers than like traditional instruments. Such instruments now make it possible to construct test systems with high channel counts that gather data and log it to a controlling computer at high throughput rates, with high measurement accuracy. Accuracy aside, plug-in boards can transform computers into multi-range digital multimeters, oscilloscopes, or other instruments, complete with user-friendly, on-screen virtual front panels.

For the sake of simplicity, this handbook uses the term “data acquisition and

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1 Data Acquisition and Measurement Overview 1

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of making measurements and controlling external processes. The term “com

-puter” is also defined rather broadly. Generally in this handbook, a PC refers to

any personal computer running Microsoft®_{Windows 98}®_{, or a later Windows}

version. However, when external instruments are combined with data acquisi

-tion boards, the result is often referred to as a hybrid system. Hybrid systems are becoming more common as engineers seek the optimum combination of throughput and accuracy for their test and measurement applications.

1.2 data Acquisition and control Hardware

Data acquisition and control hardware is available in a number of forms, which offer varying levels of functionality, channel count, speed, resolution, accuracy, and cost. This section summarizes the features and benefits generally associ-ated with the various categories, based on a broad cross-section of products,

including external instruments. Refer to Appendix A for a more detailed com

-parison of plug-in boards and external instruments.

1.2.1 plug-in data Acquisition Boards

Like display adapters, modems, and other types of expansion boards, plug-in

data acquisition boards are designed for mounting in board slots on a computer motherboard. Today, most data acquisition boards are designed for the current

PCI (Peripheral Component Interconnect) or earlier ISA (Industry Standard

Architecture) buses. ISA and associated data acquisition boards are rapidly be

-coming “legacy” products, and largely replaced by PCI. Data acquisition plug-in boards and interfaces have been developed for other buses (EISA, IBM Micro

Channel, and various Apple buses), but these are no longer considered main

-stream products. In addition, a wide range of USB data acquisition modules are now available for plugging into a PC’s USB ports, or USB hubs attached to those ports. As discussed in Section 2, newer PCs may also have PCI Express slots,

table 1-1. Features of plug-in data acquisition boards

Inexpensive method of computerized measurement and control. High speed available (100kHz to 1GHz and higher).

Available in multi-function versions that combine A/D, D/A, digital I/O, counting, timing, and specialized functions.

Good for tasks involving low-to-moderate channel counts.

Performance adequate to excellent for most tasks, but electrical noise inside the PC can limit ability to perform sensitive measurements.

Input voltage range is limited to approximately ±10V.

Use of PC expansion slots and internal resources can limit expansion potential and consume PC resources.

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often in combination with PCI slots, particularly in computers built expressly

for industrial applications.

As a category, plug-in boards offer a variety of test functions, high channel counts, high speed, and adequate sensitivity to measure moderately low

sig-nal levels, at relatively low cost. table 1-1 lists additional attributes of typical

boards.

1.2.2 external data Acquisition Systems

The original implementation of an external data acquisition system was a

self-powered system that communicated with a computer through a standard or

proprietary interface. As a boxed alternative to plug-in boards, this type of sys

-tem usually offered more I/O channels, a quieter electrical environment, and greater versatility and speed in adapting to different applications. Another

alter-native today is a system based on external USB data acquisition modules. 1 .2 .2 .1 USB Data Acquisition Modules

USB data acquisition modules offer many advantages over plug-in boards, in

-cluding plug-and-play capability, better noise immunity, and portability. USB modules do not require opening the PC, because they connect directly to the USB ports via standard cables. The PC identifies the presence of a new device,

then prompts the user for the location of the necessary driver software for

installation. table 1-2 lists features of typical USB modules.

table 1-2. Features of USB data acquisition modules

True plug and play means a simple connection between the PC and USB module. Because USB modules are external to the PC, they offer performance benefits for noise-sensitive measurements.

USB 1.1 supports data acquisition rates up to 400kHz. USB 2.0 supports data acquisition rates up to 500kHz in each direction, similar to PCI boards.

Most USB modules handle of large array of I/O connections. USB modules are compact and portable.

With USB hubs, additional modules can scale up measurement capacity. USB modules can be installed or removed while the PC is running (hot swapping). USB modules have simple power connections, either directly through the bus or through an external power source.

1 .2 .2 .2 External Box/Rack Systems

Today, external data acquisition systems often take the form of a stand-alone

test and measurement solution oriented toward industrial applications. The applications for which they are used typically demand more than a system

based on a PC with plug-in boards can provide, or this type of architecture is simply inappropriate for the application. Modern external data acquisition

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1-5 • Applications involving many types of sensors, high channel counts, or

the need for stand-alone operation.

• High sensitivity to low level voltage signals, i.e., approximately 1mV or

lower.

• Applications requiring tight, real-time process control.

Like the plug-in board based system, these external systems require the use of

a computer for operation and data storage. However, the computer can be built up on boards, just as the instruments are, and incorporated into the board rack.

There are several architectures for external industrial data acquisition systems, including VME, VXI, MXI, LXI, CompactPCI, and PXI. These systems use

mech-anically robust, standardized board racks and plug-in instrument modules that

offer a full range of test and measurement functions. Some external system designs include microprocessor modules that support all the standard PC user

interface elements, including keyboard, monitor, mouse, and standard commu-nication ports. Frequently, these systems can also run Microsoft Windows and

other PC applications. In this case, a conventional PC may only be needed to

develop programs or off-load data for manipulation or analysis. Other features

of external board-based systems are listed in table 1-3.

table 1-3. Features of external data acquisition chassis

Multiple slots permit mixing-and-matching boards to support specialized acquisition and control tasks and higher channel counts.

Chassis offers an electrically quieter environment than a PC, allowing for more sensitive measurements.

Use of standard interfaces (IEEE-488, RS-232, USB, FireWire, Ethernet) can facilitate daisy chaining, networking, long distance acquisition, and use with non-PC computers. Dedicated processor and memory can support critical “real-time” control applications or stand-alone acquisition independent of a PC.

Standardized modular architectures are mechanically robust, easy to configure, and provide for a variety of measurement and control functions. Required chassis, modules, and accessories are cost-effective for high channel counts.

1.2.2.2.1 Real-Time Data Acquisition and Control

Critical real-time control is an important issue in data acquisition and control

systems. Applications that demand real-time control are typically better suited

to the type of external system described previously, than to systems based on PC plug-in boards. Although Microsoft Windows has become the standard oper-ating system for PC applications, it is a non-deterministic operoper-ating system that can’t provide predictable response times in critical measurement and control

applications. Therefore, the solution is to link the PC to a system that can oper

-ate autonomously and provide rapid, predictable responses to external stimuli. This is accomplished with external systems that have their own dedicated

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1 .2 .2 .3 Discrete (Bench/Rack) Instruments

Originally, discrete electronic test instruments consisted mostly of single-chan-nel meters, sources, and related instrumentation intended for general-pur-pose test applications. Over the years, the addition of more channels, com-munication interfaces and advances in instrument design, manufacturing, and

measurement technology have extended the range and functionality of these

instruments. New products such as scanners, multiplexers, SourceMeter®

in-struments, counter/timers, nanovoltmeters, micro-ohmmeters, and other spe-cialized instrumentation have made it possible to create computer-controlled

test and measurement systems with a large number of channels that offer ex

-ceptional sensitivity, resolution, and throughput. Even low channel count

in-struments can be combined with switch matrices and multiplexers to lower the

cost per channel by allowing one set of instruments to service many test inputs while preserving signal integrity. These instruments can also be combined with computers that contain plug-in data acquisition boards to form a hybrid test

system. table 1-4 lists other characteristics of these systems.

table 1-4. Features of discrete instruments for data acquisition

Support measurement ranges and sensitivities generally beyond the limits of standard plug-in boards and eternal data acquisition systems.

Use standard interfaces (e.g., IEEE-488, RS-232, FireWire, USB, and Ethernet) that support long-distance acquisition, compatibility with non-IBM-compatible computers, or use with computers without available expansion slots.

Most suitable for measurement of voltage, current, resistance, capacitance, inductance, temperature, etc. May not be effective solutions for some types of specialized sensors or signal conditioning requirements.

Generally slower than plug-in boards or external data acquisition systems. More expensive than standard data acquisition systems on a per-channel basis.

1 .2 .2 .4 Hybrid Data Acquisition Systems

Hybrid systems are a relatively recent development in external data acquisition systems. A typical hybrid system combines PC plug-in boards, or an external data

acquisition system, with discrete instruments to perform the desired test and measurement functions. However, some bench and rackmounted instruments

have features that include standard data acquisition functions and expansion

capabilities in a compact, instrument-like package, making them stand-alone hybrid systems. These instruments have a user interface is much like a typical

DMM product. Hybrid systems that combine plug-in boards, external systems,

and discrete instruments may utilize a graphical user interface (GUI) that ap

-pears as a virtual instrument on the PC screen. In any case, typical hybrid test

system functions include AC and DC voltage and current measurements, tem

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process control. They allow the user to get the best combination of speed and accuracy for the application.

Figure 1-1. Keithley pxi-based hybrid test system

Keithley’s Series KPXI (Figure 1-1) is designed for high speed automated pro

-duction testing as part of a hybrid test system using precision instruments. This series consists of simultaneous data acquisition boards, multi-function analog I/O boards, high speed analog output boards, a 130MS/s digitizer module,

digital I/O modules, PXI chassis, embedded PC controllers, MXI bridges (for remote PC control), and GPIB interface cards. KPXI products are designed for optimal integration with precision instrumentation, such as Keithley’s Series

2600 System SourceMeter®_{multi-channel I-V test solutions, which feature}

TSP™ and TSP-Link™ that let users take advantage of distributed programming and concurrent execution to perform high speed, automated test sequences across multiple channels – independent of a PC operating system and its

asso-cia ted communication delays. table 1-5 summarizes the features of hybrid test

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table 1-5. Features of a hybrid data acquisition and control system

Delivers accuracy (typically 18- to 22-bit A/D), range, and sensitivity of benchtop instruments (superior to standard data acquisition equipment).

Allows optimization of throughput speed by distributing measurement and control functions to the appropriate hardware.

May have a DMM front end with a digital display and front panel controls, or a PC GUI (often a virtual instrument panel), or both.

External intruments used in these systems often have built-in data and program storage memory for stand-alone data logging and process control.

Uses standard interfaces (IEEE-488, LXI, PXI, Ethernet, etc.) that support long-distance acquisition, and compatibility with non-PC computers.

Cost-effective on a per-channel basis.

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Specifications are subject to change without notice .

All Keithley trademarks and trade names are the property of Keithley Instruments, Inc . All other trademarks and trade names are the property of their respective companies .

KeitHLeY inStRUMentS, inc. ■ 28775 AUROR A ROAD ■ CLEVELAND, OHIO 44139-1891

440-248-0400 ■ Fax: 440-248-6168 ■ 1-888-KEITHLEY ■ www.keithley.com

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