Taking full advantage of modern bench instruments

Taking full advantage of modern bench instruments

Today’s multifunctional instruments minimizes the need for large rack-and-stack systems to tackle diverse testing scenarios

BY DALE CIGOY
Senior Application Engineer
Keithley Instruments
www.keithley.com

Comprehensive component and system testing often requires a variety of test functions. In the past, this meant purchasing multiple benchtop instruments, which were cabled together for a complete set of tests. Today, cluttered test benches and racks with a maze of wires, cords, and test leads are becoming rarer. In the interest of easier test system development, lower cost of testing, and shorter time to market, test engineers are purchasing multifunction instruments and PC plug-in instrument cards that can be easily integrated as a hybrid test system.

With the flexibility of LXI, the speed of PXI, and the availability of “smart” instruments with test-script processors (TSP) allowing distributed programming, test engineers have many performance tradeoffs to consider. Nevertheless, multiple communication interfaces in these instruments allow engineers to construct a hybrid test system that best meets the needs of their application.

Depending on the devices under test (DUTs), testing requirements can often be separated into high-frequency ac on the one hand and lower-frequency ac and dc on the other. High frequency testing may require a high-end oscilloscope or spectrum analyzer, whereas lower-frequency ac and dc testing can be done with more economical instruments.

The latter may still require a variety of test functions, which in the past could have required voltmeters, current meters, ohmmeters, voltage and/or current sources, frequency generators, and switching systems. Now, newer instrumentation combines in one box many, if not most, of the functions found in standalone instruments.

Not your Dad’s DMM

A wide range of digital multimeters (DMMs), offering measurement resolutions from 4½ digits to 8½ digits, have long been ubiquitous in testing environments. For the last several years they have supplied multiple measurement functions such as ac and dc voltage and current, two- and four-wire resistance, thermocouple temperature, as well as basic diode and transistor tests. For resistance and semiconductor tests, simple source/measure functionality is included. In addition, various math functions may be available in DMMs, including calculation of minimum, maximum, average, and standard deviation for a data set, and linear curve fitting.

The latest DMMs offer increased integration of test functions. Recently available instruments provide measurement of more parameters, such as frequency, period, RTD and thermistor temperatures, and audio-signal quality measurements. In addition, multichannel scanning functions are available in some DMMs, as is multiplexer switching for data acquisition and logging. These multichannel functions are supported by buffer memories that can store thousands of readings, allowing data collection and manipulation with a PC connected to the instrument.

Along with increased integration comes the demand for more specialized instruments. For instance, some DMMs include functions for specific applications, such as those needed for automotive airbag testing. Recently, the industry has seen the introduction of a compact multislot switching mainframe with DMM functionality and an embedded graphing tool (Fig. 1 ), which allows instrument grade measurements across hundreds of channels optimized for automated testing of electronic products and components.

Fig. 1: Embedded graphing capabilities of a switching mainframe with DMM functionality. (Copyright 2010 Keithley Instruments, Inc.)

The ability to switch, read, and store data is essential for test engineers, particularly those in a production environment. They can use the full functionality of a DMM with integrated multipoint switching capability to either store and retrieve collected data, or have the instrument function more like a real-time data acquisition system.

On the cutting edge

As new device and product technologies emerge, entirely new tools and instrument features are needed for these cutting-edge applications. For example, nanotechnology R&D is being advanced with nanovoltmeters, picoammeters, and electrometers that can measure extremely small quantities accurately and repeatedly. Today’s electrometers are also designed to measure the extremely high resistances of some nanoscale devices.

Nanotechnology device research also demands highly accurate and stable sources of arbitrary waveforms, which are now available in today’s pulse/pattern generators. Dual-channel pulse generators allow the combining of channel signals to create complex waveforms for specialized testing of devices such as flash memory (Fig. 2 ).

Fig. 2: Complex waveforms created with a dual-channel pulse/pattern generator by combining channel outputs and varying pulse amplitudes, polarities, delay times, etc. (Copyright 2010 Keithley Instruments, Inc.)

Another instrument integration feature is some type of voltage and/or current source. In the past, test engineers had to link power supplies and/or signal generators together with a measuring instrument, such as a DMM. Now, there are single-box source/measure units (SMUs) that provide both source signals and measurement functionality (Fig. 3 ), along with data storage as described earlier for the latest DMMs. Some SMUs have two or more channels for even greater flexibility to carry out sophisticated I-V data collection on devices under test (DUTs). Instrument capabilities include microvolt and picoamp measurement resolutions, measurement modes for high-capacitance DUTs, 500-ns timing resolution, large buffer memories, reading rates as high as 20,000/s, and many other specialized features.

Fig. 3: Range of source/measure capabilities for a dual-channel SourceMeter instrument. (Copyright 2010 Keithley Instruments, Inc.)

Software and programming

More and more instruments now come equipped with software so that users can develop application-specific test routines and execute them from either the instrument itself or from a PC. Only a few years ago, software to control rack and stack test systems was used mostly in production applications. Now its use is increasingly required with all types of instruments to meet ISO and other standards. Connecting a computer to an instrument to log data provides detailed test records for product performance traceability and many other purposes.

Examples of software that ships with test instrumentation include TSP Express, for easy creation of test routines on Keithley instruments that have a built-in test script processor. TSP Express, which provides an intuitive user interface that resides on the instrument’s built-in Web page, is imbedded in the instrument, so there is no need to install any software on the PC. However, the software can be initiated by the PC connected to the instrument through Ethernet to set up and execute basic and advanced tests, including: nested step/sweeps, pulse sweeps, and custom sweeps for device characterization applications.

Each instrument has a unique URL, and the Web page allows users to read and set network parameters, such as an IP address, MAC address, etc., and to send commands and query data from the instrument. The resulting data can be viewed in graphical or tabular format and exported to a .csv file for use with spreadsheet applications.

Other examples include Benchlink software, which ships with 34XXX series instruments from Agilent. Benchlink controls the instrument, allows the scanning of multiple channels, and then logs and graphs data. Jitter Analysis Software from Tektronix is used with its oscilloscopes to capture and analyze data and make accurate jitter measurements. For test engineers who create specialized test programs, IVI is a level of standardization for instruments of the same type, that is, DMMs, sources, etc. It is another layer on top of VISA. For instance, this lets a DMM from Keithley respond to the same program calls as a DMM from Agilent.

Data comm interfaces

Along with increased use of software and programmability options, there are a host of standard communication interfaces available to ease transfer and collection of recorded data. One of these is the aforementioned VISA. This is a software layer that standardizes communication between test instruments and different buses such as GPIB, serial links, Ethernet, etc. It allows programmers and users to choose different buses for communication within the same program.

Growth of the Internet and Web has also impacted test systems, and Web-enabled instruments are fairly common today. For example, designers and engineers can connect instruments via Ethernet cable to a PC and test system, or create a complete data acquisition LAN.

The LXI (LAN-based eXtensions for Instrumentation) Standard created by the LXI Consortium has had a profound effect on instrument connectivity. At last count, there were more than 1,300 instruments in 162 categories with LXI compliant interfaces. In benchtop instruments these include DMM/switch systems, switching mainframes, SMUs, ac/dc current sources, and many others. The growing popularity of such instruments is due to LXI’s use of Ethernet-based data communications, and the Standard’s flexibility in designing small, modular instruments with or without front panel controls or displays.

The LXI Consortium’s goal is to maintain an evolving standard that allows flexible packaging and tight integration in proprietary instruments, without the physical constraints and added cost of card-cage architectures. The plan is to incorporate future LAN developments that go beyond the current connection capabilities of legacy test and measurement systems by taking advantage of Web-style interfacing, local and wide area networking, and precision timing synchronization opportunities.

As for other communication interfaces, IEEE-488 or GPIB is still dominant in many test environments, partly because it has been around for three decades, and is built into many, if not most, instruments on the market. It was one of the first networks specifically developed to connect and control programmable instruments. Data speeds for this parallel interface are published as 1 Mbyte/s with a maximum data rate of up to 8 Mbytes/s in burst mode.

Already widely used as a quick and easy way to connect peripherals to a PC, USB is another interface gaining popularity in test and measurement applications. Virtually all desktop and laptop PCs on the market come equipped with USB ports that have full software support under common operating systems, such as the Microsoft Windows series.

For test and measurement, USB offers significant advantages, including high data rates up to 4.8 Gbytes/s with USB 3.0. It also gives users a simple way to develop test applications using PC plug-in boards designed with measurement capabilities (PXI boards, for example). Additional advantages include plug-and-play functionality, better noise immunity, cost savings, and portability.

Based on CompactPCI, PXI (PCI eXtensions for Instrumentation) was conceived as a test solution that would be midway in complexity and cost between PC-based systems using GPIB and the more elaborate VXI systems. In terms of speed, PXI can transfer 32- and 64-bit data at 33 MHz, which results in 132- and 264-Mbyte/s peak data rates, respectively. Its typical use is in production test.

A PC functioning as a PXI controller can run programs written in a variety of programming languages. These programs can control the PXI modules, as well as subsystems of instruments connected to the system via GPIB. Using digital I/O or communication modules, the controller can trigger and communicate with other test subsystems, including those created using TSP-enabled instruments. With PXI-GPIB modules and Ethernet connectivity on the PXI controller, it’s easy to connect other instruments into a PXI subsystem. ■