Designing An Up-to-Date Electrical Test-Bench - Technology - Electronics
Written by Dale Cigoy
A variety of technologies are making it easier for test engineers to create multifunction test systems. With the flexibility of LXI, the speed of PXI, and the availability of smart instruments with test script processors (TSP) that allow 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), requirements can often be separated into high frequency AC vs. lower frequency AC and DC testing. High frequency testing may require a relatively expensive oscilloscope or spectrum analyzer, whereas lower frequency AC and DC testing can be accomplished with more economical instruments. The latter may still require a variety of test functions, which in the past could have required voltmeters, ammeters, ohmmeters, voltage and/or current sources, pulse generators, and switching systems. Now, newer instrumentation combines in one box many, if not most, of the functions found in stand-alone instruments.
Not Your Fathers DMM. A wide range of digital multimeters (DMMs) with measurement resolutions from 4 digits to 8 digits have long been available. DMMs are ubiquitous in testing environments, and for several years they have supplied multiple measurement functions such as AC and DC voltage and current, 2-wire and 4-wire resistance, thermocouple temperature measurement, as well as basic diode and transistor tests. For resistance measurements and semiconductor tests, simple source/measure functionality is included. In addition, various math functions may be available in DMMs, including the calculation of minimum, maximum, average, and standard deviation for a set of data, and linear line fitting.
Now even more functions can be found in DMMs and todays offshoots of these instruments. Increased integration of test functions includes the measurement of more parameters, such as frequency, period, RTD and thermistor temperatures, and audio analyzer functions. In addition, multi-channel scanning functions are available in some DMMs, and matrix switch system for data acquisition and logging. These multi-channel 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 multi-slot mainframe switch system with DMM functionality and an embedded graphing tool. Such an instrument can be the basis for automated test equipment that allows instrument grade measurements of electronic products and components across hundreds of channels.
The ability to switch, read, and store data is essential for test engineers, particularly those in a production test environment. They can use the full functionality of a DMM with integrated matrix switching capability to either store and retrieve collected data, or have the instrument function more like a real-time data acquisition system.
As new device and product technologies emerge, entirely new tools and instrument features are needed for these cutting-edge applications. For example, the field of nanotechnology research and development is being advanced with nanovoltmeters, picoammeters, and electrometers that can measure extremely small quantities, accurately and repeatedly. In addition, todays electrometers are 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 todays 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.
Another instrument integration feature is some type of voltage and/or current source. In the past, test engineers had to link power sources 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, along with data storage as described earlier for the latest DMMs. Some SMUs (called SourceMeter instruments by Keithley) 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, 500ns timing resolution, large buffer memories, reading rates as high as 20,000/s, and many other specialized features.
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 is imbedded in the instrument, so there is no need to install any software on the PC. However, it 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. The resulting data can be viewed in graphical or tabular format and exported to a .csv file for use with spreadsheet applications. This software can control up to eight SourceMeter instruments for collecting, time stamping, and graphing voltage and/or current readings on multiple DUTs.
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, i.e., 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 Communication 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.
The growth of the Internet and the World Wide Web has also impacted test systems, and Web-enabled instruments are fairly common today. For example, designers and engineers using instruments compatible with TSP Express can connect them via Ethernet cable to a PC and test system, or create a complete data acquisition LAN. The intuitive user interface resides on the instruments built-in LXI web page, so no other software installation is needed. 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 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 1300 instruments in 162 categories that have 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 LXIs use of Ethernet-based data communications, and the Standards flexibility in designing small, modular instruments with or without front panel control or on board display.
The LXI Consortiums 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 a dominant network 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 1Mb/s with a maximum data rate of up to 8Mb/sec in burst mode.
Another interface gaining popularity in test and measurement applications is USB. It is already widely used in computer peripherals such as printers, mice, and digital cameras as a quick and easy way to connect to a PC. Virtually all desktop and laptop PCs on the market come equipped with USB ports. These ports have full software support under common operating systems, such as the Microsoft Windows series.
For test and measurement, USB offers significant advantages, including high-speed data transfer rates up to 4.8GB/s with USB 3.0. It also allows users a simple way to develop test applications by using PC plug-in boards (PXI, for example) designed with measurement capabilities. Additional advantages include plug-and-play functionality, better noise immunity, cost savings, and portability, among others.
PXI (PCI eXtensions for Instrumentation) is based on CompactPCI. It 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 performs 32-bit and 64-bit data transfers at 33MHz, which results in 132Mbyte/s and 264Mbyte/s peak data rates, respectively. It is typically used in production test environments.
A PC functioning as a PXI controller can run programs written in 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, its easy to connect other instruments into a PXI subsystem.
Updating Your Test Bench. With shorter engineering cycles and tighter instrumentation budgets, instrument flexibility is a must-have feature. Todays designers and test engineers need test-bench instruments that can be easily and quickly customized for a number of different measurement tasks. Fortunately, as test challenges have become increasingly complex, instrumentation has continued to keep pace.
By taking advantage of integrated test and measurement functions, embedded scripting, and LXI interfaces in the latest test-bench instruments, users can tailor them for any number of measurement tasks. Moreover, todays bench-top instruments can execute thousands of measurements per second, while storing and manipulating the data on a connected PC.
References. For more information on designing and using modern test systems, downloadable handbooks are available at /knowledgecenter.