DMMs are available in a wide variety of shapes, sizes, and capabilities. For automated test applications, modular PXI, VXI, and most recently, LXI instruments provide the flexibility necessary to support different test system configurations.
Basic accuracy and functionality continue to evolve, with faster measurement speed being one of the more important improvements. Deborah Homan, product manager at Agilent Technologies, explained, “New ADC technology allows faster measurements in the Model L4411A DMM. We’ve increased performance by combining a high-speed ADC sampling technique with aperture integration time for AC and DC measurements. Previous generations of DMMs used analog AC rms converters. In addition, the L4411A has a high-speed microprocessor.”
How fast is fast? The L4411A achieves 1,000 readings/s with 6½-digit resolution, 10,000/s at 5½, and 50,000/s at 4½. Higher throughput reduces the cost of test and is one of the major performance criteria ATE designers look for. Nevertheless, speed alone doesn’t tell the complete story.
High-precision DMMs generally link the ADC integration time to the AC power frequency to reduce noise. Theoretically, if the ADC conversion time coincides with one or more complete cycles, the effect of the positive and negative half-cycles will approximately cancel. And, it really does work in practice. For integration times corresponding to two or more complete power line cycles (PLCs), the L4411A normal-mode rejection can be as high as 110 dB.
Nevertheless, depending on where you live, there are 50 or 60 PLCs in a second, so the maximum reading rate is only 25 or 30 readings/s if the integration time is as long as two PLCs. For fast reading rates, normal-mode rejection drops to 0 dB. You need to be aware that the AC-power-related noise a signal may have picked up will not be rejected at high reading rates. Some of this effect is handled by reducing the reading precision as the rate increases, but noise still can be a problem unless good measurement system design guidelines are followed.
As semiconductor technology and innovative measurement techniques continue to advance, sometimes benefits occur with little fanfare. Such is the case for the 10:1 crest factor now supported by the L4411A. Crest factor is defined as the ratio of the peak-to-rms voltage and typically is no more than 3:1 or 4:1 in most DMMs. High crest factors require error correction to achieve the best accuracy.
Because of the digital rms conversion technique used, the L4411A data sheet claims no additional error for crest factors <10:1. Crest factor may not be a pressing concern in your application. But if it is, 10:1 without requiring error correction is a big deal.
LXI on the Way Up
A general trend among test and measurement equipment manufacturers is the development of tightly integrated measurement systems. One major new factor is the availability of an increasing number of LXI products.
Keithley Instruments joined the LXI Consortium as a strategic member shortly after the organization was founded. Chuck Cimino, the company’s marketing director, described some of the benefits offered by an LXI-based test system:
“Ethernet interfaces such as LXI are fast becoming the choice for both bench and system applications due to significantly lower cost and higher bulk data throughput rates and global connectivity advantages. For time-critical applications with sensitivity to PC operating system or communications delays, moving the test control programs into the instrument via Keithley’s Test Script Processor (TSP®) technology provides ultra-high-speed testing plus the cost and convenience of LXI and Ethernet.”
The LXI Consortium was announced in 2004, but wide acceptance of the standard has been gradual since then. Nevertheless, many Class C instruments are available today as well as an increasing number of Class A and Class B products with advanced features. One of these is the IEEE 1588-based timing synchronization that establishes an accurate and consistent time reference regardless of an instrument’s physical location.
Fred Blönnigen, Bustec founder and long-time VXI bus advocate, said that Bustec had joined the LXI Consortium at the informational level and commented on the influence LXI is having. “For demanding and large applications, VXI is the logical choice today even though I believe that, in the long term, LXI will replace most other buses because of its advantages,” he noted. “These include ease of use, cost, and technical capabilities such as IEEE 1588-based synchronization.
“We tested synchronizations of better than ±35 ns. Keep in mind that this is over standard Ethernet cable without any expensive backplanes or cabling. LXI has the advantage that you can use the instrument without any overhead in infrastructure,” Mr. Blönnigen said.
A major difference between LXI and other test instrumentation stand-ards is the lack of centralized control. Certainly, you can design an LXI system to operate in the same way as a central controller-based VXI or PXI system, but you don’t have to.
Keithley’s Mr. Cimino explained that with the company’s TSP technology, all test routines, including advanced decision-making algorithms, can be performed by the relevant instrument. As a result, delays caused by PC interface traffic congestion are eliminated, and overall test times are greatly reduced, bringing both ease of use and performance advantages to the user.
Keithley’s Model 3706 System Switch/DMM is a Class B LXI instrument that integrates switching and measurement functions. Because it includes IEEE 1588-based synchronization, multiple instruments enjoy tight time alignment for shared triggers, timestamping applications, or synchronizing system execution with GPS or other master reference clocks. With six slots for switch cards and an integrated 7½-digit DMM, measurements can be made on up to 576 channels.
EADS North America Defense Test and Services has introduced the Model 1830 Source/Measure Switch System. As Charles Greenberg, senior product marketing manager, described it, “The 1830 is an LXI instrument container that provides advantages in the interconnectivity of the internal instruments and switches. It contains no instruments or switches until you plug in switch cards from among types 1170/80, 1220, 1380, and 1450 and either the 4101 7½-digit DMM or the 4102 7½-digit source/measure meter (SMM).”
Although it’s true that most of the 1830’s functionality is determined by nine plug-in cards, the chassis includes alarm capability; thermal monitoring; a real-time clock and timestamp capability; a state machine to control channel scanning including parallel scanning; triggering; USB, GPIB, LAN, and RS-232 control port interfaces; and the SignalExpress Bus, a 16-channel single-ended or eight-channel differential analog bus.
Up to four DMM/SMM modules can be installed in a single 1830, each of which can simultaneously gain direct access to switching channels for four-wire measurements. The Signal ExpressBus facilitates internal connectivity and eliminates two-thirds of the system cabling that otherwise would be required.
Agilent’s Model 34980A Multifunction Switch/Measure Unit predates other manufacturers’ LXI-based system platforms, having been introduced late in 2005. It offers an eight-slot mainframe with an optional built-in 6½-digit DMM. A selection of 21 plug-in modules includes multiplexers, matrix switches, high-current general-purpose switches, RF and microwave switches, and system control modules.
Up to 560 two-wire multiplexer channels or 4,096 matrix crosspoints can be accommodated in one mainframe. I/O includes GPIB, USB 2.0, and Ethernet. This and other PXI/VXI/LXI DMMs are presented in the comparison chart accompanying this article.
VXI and PXI Remain Popular
Although several test platforms recently have been introduced based on LXI, that doesn’t mean that PXI and VXI shouldn’t be considered for new projects. As EADS’ Mr. Greenberg commented, “The PXI instrument subsystem forms the core of a test system when a vast array of instrument choices is needed to perform a variety of test system functions. In contrast, VXI is the ultimate test platform for demanding test applications that must be supported for a long period of time.”
National Instruments’ (NI) Travis White, the company’s product manager for precision DC instruments, agreed that VXI still had a strong presence in military and defense applications but said that recent growth and innovation has been primarily in LAN-based and PXI DMMs, “LAN-based DMMs offer an alternative to GPIB bench DMMs when remote access or LAN connectivity is desired. PXI DMMs, on the other hand, have been more widely adopted in automated test systems where system throughput and synchronization are important as well as in situations where complimentary mixed-signal and RF instrumentation need to be integrated with DMM measurements.
“For instance, DMMs often are used to accurately measure voltages or current consumption on consumer electronics devices, but increasingly tests need to be correlated with audio/video measurements, digital data, and RF measurements. This trend has fueled the adoption of PXI DMMs due to a need for better integration and synchronization of multiple instruments as well as a need for space savings in these larger test systems.”
Keithley’s Mr. Cimino didn’t fully agree. In his view, “The types of applications benefiting from PXI and VXI formats tend to have very high-speed digitizing requirements and involve a fairly specialized range of signal sources and measurements. Such card-cage-based formats often impose limitations on instrument performance because of constraints such as power, frequency, and connectors. For the vast majority of DMM applications, we see most users heading toward the cost, performance, flexibility, and connectivity advantages of LXI/Ethernet and have structured our plans for TSP and LXI technologies accordingly.”
Summary
Some of the differences of opinion expressed by vendors relate to instrument availability as much as they do to actual capabilities. As Bustec’s Mr. Blönnigen commented, LXI eventually could displace both VXI and PXI. However, it’s only very recently that Class A and B LXI instruments began to be produced. A much greater number of Class C instruments are available, but the really innovative aspects of LXI that support high-performance test system architectures are in Class A and B products.
Agilent’s Ms. Homan added that LXI, USB, and GPIB interfaces are being included on many new instruments to allow customers to move away from GPIB at their own rate. Many LXI units are replacing older GPIB products, especially for R&D users that typically work with front-panel controls and can easily change I/O type if required.
It takes time to displace existing standards, as evidenced by Ms. Homan’s reference to GPIB, even if the new technology has obvious benefits. Part of the inertia is provided by the existing customer base such as the mil/aero VXI users buying EADS test systems or the industrial companies basing ATE systems on NI PXI products. In both cases, the solutions are well proven, and it would be hard to justify changing course for existing applications. There’s a lot more to a test system than hardware, and the huge investment made in test program sets is a major reason that large VXI-based systems are repeatedly upgraded but seldom totally replaced.
On the other hand, there are very strong reasons to consider LXI-based systems for new projects. A comment echoed by almost all the companies represented in this article relates to the great reduction in PC-to-instrument communications traffic that LXI makes possible. Keithley’s TSP scripting language is a prominent initiative in this area, effectively turning each instrument into a largely self-contained test subsystem.
Another advantage that simply cannot be easily duplicated by the other technologies is tight synchronization across large physical distances. PXI and VXI systems have hardware trigger buses, but these cannot be extended to test instruments separated by long distances. Instead, each local group of hardware must have a separate GPS reference, an approach NI has used in structural health monitoring of large buildings in China:
“…The nine 64-channel units each consist of three CompactRIO systems while the two 36-channel devices each contain two. Each device also incorporates multiple accelerometers for vibration measurements as well as a GPS receiver for real-time synchronization. We use the LabVIEW FPGA Module and the GPS disciplined clocks to achieve real-time, intrachassis synchronization within ±10 µs.” 1
Instead of the absolute accuracy of GPS, LXI requires only stable local clocks. Class A and B instruments with IEEE 1588 synchronization interchange messages, first determining which clock should be the master and then aligning all other clocks to it. The result is a simpler and equally accurate system. Also, Ethernet isn’t prone to the reception problems that can affect GPS operation.
These are only two examples of the advantages that LXI offers and that have led to the introduction of today’s integrated DMM/switch products. Watch for these and similar units to gain extended capabilities in the future, becoming nearly autonomous, very high-performance test systems with the host PC primarily used for initialization and display.
Reference
1. Performing Structural Health Monitoring of the 2008 Olympic Venues Using NI LabVIEW and CompactRIO, National Instruments, http://sine.ni.com/cs/app/doc/p/id/cs-11279
February 2009