PXI DMMs gain a digit
Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 6/1/2005
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| A PXI DMM provides extended utility as well as high accuracy. Courtesy of National Instruments. |
"By increasing precision and resolution to 7.5 digits, we have opened up new applications," said Zwie Amitai, Signametrics' director of operations. "These include IC pin leakage measurements, and cable manufacturing, which uses both high-ohm and low-ohm resistance measurements in production." Kevin Bisking, DMM product marketing manger for National Instruments, agreed, noting "These instruments offer picoamp and nanovolt sensitivity, now allowing them to measure parameters such as transistor current-voltage transfer characteristics."
Achieving this level of precision was a challenge, especially in light of the backplane noise in PXI systems. According to Amitai, some of the techniques that Signametrics used included extensive isolation. The design incorporates an isolated power supply and optical isolation for its measurement circuitry to keep the PXI bus noise at bay. In addition, the unit's internal layout included extra spacing to separate the sensitive analog circuitry from the digital section.
Bisking said a key element in the NI design is a high-precision thermally stabilized voltage reference that allows the instrument to perform self-calibration to correct for time and temperature-related drift. In addition, he noted, the small form factor of the PXI board helps keep traces short to reduce noise pickup.
Both companies point out that the versatility of a DMM is a particular advantage in the PXI format. For example, the triggering capabilities of PXI can combine with the DMM's sampling ability to measure parameters at specific events in an oscilloscope-like function. This ability can also extend to perform rough digital oscilloscope functions at modest sample rates.
The NI PXI 4071 Flex, for example, can sample as fast as 1.8 Msamples/s with 3-digit accuracy and 5 ksamples/s with 5.5 digits. The Signametrics SMX2064 offers selectable measurement time aperture to permit a balance between measurement speed and noise and a top sampling rate of 20 ksamples/s. This sample rate/accuracy tradeoff is particularly important in manufacturing applications where measurement times are critical.
While these sample rates represent only modest performance for an oscilloscope, they are enough for many applications. The DMMs also offer a more extended measurement range and higher isolation than a traditional oscilloscope. According to Bisking, the NI design achieves an internal common mode voltage rating of 500 V—a wide enough range, for instance, to test a 400-V fuel cell stack as an entire unit as well as each individual 1-V cell with the same instrument.
The ability to replace multiple instruments is one of the most valuable aspects of the PXI-based DMMs. The Signametrics device offers full LCR measurements, current and voltage sourcing for circuit simulation, and support for thermocouples for temperature measurements. NI has incorporated its DMM as part of its virtual instrument program, which allows users to configure a variety of instruments that have functions in common from units that contain those functions. In either case, the range and versatility of the PXI DMM has grown significantly with these latest releases.
For more on how manufacturers calculate the number of digits in a PXI DMM, see "DMM digits depend on performance."
