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PXI plug-in is a 7-1/2-digit DMM, digitizer

Posted: 03 Mar 2005 ?? ?Print Version ?Bookmark and Share

Keywords:national instruments? ni? instrument? pxi/cpci 7-1/2? dmm?

National Instruments (NI) never sleeps. NI not only offers a dazzling array of modular instrumentsinstruments for making measurements anywhere from DC into the microwave domainbut it's also constantly dishing up new products such as this new 3U-sized single slot PXI/cPCI 7-1/2 digit DMM (digital multimeter).

Right now, NI's wares include high-resolution digitizers (up to 24 bits, and up to 200MS/s), as well as signal generators (up to 16 bits, and 200MS/s) and waveform generators/analyzers (with plug-in renditions working at rates to 400Mbps).

NI also offers PC-hosted instruments such as RF vector signal generators and signal analyzers (some useful up to 2.7GHz), and dynamic signal analyzers (with resolutions of 24 bits, and speeds to 205kS/s), as well as any required MUXs and/or switching matrices needed to stitch these instruments together into a system.

The competition
Prior to the arrival of the Model PXI-4071 FlexDMM, the only other 7-1/2 digit plug-in bus-oriented instrument you could buy was Signametrics's PCI bus SM2060 and SM2064 instruments. These DMMs provided 20,000 reading/s. By the way, Signametrics argues it's more accurate to evaluate a DMM's dynamic range in terms of counts, rather than to characterize a DMM in terms of digits. Wading through the way a DMM vendor specs its products can be daunting though. Signametrics's SM2064, for example, conveys a 24,000,000 count dynamic range, translating to a resolution capability of 10nV (which is right where NI's new DMM appears to be, too).

Like NI's, Signametrics's DMMs can measure very low current levels as well (down to 50pA). That's just what's needed for IV curve tracing (as pointed out in NI's press release, on the left). For its part, NI's unique solid-state shunt in the competing PXI-4071 delivers sensitivity as low as a picoamp for current measurements.

Sampling-rate tradeoffs
Based on the PXI bus, at 7-1/2 digits, a PXI-4071 operates at a DC reading rate of 7kS /s, but if you need higher speed, the instrument can operate at the DC reading rate of 10kS/s, for 4-1/2 digits, as noted in the press statement.

The PXI-4071 can also operate as a front-end digitizer. In its isolated digitizer mode, it can acquire DC-coupled waveforms at sample rates up to 1.8MS/s in all native voltage and current modes. Then, using NI's popular LabVIEW package on your PC, you can analyze captured waveforms in both time and frequency domains, looking at signals as you might on an oscilloscope and a spectrum analyzer.

As a programmable digitizer, you can also measure differential waveforms, even if they're riding on high common-mode levels. By using LabVIEW, you can also analyze fleeting and transient signals, and high-voltage fly-back signals such as those you might encounter in scopes and CRTs.

You can also capture aperiodic high-voltage AC waveforms. Best of all, you can analyze these digitized waveforms in both time and frequency domains. NI claims that no other 7-1/2 digit DMM can do that.

A multifunctional card
With variable resolution from 10 bits to 23 bits (it's adjusted by changing sampling rate), the PXI-4071 FlexDMM operates as a multi-function card. That can help you save money by sidestepping the purchase of separate data-acq hardware, as well as simplifying front-end signal conditioning.

Noteworthy is the fact that, when conditioning industrial or lab sensors, the PXI-4071 meets the safety and electrical standards requirements of IEC 61010-1, EN 61010-1, UL 61010-1 and CAN/CSA C22.2 No. 61010-1.

Input protection on the DMM's resistance ranges is 1kV (in 2-wire mode), and 500V (in 4-wire). Maximum common-mode voltage is 500V.

Earlier, I cited NI's extensive product line, referring to signal MUXing in automated applications. The PXI-4071, with its imported and exported triggers, is also readily integrated with MUX and matrix switch modules. NI's DMM can be seamlessly integrated with the company's PXI-2530 multiplexer, for example, or its SCXI-1129 high-density switching matrix.

Manage 1000s of channels
If you use the DMM/digitizer with these products, you can also invoke NI's Switch Executive switch management software. The Switch Executive executables let you manage and measure even thousands of channels.

Speaking of software, the new PXI-4071 ships with drivers; the NI-DMM IVI-compliant driver lets you drive the DMM through an API (application programming interface), and NI-DMM 2.4 (or later) contains a DMM Express VI (virtual instrument). The VI lets you develop a FlexDMM app in LabVIEW or NI's SignalExpress. It does that with interactive configuration dialogs.

NI-DMM also includes what's called an SFP (DMM Soft Front Panel). The DMM SFP is an interactive executable that provides a way to test input signals or debug your system. As you'd expect, it's optimized for use with LabVIEW and LabWindows/CVI, but also works with other popular software suites, such as Measurement Studio, and Visual Studio .NET.

A unique AC algorithm
For AC measurements, the FlexDMM gets its speed by performing a unique rms-to-DC conversion, doing it entirely in the digital domain. NI's algorithm requires only a few cycles of a waveform to compute rms. The algorithm also automatically rejects any DC component, letting NI bypass the use of settling-time-limiting input capacitors.

However, to measure small AC levels in the presence of DC (such as ripple on a power supply), the DMM does offer a standard AC Volts mode that uses a coupling cap; eliminating DC offsets by coupling through a capacitor lets the DMM use its most sensitive range.

NI claims its all-digital approach to rms computation also conveys accuracy benefits. For example, NI claims its algorithm makes the DMM insensitive to crest factor, and yet still delivers quiet and stable readings. The proof of the pudding is that the PXI-4071 spec sheet guarantees AC accuracy down to 1 percent of FS (full-scale). It can also achieve usable readings below 0.1% of FS.

Not mentioned in NI's press release is the fact that this smart DMM packs built-in self-calibration, relying on a high-stability internal voltage reference. No other equipment or hook-ups are needed during calibration. The system is NIST traceable, and self-calibration takes less than a minute.

Touting a 2-year calibration cycle, the PXI-4071's self-cal routines correct for DC gain drift and offset drift. Self-calibration compensates for resistance and current source drift, too. In resistance, all errors are corrected to a single internal high-stability foil resistor. It's stable to within 0.8ppm/C over the DMM's full operating range.

When you consider the PXI-4071's price of less than $2500, it shapes up as a DMM that's likely just at home making automated tests on the factory floor as it is in the lab.

- Alex Mendelsohn

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