PXI's innovative content description

PXI's innovative content description

Since its birth in 1997, PXI has been in a stage of rapid growth. One of the important reasons for the growth of PXI is that manufacturers release a large number of PXI modules every year. In 2005, more than 1,200 PXI products were launched, including vision, motion, digital multimeters, oscilloscopes, power supplies, RF signal analyzers, high-density switches, etc.

These modules are released by more than 70 PXI system alliance vendors, and many of them are best in class. In fact, the common platform elements under the open standard such as PXI have fostered many innovations and promoted the development of measurement technology. This article will reveal the development trend of this innovation and introduce some of the most advanced PXI products in the industry.

Figure 1 PXI product options (data provided by the PXI Systems Alliance)
An open standard can bring us a range of innovative technologies. PXI is an open industry standard that is controlled by the PXI Systems Alliance (PXISA).
PXISA has developed a complete standard that specifies the driver software architecture, mechanical packaging, internal communication bus, and module power requirements. Using these predefined system components, instrument design engineers can focus more on the most critical issue of the product, that is, the development of measurement circuits.
It may not be easy to understand that a standard like PXI can bring innovation. However, building a PXI module requires many strict design requirements. For example, the power supply power of each module cannot exceed 30 watts, and the entire design must be able to be inserted into a 3U slot (562 cm3) or 6U slot (1124 cm3), while traditional benchtop instruments have no power restrictions and the minimum footprint limit is 7500 cm3 (which is 13 times larger than PXI modules). These requirements are very stringent, but they also drive innovation among design engineers—adopting the latest chip technology, developing new design methods, and performing measurement functions in software that usually require discrete analog and digital devices.
For example, when NI design engineers set out to develop the first NI PXI digital multimeter, they evaluated the ADCs used in existing digital multimeters, including dual-slope, charge-balance-with-ramp-down, and sigma-delta ADCs. None of the existing ADC implementations met the requirements of the PXI standard and could not meet the product's requirements for noise, linearity, speed, and flexibility. So the design engineers developed a new solution that combined off-the-shelf high-speed ADC technology with patented sigma-delta ADC technology, see Figure 2.

Figure 2 Patented A/D converter in FlexDMM
At low speeds, the circuit takes advantage of the sigma-delta converter. The feedback DAC is designed to achieve very low noise and excellent linearity. The low-pass filter provides noise shaping, which allows very good performance at all resolutions. Because the ultra-high-precision 1.8 MS/s modulator can provide conversion at very high resolutions, there is no need for ramping. At high speeds, the 1.8 MS/s modulator is combined with a fast sampling ADC to provide continuous sampling digitization. The DSP provides real-time sequencing, calibration, linearization, AC RMS calculations, decimation, and weighted noise filtering for DC functions. The resulting design, the PXI-4071 FlexDMM, is the industry's most accurate 7 ½-digit DMM on any platform. The PXI-4071 DMM is
just one of many top PXI products in the industry. Let's take a look at other PXI products that are pushing the boundaries of measurement technology.