Optimizing resolution and sampling rate

2006 AWARDS: Test Engineer of the Year Test Product of the Year Test of Time READ OTHER MARCH ARTICLES:  Contents, March 2006

Given the PXI-5922's flexible performance and suitability for use in such a wide range of applications, it's not surprising that Test & Measurement World readers have chosen it as the 2006 Test Product of the Year (www.tmworld.com/awards).

The PXI-5922 single-slot PXI card employs a custom multibit delta-sigma ADC to support high acquisition speeds at high resolution. Courtesy of National Instruments.

But achieving the 5922's high resolution for a wide range of frequencies proved to be challenging. Typically, Wagle said, NI leverages off-the-shelf components. To that end, he said, "We took a survey of all available ADC technologies out there—flash, SAR, delta-sigma. All these ADCs have different pros and cons, but none gave the sampling rate and resolution that we needed." For example, single-bit delta-sigma converters provide the necessary resolution but not sufficient speed. The solution? "We designed our own ASIC, called the Flex II ADC."

The Flex II ADC, he explained, employs a multibit delta-sigma conversion approach that provides the resolution of single-bit delta-sigma devices while offering higher conversion speeds. The multibit approach, however, exhibits nonlinearities that generate unwanted harmonics. To deal with that problem, Wagle said, the designers implemented a patented linearization technique. The result is an instrument offering –120-dBc (typical) spurious free dynamic range to 100 kHz.

Niels Knudsen, principal hardware architect (left), and Kaustubh Wagle, product marketing manager for digitizers, with a PXI-5922. Courtesy of National Instruments.

Wagle cited the audio field as one area where the 5922 will find use. "If you look at audio, you might be surprised that a card like the 5922 would be required. After all, the audible frequency range is only 20 kHz." But, he said, "there is this notion out there among audio chip and equipment manufacturers that people not only hear but also feel the music by means of ultrasonic waves."

To ensure that they can measure any anomalies that a listener might in any way detect, Wagle said, "These companies want to test high-order harmonics way beyond 20 kHz," and they need to do it at 24-bit resolution. Existing 24-bit products, Wagle said, top out at 200 ksamples/s, while the 5922 continues working to 500 ksamples/s.

There's more to the NI PXI-5922 than the Flex II ADC. For example, it incorporates an analog front end that complements the Flex II ADC, eliminating input bottlenecks that could degrade dynamic performance. The input amplifier has software-selectable input impedances of 50 ohms and 1 megohm.