Under the Hood: PXI-5152 scope a design hotbed

Precision oscilloscopes and laboratory instruments occupy a special place in the design flow for a new circuit or system. This is the instrument that verifies the performance of your prototype. It captures the signals your circuitry puts out and displays them on a calibrated screen. No matter how fast you think your high-speed circuit performs, the oscilloscope must run faster.

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In the case of National Instruments' PXI-5152, the front-end digitizer will grab coarse signals at a breathtaking 20 Gsamples/second when operating in a random interleaved sampling mode. The PXI digitizer module and chassis, used with a separate computer, form the front end of a digital oscilloscope. The instrument digitizes signals with 8-bit resolution, relying on software on the computer host for data analysis, information display and measurement control.

Introduced in 2006, the PXI-5152 is the fastest signal-capture card made for NI's PXI measurement-and-control chassis. The design of the chassis is entirely modular: Cards inserted with Euro-card connectors alter the speed and resolution of the input and output circuits, and configure the PXI chassis for a variety of analysis applications. Configured as an oscilloscope, the PXI provides control over trigger points, timing windows and amplitude adjustments for captured signals.

As with the semiconductors that perform analog-to-digital conversion, test instruments embody a trade-off between speed and bit resolution. With a 10-volt input range, a front-end module like NI's PXI-5922 will resolve 16 bits—down to about 150 microvolts—but its sample rate is only 15 Msamples/s and its bandwidth 6 MHz. The PXI-5152 will capture high-speed signals with 8-bit resolution; that is, over a 10-V input range, it will resolve signals down to 39 mV. This will depict repetitive signals (like sine waves) with no artifacts, though the spurs on an RF signal might appear grainy on the display screen. The 2-Gsamples/s bandwidth, however, lets designers at least capture the spurs, whereas a lower-bandwidth scope would miss them entirely. The 300-MHz bandwidth allows intermediate frequencies—to outputs of downconverters, for example—to be captured cleanly.

The secret sauce
Removing many layers of RF shielding on the 5152 reveals a small arsenal of analog ICs with all the famous labels: Analog Devices, Linear Technology, Texas Instruments and so on. The key to the digitizer's high-speed performance, though, is in the center of the board: a National Semiconductor ADC08D1000, a dual 8-bit converter with a 1-Gsample/s update rate. The dual-converter architecture lets users such as NI interleave sampling cycles, effectively doubling the signal capture rate to 2 Gsamples/s.

Each converter on the ADC08D1000 is a CMOS A/D converter with 8-bit sampling rates that reach 1.3 Gsamples/s. The total part consumes 1.6 watts typical at 1 Gsample/s from a single 1.9-V supply (3.5 mW in powerdown). Each converter has a 1:2 demultiplexer that feeds two low-voltage, differential-signaling (LVDS) buses and reduces the output data rate on each bus to half the sampling rate.

While data converters with gigasample rates are also made by Maxim Integrated Products and Atmel, two factors led NI to choose the National Semiconductor part, said Brian Leonard, the engineer who leads NI's high-speed design team. One was the data sheet specifications, such as that for power consumption, which often equates to heat generation. Anything that runs at gigahertz speeds will generate some heat. The tight spacing between the PXI-5152's boards does not leave much room for heat sinks, and the National Semiconductor part (in low-profile QFP packaging) allows heat to escape through a tab on the bottom of the part. Leonard also liked the spec of 7.5 effective number of bits, a measure of how little noise he has to cope with when the device is running at full speed.

The second factor he cited was his relationship with National Semiconductor's field applications team. The interleaved sampling capability of the ADC08D1000, Leonard believes, was something National Semiconductor cultivated in response to NI's needs.

A Xilinx Virtex-II (XC2VP20) is prominent on one of the three circuit cards that make up the PXI-5152. The FPGA interfaces with National Semiconductor's A/D converter and performs a data-formatting function. The synchronization memory core (SMC) board, on which the FPGA resides, also performs synchronization between the 5152 and other modules in the PXI chassis. With timing synchronization to within 10 picoseconds, a PXI chassis can accommodate up to 34 gigasample channels, said Kaustubh Wagle, NI's product manager for high-speed digitizers. An 18-slot PXI chassis, for example, can accommodate 17 two-channel modules, each synchronized by the SMC cards.

The design of the ADC08D1000 also supports an FPGA interface. Most FPGAs are not equipped to pull data in at gigasample rates. A multiplexed LVDS interface allows data to flow out of the A/D to be timed on the same 250-MHz clock that runs the FPGA. To sample at 2 Gsamples/s, you take four samples per clock edge, on both the rising and falling edges of the clock (eight samples per clock), Leonard explained.

Stephan Ohr stephan.ohr@gartner.com is research director for Analog and Power Semiconductors at Gartner Dataquest Research.