Electronic Design

Do They Play Nice Together? Don't Look At Chips And Reference Desgins In Isolation

What's more important in new product announcements- new levels of performance, or the availability of design tools that make those performance levels accessible? Many designers no longer have the luxury of enough development time to breadboard, refine, and test circuits assembled from parts from multiple chip vendors-assuming designers aren't paying too much of a premium for what the marketing people call a "platform" or a "solution."

During a recent product briefing with National Semiconductor, I asked the company rep about the importance of the new products' reference designs. "Are they really vital, or are they just sales gimmicks?" I asked.

"No gimmicks," the representative said. "The designers are as proud of them as they are of the parts themselves."

"Then lead with the reference designs," I said. But of course, that didn't happen. Instead, I learned that the new parts are three energy-efficient, high-speed differential amplifiers that respectively target communications, test and instrumentation, and defense and space applications. Energy efficiency is their claim to fame, so National has tagged the line "PowerWise" (see the figure).

National quickly moved to a description of the three reference designs: one for WiMAX and 2G/3G wireless basestations, one for test and measurement equipment, and one for high-speed communications systems. National's WaveVision hardware and software, application specifications, schematic diagrams, bills of materials, and layout guidelines all support the boards.

The LMH6552 is a 1.5-GHz, current-feedback differential amplifier that consumes 112 mW. The LMH6515 is a digitally controlled variable-gain amplifier (DVGA) with an 8-dB noise figure and a 40-dBm output intercept point (OIP3) at 70 MHz, with similarly low power consumption. The LMH6555 differential amp suits signal conditioning ahead of an 8-bit, 3-Gsample/s data-acquisition system.

The reference designs
The first reference design is a low-IF receiver subsystem that uses a pair of LMH6552 differential drivers and a dual analog-to-digital converter (ADC) for quadrature direct conversion or for a near zero-IF receiver for signal frequencies from dc to 40 MHz.

With a 1-GHz input bandwidth for the ADC, the design achieves 73.3 dBFS and a signal-to-noise ratio (SNR) better than 85 dBFS. The ADC is National's ADC14DS105 dual, 14-bit, 105-Msample/s differential-out chip. Other components include a low-jitter clock conditioner and several power- management ICs.

The high-IF receiver reference design highlights the Power- Wise DVGA and National's ADC14V155 14-bit, 155-Msample/s ADC. In a wireless infrastructure system or digital spectrum analyzer, it would provide variable-gain IF amplification and digitization. Small-signal SNR is 72 dBFS, and spurious-free dynamic range (SFDR) is greater than 90 dBFS with a 169-MHz input.

Large-signal SNR is 68.3 dBFS, with a 77-dBFS SFDR at 169 MHz. Other chips on the board include a low-jitter precision clock conditioner with an integrated voltage-controlled oscillator (VCO) that provides 190-fs jitter over an integration bandwidth of 100 Hz to 20 MHz, as well as those power-management ICs.

The final reference design is an 8-bit, 3-Gsample/s dataacquisition board for test and measurement equipment and the kind of very high-speed communications systems used in defense and space applications. It receives an ac- or dc-coupled analog input and captures the digital output of the ADC into an FPGA. For evaluation, the captured data can be analyzed using National's WaveVision software on a Windows PC via a USB 2.0 connection.

When ac-coupled, the board achieves better than 7-bit effective number of bits (ENOB) and 53.5-dB SFDR at frequencies up to 1.4 GHz. The ADC is National's ADC083000 8-bit, 3-Gsample/s performer. Also on the board is the LMX2531 single-chip phase-locked loop/VCO, which provides â??116-dBc/Hz phase noise at a 100-kHz offset.

How "PowerWise" are the boards? The high-IF reference design draws a bit over 600 mA at 5 V; the low-IF draws around 500 mA; and the data-acquisition board draws approximately 600 mA at 12 V.

In 1000-unit lots, the LMH6515 costs $5.25 each and the LMH6552 is $3.95. The LMH6555 costs $5.95 each in 100-unit lots. The ADC14V155KDRB (High-IF) and ADC14DS105KARB (Low-IF) receiver reference-design boards cost $800 each. The 3-Gsample/s ADC083000RB dataacquisition board costs $4900.

National Semiconductor www.national.com

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