When I report on new products, I usually avoid claims that chips are “so many percent” better in some way than their competitors. That’s because specsmanship is a constant game of leapfrog. Sometimes a focus on specs can lead to an awkward situation.
For example, Texas Instruments and National Semiconductor demonstrated the perils of dueling specifications last January by announcing new analog-todigital converters (ADCs) for the same application space (high-end medical imaging) with nearly identical groundbreaking performance within days of each other (Fig. 1).
A TIE FOR “BEST” So I’m calling that one a tie. I felt sorry for the folks at TI, who announced their chips first. Their products also substantially reduced power consumption compared to what had been out there. Still, by using a brand-new topology, National squeezed its power by that much again. Number-wise, it was a case of a third of a third of what was already a very small quantity, but National earned the right to crow—and that’s marketing.
Those two chips were TI’s pipeline ADS5281 and National’s continuoustime (CT) delta-sigma ADC12EU050. Both are 12-bit, 50-Msample/s ADCs, though TI promised a future 65-Msample/s version. TI also claimed 70 dB full-scale at a 10-MHz IF, while National claimed 70 dB at 3.5 MHz. Power was the big issue. TI’s part drew 64 mW, National’s 44 mW. Pricewise, though, TI came in at $60 (in sample quantities), and National at $64. Packaging for the National part was a millimeter bigger in each horizontal direction.
With that tight a race in specifications, it became more interesting to examine implicit differences between the pipeline and CT delta-sigma architectures. Although CT was previously used in delta-sigmas integrated into more complex chips, the National chip was the industry’s first “discrete” commercial CT delta-sigma ADC. That had both plusses and minuses.
National took advantage of the oversampling architecture’s low-pass, brickwall, anti-aliasing filter to obviate the need for an external AA filter. Also, the CT architecture meant the IC had an easy-to-drive, purely resistive input stage that required no sample-and-hold amplifier. On the other hand, National had to overcome the CT architecture’s traditional susceptibility to clock jitter with an integrated phase-locked loop (PLL) and voltage-controlled oscillator for clock conditioning.
Meanwhile, TI’s engineers were busy building in a low-frequency noise-suppression mode to eliminate 1/f noise. As a result, signal-to-noise ratio (SNR) improved by up to 4.2 dB over a 1-MHz band in baseband and time-domain applications. TI also included overloadrecovery circuitry for ultrasound applications and programmable gain.
DATA CONVERTERS: INTERFACE DESIGN Back in April, Linear Technology focused on the communication between high-speed ADCs and FPGAs with its LTC2274 16-bit, 105-Msample/s ADC, which offered a single, self-clocking, differential- pair serial interface, communicating at 2.1 Gbits/s (Fig. 2). While lots of ADCs have serial outputs, the trick is making them that fast. Until recently, typical serial transmission could not be accomplished above 1.2 GHz, forcing a tradeoff between speed or resolution of an ADC.
In 2006, the JEDEC group formulated a serial interface specification ( JESD204) that enables a high-speed serial connection between data converters and logic devices over two wires. The electrical layer of the specification supports code rates of 312.5 Mbits/s to 3.125 Gbits/s across a current-mode logic (CML) pair.
This self-clocked serial data stream is encoded using 8B/10B coding, which provides benefits over conventional serial transmission by using a running disparity to eliminate dc imbalance in the signal. In noise-sensitive applications, this serial interface can be transmitted across an isolation barrier between digital and analog circuitry. It also serves to eliminate digital feedback.
The LTC2274 is the first ADC to adopt the JEDEC serial interface, making it compatible with many FPGA high-speed interfaces like Xilinx’s Rocket IO, Altera’s Stratix II GX I/O, and Lattice’s ECP2M I/O. With fewer output pins than a paralleloutput device, the LTC2274 fits in a 6- by 6-mm quad flat no-lead (QFN) package.
As was to be expected, Linear’s engineering team combined high ac performance and the high-speed serial interface on the same die. That means a 77.5-dBFS SNR and 100-dB spuriousfree dynamic range (SFDR) at baseband. Some special features also facilitate system design.
For high-sensitivity receiver applications, an internal transparent dither circuit improves the ADC’s SFDR response well beyond 100 dBc for low-level input signals. To avoid any interference from the serial digital outputs, an optional data scrambler can randomize the spectrum of the serial link. Serial test patterns can facilitate testing of the serial interface. Pricing starts at $68.00.
SPEED AND ACCURACY IN MEDICINE Analog Devices’ 2008 additions to its PulSAR family of precision 16-bit successive-approximation register (SAR) ADCs upped the ante for speed and accuracy in that class of converter, which targets medical MRI and digital X-ray systems.
At a 10-Msample/s throughput, the AD7626 achieved a 15-bit effective number of bits (ENOB). That’s due to its 92-dB SNR, which is 8 dB higher than any ADC running at that rate, regardless of architecture.
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