16-bit, 105M-sample/sec serial-output ADC has two-wire data stream

By Paul Rako, Technical Editor -- 4/24/2008

Linear Technology Corp’s new, 16-bit, 105M-sample/sec LTC2274 ADC has a two-wire serial interface, which greatly reduces the number of data lines between the ADC and the FPGA. The data interface is a single self-clocking, differential pair communicating at 2.1 Gbps using 8B/10B encoding. The LTC2274 serial output is compatible with many high-speed-FPGA interfaces, including Xilinx’s Rocket IO, Altera’s Stratix II GX I/O, and Lattice Semiconductor’s ECP2M I/O.

The part is suitable for communications systems, including cellular base stations, WiMax (worldwide interoperability for microwave access), WCDMA (wideband-code-division multiple access), TDSCDMA (time-division-synchronous CDMA), and multicarrier GSM (global-system-for-mobile) communication. It also finds use in multichannel ADC systems in ATE (automatic-test equipment), medical imaging, and instrumentation. The device comes in a 6×6-mm QFN-40 package, whose size makes the part useful in space-constrained data-acquisition systems.

The LTC2274 provides an internal transparent dither circuit that improves the ADC’s SFDR (spurious-free-dynamic-range) response to beyond 100 dBc for low-level input signals. A user can enable an optional data scrambler to randomize the spectrum of the serial link, reducing interference from the serial-digital outputs. A programmable-gain amplifier in the front end allows either a 2.25 or a 1.5V p-p input range. The part outputs data in two’s complement or offset-binary formats. The device has separate shutdown pins for the analog and the digital sections to conserve power. An on-chip duty-cycle-stabilizer circuit facilitates non-50%-clock-duty cycles.

The LTC2274’s ac specifications include a 77.5-dBFS (decibels relative to full-scale) SNR (signal-to-noise ratio), a 77.8-dBFS noise floor, and a 100-dB SFDR at baseband. The sample-and-hold section has a 700-MHz full-power bandwidth. The part exhibits 80-fsec-rms jitter, which enables undersampling performance of input frequencies as high as 500 MHz. The LTC2274 consumes 1.3W from a 3.3V analog supply.

The company plans this summer to release pin-compatible 80M- and 65M-sample/sec versions. Linear can screen products to order at sampling rates higher than the rated data-sheet specifications. The LTC2274 sells for $68 (1000). Production quantities will be available in July in both commercial- and industrial-temperature grades. Demonstration boards and samples are available online now.

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