Direct RF-sampling ADCs aims to revolutionize radio architectures

July 12, 2011 // By Paul Buckley
National Semiconductor Corp., has introduced a family of analog-to-digital converters that the company claims are the industry’s first to directly sample RF signals beyond 2.7 GHz, with third-order intermodulation distortion (IMD3) up to -71 dBc and sampling at up to 3.6 GSPS.

The ADC12Dxx00RF family includes five 12-bit ADCs that enable system designers to eliminate multiple intermediate frequency (IF) down-conversion stages including amplifiers, mixers and filters. A single ADC12Dxx00RF replaces entire radio signal path subsystems, reducing bill of materials (BOM) cost, board size and weight in 3G/4G wireless basestations as well as microwave backhaul, military and wideband software-defined radio (SDR) applications.

“Radio designers are seeking new ways to increase radio capacity while reducing cost, size and weight. Architectures based on RF sampling ADCs provide these benefits, but it has long been believed that this technology was years away from reality,” said Jon Baldwin, marketing director for National’s High-Speed Signal Path Division. “With its combination of unparalleled high frequency dynamic range and industry’s highest sampling rates, the ADC12Dxx00RF family of ADCs provides the core technology that enables our customers to begin development of direct RF-sampling radios today.”

The footprint-compatible ADC12Dxx00RF ADCs join National’s industry-leading GSPS ADC family, allowing an easy upgrade path from 500 MSPS to 3.6 GSPS:

•    The ADC12D1800RF provides interleaved single-channel sampling rates up to 3.6 GSPS, or dual-channel rates up to 1.8 GSPS, with IMD3 of -64 dBc at 2.7 GHz.

•    The ADC12D1600RF provides interleaved single-channel sampling rates up to 3.2 GSPS, or dual-channel rates up to 1.6 GSPS, with IMD3 of -70 dBc at 2.7 GHz.

•    The ADC12D1000RF provides interleaved single-channel sampling rates up to 2.0 GSPS, or dual-channel rates up to 1.0 GSPS, with IMD3 of -69 dBc at 2.7 GHz.

•    The ADC12D800RF provides interleaved single-channel sampling rates up to 1.6 GSPS, or dual-channel rates up to 800 MSPS, with IMD3 of -71 dBc at 2.7 GHz.

•    The ADC12D500RF provides interleaved single-channel sampling rates up to 1.0 GSPS, or dual-channel rates up to 500 MSPS, with IMD3 of -69 dBc at 2.7 GHz.

The excellent linearity of the direct RF-sampling ADCs across all frequencies allows more system margin to be shifted to other