RF A/D converter samples inputs to 2.7 GHz

EDN Europe, 15 Jul 2011

With a series of analogue/digital converter ICs from National Semconductor you can approach many radio design exercises in a new way, by directly digitising input RF at higher frequencies than has previously been possible; National claims the chips will, “revolutionise radio architectures”.

Five parts comprise the new series – like a predecessor, 1-GHz, series released in 2010, they are 12-bit chips – coded ADC12DxxxRF. “xxx” in the part numbers takes the values 1800, 1600, 1000, 800 or 500, indicating the basic sampling rate in Gsamples/sec. Each IC is a dual device, and interleaving the two ADCs gives a maximum sample rate of 3600 Gsamples/sec. Compared to earlier chips, the parts also have lower noise floor, and lower power, National says: they remain pin-compatible with earlier ADC12D1x00 and ADC10D1x00 families, and across the range.
You can sample inputs at or even beyond 2.7 GHz in 3G/4G basestation, military and software-defined radio applications. Third-order intermodulation distortion (IMD3) figures vary from -64 dBc to -71 dBc over the five parts; noise floor varies in the range -155 to -150.5 dBm/Hz. This allows you to get good noise and linearity performance beyond the 7th Nyquist zone; National claims that noise floor is as good as 16-bit parts already in the market. Undersampling allows any of the chips to digitise RF at 2.7 GHz; the slower parts will capture a correspondingly narrower bandwidth at the RF frequency. Power ranges from 2W for the 500 Msample/sec part to 4.4W for the fastest. On-chip clocking and de-skewing manages the interleaving when the two channels are used as a single converter. You can use the chips in a double-edge-sampling (DES, sampling on both edges of the clock) mode for I and Q channels: new is a mode called DESCLKIQ that removes the need for an input multiplexer when doing so, which contributes to a higher input frequency range. In addition to the RF path itself, you can also use the parts in the digital pre-distortion loop in a basestation RF power amplifier, to improve overall linearity. One further application that National suggests is in microwave back-haul – there is a growing demand for economical point-point small microwave links to support escalating basestation bandwidth demands, and you can build a much simpler receiver with this ADC, the company says.
Compared to architectures that convert at IF, or direct-conversion to baseband configurations, you can eliminate mixers, local oscillators, amplifiers and filters from radio designs, effectively making them digital immediately following the antenna/LNA/band filtering input path. Use of the fastest parts also enables digitising of a much wider slice of spectrum, potentially allowing you to reduce the number of channels needed to monitor a given band.
National builds the chips in 1.8-micron, pure-CMOS process; all five 12-bit RF-sampling ADCs come in pin-compatible, thermally-enhanced 292-ball BGA packages.

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