Electronic Design

Simpler Solutions Spark Active ADC Arena

Expert designers are hard to find. So, converter suppliers are making their products easier to use. Some suppliers simplify analog front-end design by choosing alternative architectures. Delta-sigma, both switched-capacitor and continuous-time, simplifies front-end filtering compared to successive-approximation registers (SARs) and pipelines. Another trend sees amplifiers custom-designed for targeted converters. Alternatively, variable-gain amplifiers (VGAs) are being built into the converter.

On-chip voltage references also are getting more stable over temperature extremes. A 25-ppm/°C temperature coefficient (TC) used to be the best an on-chip reference could do. Now, guaranteed 10-ppm/°C maximum specs are common. (If a design requires 3 ppm/°C, TCs will require external references.) Yet not all applications are subject to temperature extremes, and TC is less critical in frequency-domain applications. Signaling another possible trend, Analog Devices is offering customized clock chips.

Most of the really challenging application areas for analog-to-digital converters (ADCs) can be predicted rather easily. But some low-bandwidth apps, such as high-end weigh-scales, are bears to design because both power and noise—the two characteristics traditionally at opposite poles of the design tradeoff decision—must be reduced simultaneously. This raises its own challenges for chip designers.

Formerly, basestations were designed for a single carrier, with relatively narrow bandwidth requirements. Today, they need more bandwidth—not just for CDMA, but to handle up to four 20-MHz carrier signals.

Basestation designers also want fewer mixer stages, so newer converters must operate at 2- or 3-MHz IFs. Meanwhile, when sample rates run from 100 to 200 MHz, there's a need for new interfaces on the digital side of the chip: low-voltage differential signaling (or an even lower voltage differential standard), rather than CMOS.

Low-power pico- and micro-cell basestations provide uniform coverage in tunnels and city canyons. Where traditional basestations that service relatively large geographical areas can afford more expensive converters, pico- and micro-cell basestations need converter products that integrate as much of the whole receive chain as possible.

While basestation applications deal with band-limited 20- to 40-MHz signals, system designers need to digitize signals as fast as 70 or even 100 MHz in broadband and some wireless applications. This requires ADCs with 200- to 250-Msample/s capabilities.

In instrumentation applications, which use ADCs optimized for precision rather than high speed, most system designers want sample rates from 100 to 150 Msamples/s and resolutions from 12 to 14 and sometimes even to 16 bits. Overall, throughput demand ranges from 20 to 200 Msamples/s.

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