Electronic Design

Direct Conversion Creates Pull For Continuous-Time Sigma Delta

It’s curious how a nascent trend can unleash a breakthrough technology with far-reaching effects. The case at hand involves an evolving movement toward the commoditization of basestations that tends to favor direct-conversion receivers, according to Nitin Sharma, product marketing manager for high-speed converters at Analog Devices.

This trend led ADI to look at the continuous-time (CT) sigma-delta architecture for a standalone analog-to-digital converter (ADC), an architecture the company had used in clock and data recovery chips and modems but never as an ADC per se.

Yet according to Sharma, a properly designed CT sigma-delta could neatly fill a performance gap between successive-approximation ADCs, with their decent noise performance (low-90s dBs) and so-so bandwidth (5 or so MHz) and pipelines with their less spectacular noise performance (mid-80s) and better bandwidth (20 MHz and up, but with noise falling off every decade). So that’s what ADI did, creating three devices.

The AD9262 is a 16-bit, 10-MHz bandwidth, 30- to 160-Msample/s dual device, obviously intended for direct I/Q decoding. It has onboard, low-pass decimation filters, sample-rate converters, and dual 16-bit serial outputs. Its signal-to-noise ratio (SNR) is 84.5 dBFS to 10 MHz, and its two-tone spurious-free dynamic range (SFDR) is greater than 87 dBc.

The AD9267 is just like it, but without the decimation filters, sample rate converters, and wide bus outputs. It’s designed to feed right into an FPGA, where those functions would be handled internally. It offers 85-dBFS SNR to 10 MHz and better than 87-dBc SFDR.

The AD9261 is a single version of the AD9262 with decimation and sample rate conversion. Sharma says the bandwidth and low noise that make CT attractive for RF work also make it appealing in industrial applications, particularly in robotics, where it can make robotic arms that are both faster and more precise. Other potential applications include MRI and other forms of medical imaging, radar, softwaredefined radio, and spectrum analyzers.

Analog Devices

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