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
