Over the last several years, we've
seen ICs wrap lots of applicationspecific functionality around a
mixed-signal core, ultimately creating a canned solution for consumer-products OEMs. Expect
more of the same this year.
For example, there's Wolfson Microelectronics' WM8781 (see "Digital TV,
Audio Boost Analog/Mixed-Signal"). This high-input-voltage,
24-bit, sigma-delta analog-to-digital converter (ADC) suits audio gear. Its application-specific customization includes the
ability to accept stereo line-level inputs
and a clocking scheme that facilitates
running the clock of an attached processor clock at an MPEG-friendly 384 times
the audio sampling frequency.
ASSPs Swallow DACs
In 2007, look
for sampling rates to push far beyond
audio. It's been getting difficult to find new
discrete digital-to-analog converters
(DACs), except in the upper reaches of the
radio frequency spectrum. Now,
even there, subsystem integration is starting to happen.
In December, Analog Devices
announced its AD9910, a 1Gsample/s, 14-bit direct-digitalsynthesis (DDS) chip. (Allowing
for anti-aliasing, that implies
output frequencies of up to 400
MHz with 0.23-Hz resolution.)
The DAC is part of the IC, and its
1 Gsample/s is almost as fast
as ADI's fastest standalone 14bit DAC, the AD9736, which
goes to 1.2 Gsamples/s ().
Many of the ICs that integrate
DACs are consumer audio products. It could be argued that integration there is necessary simply
because the chips are being
designed for OEMs that specialize in manufacturing, and that
what these OEMs are, in effect, doing is outsourcing subsystem design to
the chipmakers. That isn't true with the
AD9910, though. The AD9910 was developed for high-performance applications as
varied as radar systems, military communications, and high-end test equipment.
Companies in those businesses generally
do their own subsystem design.
Still, for a circuit designer in that kind of
environment, integrating the DAC in a chip
that functions as a complete subsystem
buys many of the same advantages it does
for consumer-products OEMs. These
include a shorter development cycle, a
smaller footprint, and a more sophisticated design achieved at little risk.
Integration's advantages to
the chip maker include the ability to achieve differentiation by
giving subsystem designers
almost as much circuit versatility as they would have if they
started with a blank slate.
For instance, consider some of
the sophisticated architecturally
features of the AD9910: It takes
data in via a 250-MHz serial I/O
port and stores that data in internal control registers; on-chip static RAM supports various combinations of frequency, phase,
and/or amplitude modulation, or
alternatively, the chip will support
a user-defined, digitally controlled, linear sweep mode; and
for more advanced modulation
functions, a high-speed parallel
data-input port enables direct frequency, phase, amplitude, or
polar modulation.
A related advantage for
chip makers that have developed a high-performance
DAC core is the ability to
reuse it in other designs. ADI
utilized the same core in the
AD9957 1-Gsample/s quadrature digital upconverter.
This "QDUC" accepts an I
and Q data stream from a
serial or an 18-bit parallel
port and outputs a modulated signal from the DAC. This
moves modulation into the
digital domain, which has
advantages in WiMAX and
cable modem applications.
Bettering The Best
Naturally, chip makers still
address raw performance.
But it takes a strenuous
effort to top your own (and your competitors') previous personal
bests time after time. TI just
announced a monolithic 12-bit,
500-Msample/s pipeline ADC with a
2-GHz input bandwidth and low-voltage differential signaling outputs.
The ADS5463 targets communications, amplifier linearization, test
and measurement instrumentation,
software-defined radio (SDR), and
radar and imaging systems. It
boasts a guaranteed minimum 63.5-dBFS signal-to-noise ratio
(SNR) at a 100-MHz input frequency.
Other specs at the same input frequency include a 70-dBc spuriousfree dynamic range (SFDR) and 64dBc second- and third-order
harmonic distortion. These contribute to a 62-dBc signal-to-noise
and distortion (SINAD), which yields
a guaranteed 10-bit effective number of bits (ENOB) resolution.
This is best-in-class performance, but
it's a very close race. I got a sense of this
from TI's press briefings, in which the
company compares the "typical" specs
for the new chip with those of its closest
competitor, another 12-bit, 500-Msample/s ADC.
Looking at SNR at the full conversion
rate, with a 250-MHz IF, TI scores 64.5
dBFS versus 62 dBc for the competitor.
For SFDR, it's 76 versus 72 dBc. Those 2.5- and 4-dB differences are significant
in high-end designs. But as deep as they
are below carrier level, every decibel of reduced noise or distortion must represent a hard-fought engineering battle
and some serious anxiety in the testing
lab as the results are confirmed.
In fact, TI isn't relying on just performance numbers to promote the ADS5643.
It's also emphasizing that the device is
available at introduction in industrial and
military temperature-range versions, that
a space-qualified version is also available, and that its packaging has a much
smaller footprint than the competition.