GENERAL DESCRIPTION

The AD6644 is a high speed, high performance, monolithic 14-bit analog-to-digital converter (ADC). All necessary functions, including track-and-hold (TH) and reference, are included on-
chip to provide a complete conversion solution. The AD6644
provides CMOS-compatible digital outputs. It is the third generation in a wideband ADC family, preceded by the AD9042
(12-bit 41 MSPS) and the AD6640 (12-bit 65 MSPS, IF
sampling).

FEATURES

65 MSPS guaranteed sample rate
40 MSPS version available
Sampling jitter < 300 fs
100 dB multitone SFDR
1.3 W power dissipation
Differential analog inputs
Pin compatible to AD6645
Twos complement digital output format
3.3 V CMOS compatible
Data-ready for output latching

APPLICATIONS

Multichannel, multimode receivers
AMPS, IS-136, CDMA, GSM, WCDMA
Single channel digital receivers
Antenna array processing
Communications instrumentation
Radar, infrared imaging
Instrumentation

How to use AD 6644 :

DEVELOPMENT OF A DIGITAL IF RADAR TRANSCEIVER SYSTEM :

Recent rapid growth in the microelectronics industry
has seen the development of high performance
analogue-to-digital converters, reconfigurable logic
devices and digital signal processors, such that it is
possible to quantize down-converted radar signals
directly at IF. Using such techniques it is now possible
to replace substantial portions of the analogue
receiver system with digital signal processing (DSP).
In doing so, many of the inherent alignment,
maintenance and temperature sensitivity problems
can be drastically reduced. In addition to solving many
of the traditionally difficult problems associated with
analogue receiver systems, a digital system provides
an improved linearity, increased linear dynamic range
and reduced phase-jitter through reduced AM/PM
conversion inherent to IF limiting often used in linear
receiver systems.
The key advantages of digital IF processing over
equivalent function analogue processing are:
• Zero in-phase and quadrature phase error, gain
imbalance and offset (DC bias) errors – which
manifest themselves as image spectra or clutter
echoes in the Doppler spectra.
• Wider dynamic range with a truly linear response.
• Any number of channels can be constructed with a
high degree of repeatability – cross covariance
parameters in polarimetric radars are more a
function of the target and less a function of the
receiver.
• Massively reduced temperature sensitivity.
• Reconfigurable in real-time– For example, the final
IF filtering characteristics can be changed on a
pulse-to-pulse basis if required.

The basis of this system is a digital to
analogue converter card based upon the Analog
Devices AD6644 (14-bit 65 MSPS) converter (Echotek
ECDR-214-PMC) coupled to a signal-processing card
using four Analog Devices TigerSHARC processors
(Transtech DSP, TS-36N).