Low-noise VGA with 24 dB gain range
10 March 2009
Faster signal transition times in advanced video applications.
An integrated device designed for PET (Position Emission Tomography) scanners, communication systems, and advanced video equipment was released by Analog Devices. The AD8264 quad-channel VGA (Variable Gain Amplifier) integrates four ADC (Analogue-to-Digital Converter) drivers, enabling radiological equipment designers, for example, to reduce printed-circuit board area and simplify layout by replacing eight components with a single device.
The AD8264 quad VGA is suited for wireless infrastructure equipment where its four DC-coupled channels and precise gain control allow communication-systems designers to more easily adjust output-signal levels and eliminate the time and cost associated with matching four discrete VGAs. For advanced broadcast and video-security applications, ADI’s new VGA improves picture quality by more accurately white-balancing RGB (red-green-blue) signals and better managing gain-trim calibration over the system’s life.
Saving space is a key design requirement for advanced systems in many applications. Next-generation PET scanner designs are increasing channel densities to enhance image resolution and improve patient diagnostics, despite limits to the physical size of the scanning element, or gantry. The high-speed AD8264 VGA has a single-ended output for wide-bandwidth applications, and auxiliary differential ADC drivers that operate with ADI’s data converters, including the AD9222 octal and AD9228 quad 12-bit ADCs.
Each of the AD8264’s four linear-in-dB VGA channels has independent gain control. Each channel comprises a high-impedance preamplifier, a high-speed VGA, and a differential output amplifier. The AD8264’s pinout provides single-ended output taps directly from the VGAs, which have a -3 dB bandwidth of 235 MHz, and balanced outputs from the 80 MHz output amplifiers. Dual supply operation enables gain control of negative-going pulses such as those that photodiodes, photo-multiplier tubes, and video signal-sources generate. With a 24 dB gain range, the AD8264 features a gain-control interface that provides a precise 20 dB/V linear-in-dB scale. A common gain-control reference for all channels simplifies wiring. The differential gain-control structure accommodates a wide range of common-mode operating points, simplifying the interface to any analogue or digital control source. A VOCM pin sets the common-mode voltage of the differential output stage to match most modern single-supply ADCs, and allows for dc level-shift of the output. Power consumption is 125 mW per channel at ±3.3 V.
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