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Dual 14-bit ADC supports 3G and WiMAX basestations

With sampling rates to 150 Msamples/s, the AD9640 dual 14-bit analog-to-digital converter (ADC) meets the sampling rate requirements for all wireless standards, including 3G (W-CDMA, CDMA2000 and TD-SCDMA) and WiMAX.

In the case of WiMAX, rates of at least 135 Msamples/s are required, but according to Analog Devices Inc. (Norwood, MA), the manufacturer of the AD9640, rates beyond 135 Msamples/s could only be achieved using two single-channel ADCs. With the AD9640, such rates—and higher—can be achieved using a single dual-channel device, giving a 50 percent savings in board space. The AD9640 also simplifies the design of multicarrier wireless infrastructure systems by decreasing the amount of analog filtering in front of the ADC and offering low power consumption.

“Today's basestation manufacturers are faced with designing for a variety of 3G wireless standards, as well as the emerging WiMAX standard, while also focusing on reducing costs,” said Kevin Kattmann, product line director, High Speed Signal Processing Group at ADI. “Addressing this challenge, the AD9640 offers the performance and sample rate required for these standards, but also provides the low power, small size and digital features to lower overall system costs.”

The new ADC consumes 390 mW per channel, has a signal-to-noise ratio (SNR) of 72.7 dBFS and a spurious free dynamic range (SFDR) of 85 dBc with a 70-MHz intermediate frequency (IF). It is also capable of supporting IFs as high as 450 MHz, enabling WiMAX designers to use a single downconversion receiver design instead of the traditional two downconversion stages. For optimal performance, the AD9640 can be driven with ADI's AD8352 low distortion differential amplifier.

The AD9640 divides the input clock by an integer between 1 and 8, reducing system complexity and improving clock jitter. In a traditional base station transmitter, the digital-to-analog converters (DACs), such as ADI's AD9779, will have a higher clock rate, which is divided down to match the ADC. By implementing the clock divider on-chip, the AD9640 eliminates the need for an external divider. In
addition, the phase noise of the source improves in proportion to the divide ratio.

Digital features lower system cost
Incorporated on the AD9640 are digital features that reduce system costs by simplifying the automatic gain control (AGC) loop in the receiver. It includes a block that monitors the incoming composite signal power and indicates whether the gain of the receiver needs to be increased or decreased. While the signal monitor block can act as a slow mechanism to change the receiver gain, the AD9640 also includes fast detect (FD) modes, allowing designers to detect an input overrange condition in as little as 2 clock cycles, immediately reducing the gain to avoid overdriving the analog front end. In addition to providing clipping information, the FD bits have programmable thresholds that are useful in optimizing the receiver AGC loop.

The AD9640 is sampling now with production quantities available in April 2007. It is available in 12- and 14-bit resolutions with sample rates of 80, 105, 125 and 150 MSPS. CMOS and LVDS output modes are supported. The 14-bit ADC is priced between $37.50 and $87.50 and the 12-bit ADC is priced between $25.05 and $47.97 in 1,000-piece quantities. Both versions are packaged in 9- – 9-mm, 64-lead LFCSP (lead-frame chip-scale package) packages. For more information or to
download the ADIsimADC behavioral design model for the AD9640, visit www.analog.com/pr/AD9640.

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Understanding analog-to-digital converter specifications

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