Electronic Design

Dual 14-Bit 150-Msample/s ADC Provides AGC Input For Receivers

Meeting the need for analog-to-digital converter (ADC) sampling at rates beyond 135 Msamples/s in WiMAX applications, Analog Devices' AD9640 is the first dual 14-bit analog-todigital converter (ADC) to offer 150 Msamples/s. It provides two ADCs in the same footprint as previous single ADCs with the same performance. Also, it offers an onchip clock divider and digital inputs to the receiver-system automatic gain control (AGC) circuitry (see the figure).

That AGC block monitors the incoming composite signal power so receivers that use the chip can implement fast or slow AGC. "Slow" mode is for conventional AGC. "Fast detect" (FD) mode enables the system to detect an input over-range condition in as little as two clock cycles so it can immediately reduce 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 on-chip clock divider in the AD9640 simplifies system layout and takes another chip off the board. In basestation transmitters, the digital-toanalog converters (DACs) usually operate at a higher clock rate than the ADC, and the DAC clock is divided down for the ADC. By implementing the clock divider on-chip, the AD9640 eliminates the need for an external divider.

Besides WiMAX, the AD9640 can support W-CDMA, cdma2000, and TDSCDMA. Sampling now, production quantities will be available in April. It will be available in 12- and 14-bit resolutions with sample rates of 80, 105, 125, and 150 Msamples/s.

In 1000-piece lots, 14-bit versions of the AD9604 cost between $37.50 and $87.50, and 12-bit versions cost between $25.05 and $47.97, depending on sample rate.

Analog Devices
www.analog.com

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