IC QUADRATURE DEMOD 28-TSSOP
Baseband level detectors are included for use in an AGC loop to maintain the output level. The demodulator dc offsets are minimized by an internal loop, whose time constant is controlled by external capacitor values. The offset control can also be overridden by forcing an external voltage at the offset nulling pins. The AD83471 is a broadband direct quadrature demodulator with RF and baseband automatic gain control (AGC) amplifiers. It is suitable for use in many communications receivers, performing quadrature demodulation directly to baseband frequencies. The input frequency range is 800 MHz to 2.7 GHz. The outputs can be connected directly to popular A-to-D converters such as the AD9201 and AD9283. The baseband variable gain amplifier outputs are brought off- chip for filtering before final amplification. By inserting a channel selection filter before each output amplifier, high level out-of-channel interferers are eliminated. Additional internal circuitry also allows the user to set the dc common-mode level at the baseband outputs. The RF input signal goes through two stages of variable gain amplifiers prior to two Gilbert-cell mixers. The LO quadrature phase splitter employs polyphase filters to achieve high quadrature accuracy and amplitude balance over the entire operating frequency range. Separate I and Q channel variable gain amplifiers follow the baseband outputs of the mixers. The RF and baseband amplifiers together provide 69.5 dB of gain control. A precision control circuit sets the linear-in-dB RF gain response to the gain control voltage.