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The AD8322 is a low-cost, digitally controlled variable gain amplifieropTImized for coaxial line driving applicaTIons such as cablemodems that are designed to the MCNS-DOCSIS upstreamstandard. An 8-bit serial word determines the desired outputgain over a 42.14 dB range, with gain steps of 6.02 dB/majorcarry.The AD8322 comprises a digitally controlled variable attenuatorof 0 dB to –42.14 dB, which is preceded by a low-noise, fixed-gainbuffer and is followed by a low-distorTIon, high-power amplifier.The AD8322 accepts a differenTIal or single-ended inputsignal. The output is specified for driving a 75 Ω load, such ascoaxial cable.Distortion performance of –58 dBc is achieved with an output levelup to 60 dBmV at 21 MHz bandwidth. A key performance andcost advantage of the AD8322 results from the ability to maintaina constant 75 Ω output impedance during power-up and powerdownconditions. This eliminates the need for external 75 Ωtermination resulting in twice the effective output voltage whencompared to a standard operational amplifier.The AD8322 is packaged in a low-cost 28-lead TSSOP, operatesfrom a single 5 V supply, and has an operational temperaturerange of –40°C to +85°C.
