The AD8322 is a low-cost, digitally controlled variable gain amplifier
optimized for coaxial line driving applications such as cable
modems that are designed to the MCNS-DOCSIS upstream
standard. An 8-bit serial word determines the desired output
gain over a 42.14 dB range, with gain steps of 6.02 dB/major
carry.
The AD8322 comprises a digitally controlled variable attenuator
of 0 dB to –42.14 dB, which is preceded by a low-noise, fixed-gain
buffer and is followed by a low-distortion, high-power amplifier.
The AD8322 accepts a differential or single-ended input
signal. The output is specified for driving a 75 Ω load, such as
coaxial cable.
Distortion performance of –58 dBc is achieved with an output level
up to 60 dBmV at 21 MHz bandwidth. A key performance and
cost advantage of the AD8322 results from the ability to maintain
a constant 75 Ω output impedance during power-up and powerdown
conditions. This eliminates the need for external 75 Ω
termination resulting in twice the effective output voltage when
compared to a standard operational amplifier.
The AD8322 is packaged in a low-cost 28-lead TSSOP, operates
from a single 5 V supply, and has an operational temperature
range of –40°C to +85°C.