The AD8325 is a low-cost, digitally controlled, variable gain amplifier optimized for coaxial line driving applications such as cable modems designed to the MCNS-DOCSIS upstream standard. An 8-bit serial word determines the desired output gain over a 59.45 dB range, resulting in gain changes of 0.7526 dB/LSB. The AD8325 comprises a digitally controlled variable attenuator of 0 dB to -59.45 dB, which is preceded by a low noise, fixed gain buffer and is followed by a low distortion high power amplifier. The AD8325 accepts a differential or single-ended input and has a 75 ohm output impedance. The AD8325 supports SPI interfaces for controlling gain, output enable and power consumption.
The AD8325 is a low‑cost, digitally controlled, variable‑gain amplifier optimized for coaxial line‑driving applications such as DOCSIS upstream cable modems. It provides a programmable gain range of about 59 dB in 0.7526 dB steps and maintains a constant 75 Ω output impedance.
The document includes full electrical specifications, pin‑out diagrams, absolute maximum ratings, timing requirements for the SPI interface, and recommended PCB layout and grounding practices. It also describes the evaluation board and software used for testing and programming the device.
| Gain Range | 58.45 – 60.45 dB |
|---|---|
| Gain Scaling Factor | 0.7526 dB/LSB |
| Single‑Ended Input Resistance | 800 Ω |
| Differential Input Resistance | 1600 Ω |
| Input Capacitance | 2 pF |
| Supply Voltage Operating Range | 4.75 – 5.25 V |
| Quiescent Current (TXEN = 1) | 123 – 140 mA |
| Output Noise Level | –48 dBmV in 160 kHz |
| Output Impedance | 75 Ω |
The AD8325 provides a programmable gain from about –29.45 dB to +30 dB, covering roughly 59.45 dB.
Gain is set by sending an 8‑bit serial word over an SPI interface using the DATEN, SDATA, and CLK pins.
The device operates from a 5 V supply with an acceptable range of 4.75 V to 5.25 V.
In transmit‑enable mode the quiescent current is approximately 123 mA to 140 mA.