GENERAL DESCRIPTION
The AD604 is an ultralow noise, very accurate, dual-channel, linear-in-dB variable gain amplifier (VGA) optimized for time based variable gain control in ultrasound applications; however, it supports any application requiring low noise, wide bandwidth, variable gain control. Each channel of the AD604 provides a 300 kΩ input resistance and unipolar gain control for ease of use. User-determined gain ranges, gain scaling (dB/V), and dc level shifting of output further optimize performance.
Each channel of the AD604 uses a high performance preamplifier that provides an input-referred noise voltage of 0.8 nV/√Hz. The very accurate linear-in-dB response of the AD604 is achieved with the differential input exponential amplifier (DSX-AMP) architecture. Each of the DSX-AMPs comprises a variable attenuator of 0 dB to 48.36 dB followed by a high speed fixed gain amplifier. The attenuator is a 7-stage R-1.5R ladder network. The attenuation between tap points is 6.908 dB and 48.36 dB for the ladder network.

FEATURES
  Ultralow input noise at maximum gain
     0.80 nV/√Hz, 3.0 pA/√Hz
  2 independent linear-in-dB channels
  Absolute gain range per channel programmable
     0 dB to 48 dB (preamplifier gain = 14 dB) through 6 dB to
        54 dB (preamplifier gain = 20 dB)
  ±1.0 dB gain accuracy
  Bandwidth: 40 MHz (−3 dB)
  Input resistance: 300 kΩ
  Variable gain scaling: 20 dB/V through 40 dB/V
  Stable gain with temperature and supply variations
  Single-ended unipolar gain control
  Power shutdown at lower end of gain control
  Drive ADCs directly

APPLICATIONS
  Ultrasound and sonar time gain controls
  High performance AGC systems
  Signal measurement