The AD604 is an ultralow noise, very accurate, dual-channel, linear-in-dB variable gain amplifier (VGA) optimized for timebased 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 k 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. The equation for the linear-in-dB gain response is G (dB) =