AD604_08

Dual, Ultralow Noise Variable Gain Amplifier AD604 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 FUNCTIONAL BLOCK DIAGRAM PAO –DSX +DSX VGN GAIN CONTROL AND SCALING VREF DIFFERENTIAL ATTENUATOR R-1.5R LADDER NETWORK 0dB TO –48.4dB PROGRAMMABLE ULTRALOW NOISE PREAMPLIFIER G = 14dB TO 20dB PRECISION PASSIVE INPUT ATTENUATOR PAI AFA OUT VOCM Figure 1. APPLICATIONS Ultrasound and sonar time gain controls High performance AGC systems Signal measurement GENERAL DESCRIPTION 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 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. The equation for the linear-in-dB gain response is G (dB) = (Gain Scaling (dB/V) × VGN (V)) + (Preamp Gain (dB) – 19 dB) Preamplifier gains between 5 and 10 (14 dB and 20 dB) provide overall gain ranges per channel of 0 dB through 48 dB and 6 dB through 54 dB. The two channels of the AD604 can be cascaded to provide greater levels of gain range by bypassing the preamplifier of the second channel. However, in multiple channel systems, cascading the AD604 with other devices in the AD60x VGA family that do not include a preamplifier may provide a more efficient solution. The AD604 provides access to the output of the preamplifier, allowing for external filtering between the preamplifier and the differential attenuator stage. Note that scale factors up to 40 dB/V are achievable with reduced accuracy for scales above 30 dB/V. The gain scales linearly-indB with control voltages of 0.4 V to 2.4 V with the 20 dB/V scale. Below and above this gain control range, the gain begins to deviate from the ideal linear-in-dB control law. The gain control region below 0.1 V is not used for gain control. When the gain control voltage is