AD605AN

Dual, Low Noise, Single-Supply Variable Gain Amplifier AD605 FEATURES 2 independent linear-in-dB channels Input noise at maximum gain: 1.8 nV/√Hz, 2.7 pA/√Hz Bandwidth: 40 MHz (–3 dB) Differential input Absolute gain range programmable –14 dB to +34 dB (FBK shorted to OUT) through 0 dB to 48 dB (FBK open) Variable gain scaling: 20 dB/V through 40 dB/V Stable gain with temperature and supply variations Single-ended unipolar gain control Output common mode independently set Power shutdown at lower end of gain control Single 5 V supply Low power: 90 mW/channel Drives ADCs directly VGN FUNCTIONAL BLOCK DIAGRAM GAIN CONTROL AND SCALING PRECISION PASSIVE INPUT ATTENUATOR FIXED GAIN AMPLIFIER +34.4dB OUT FBK VREF VOCM +IN –IN DIFFERENTIAL ATTENUATOR 0 TO –48.4dB Figure 1. APPLICATIONS Ultrasound and sonar time-gain controls High performance AGC systems Signal measurement GENERAL DESCRIPTION The AD605 is a low noise, accurate, dual channel, linear-in-dB variable gain amplifier, optimized for any application requiring high performance, wide bandwidth variable gain control. Operating from a single 5 V supply, the AD605 provides differential inputs and unipolar gain control for ease of use. Added flexibility is achieved with a user-determined gain range and an external reference input that provides user-determined gain scaling (dB/V). The high performance linear-in-dB response of the AD605 is achieved with the differential input, single-supply, exponential amplifier (DSX-AMP) architecture. Each of the DSX-AMPs comprise a variable attenuator of 0 dB to −48.4 dB followed by a high speed fixed gain amplifier. The attenuator is based on a 7-stage R-1.5R ladder network. The attenuation between tap points is 6.908 dB, and 48.360 dB for the entire ladder network. The DSX-AMP architecture results in 1.8 nV/√Hz input noise spectral density and accepts a ±2.0 V input signal when VOCM is biased at VP/2. Each independent channel of the AD605 provides a gain range of 48 dB that can be optimized for the application. Gain ranges between −14 dB to +34 dB and 0 dB to +48 dB can be selected by a single resistor between Pin FBK and Pin OUT. The lower and upper gain ranges are determined by shorting Pin FBK to Pin OUT, or leaving Pin FBK unconnected, respectively. The two channels of the AD605 can be cascaded to provide 96 dB of very accurate gain range in a monolithic package. The gain control interface provides an input resistance of approximately 2 MΩ and scale factors from 20 dB/V to 30 dB/V for a VREF input voltage of 2.5 V to 1.67 V, respectively. Note that scale factors up to 40 dB/V are achievable with reduced accuracy for scales above 30 dB/V. The gain scales linearly in dB with control voltages (VGN) of 0.4 V to 2.4 V for the 20 dB/V scale and 0.20 V to 1.20 V for the 40 dB/V scale. When VGN is