AD603ARZ-REEL7

The AD603ARZ-REEL7 from Analog Devices is a low-noise, voltage-controlled variable gain amplifier (VGA) optimized for applications requiring precise gain control over a wide frequency range. The device provides a gain range of 0dB to +40dB (AD603A version) with a linear-in-dB response controlled by a differential voltage on the GNEG and GPOS pins. The gain scaling factor is 25mV/dB (equivalently, 40dB per volt of control voltage), enabling precise gain programming from a DAC or other voltage source. A key feature of the AD603 is its constant 90MHz bandwidth across the entire gain range, unlike traditional VGA circuits where bandwidth varies with gain. This is achieved through ADI’s proprietary X-AMP (eXponential AMPlifier) architecture, which uses a fixed-gain amplifier preceded by a variable attenuator based on a patented interpolating DAC. The ultra-low 1.3nV/rtHz input-referred noise at maximum gain makes the AD603 suitable for receiving weak signals in ultrasound, radar, and communications systems. The gain accuracy of +/-0.5dB (typical) ensures repeatable performance across devices and temperature. The differential gain control interface provides 30dB of common-mode rejection on the control lines, rejecting noise and offset. Two AD603 devices can be cascaded in series (output of one feeding the input of the next) to achieve 0-80dB gain range, or in parallel for improved noise performance.

The AD603ARZ-REEL7 operates using ADI’s proprietary X-AMP architecture. Key subsystems include: (1) X-AMP Architecture – the signal path consists of a passive 42dB R-2R ladder attenuator followed by a fixed-gain amplifier; the attenuator has multiple tap points; an interpolating circuit smoothly transitions between adjacent taps, controlled by the differential voltage on GPOS/GNEG; this provides continuous gain adjustment with a linear-in-dB response; (2) Gain Control – the gain is determined by the differential voltage VCTL = VGPOS – VGNEG; the gain in dB = 40 x VCTL + gain_offset; for the AD603A, the gain range is 0-40dB corresponding to VCTL of 0V to 1V; the 25mV/dB scaling means that a 1V change in control voltage produces a 40dB gain change; (3) Constant Bandwidth – because the variable attenuation occurs before the fixed-gain amplifier, the bandwidth is determined solely by the fixed-gain amplifier characteristics; the attenuator is purely resistive and has no bandwidth limitation; this results in a constant 90MHz -3dB bandwidth regardless of the gain setting; (4) Low Noise – the input-referred noise voltage density of 1.3nV/rtHz at maximum gain is set by the thermal noise of the attenuator ladder resistance at the lowest attenuation setting; as gain decreases, the attenuator increases the source resistance seen by the amplifier, increasing the input-referred noise; (5) Cascading – two AD603 devices can be cascaded by connecting the output of the first stage to the input of the second; in this configuration, the gain ranges add (0+0=0dB minimum, 40+40=80dB maximum), and the total gain control range is 80dB.

• Ultrasound TGC: time-gain compensation sweeps 0-40dB during echo reception to equalize near/far signal amplitudes
• RF AGC loop: AD603 as IF VGA with detector feedback for constant-output automatic gain control in 10.7MHz IF stage
• Radar receiver: cascaded 2x AD603 for 80dB dynamic range in X-band downconverted signal chain
• Sonar processing: 1.3nV/rtHz noise allows detection of weak acoustic returns at long range