The AD604 is a low-noise, high-precision, dual-channel variable gain amplifier developed by Analog Devices. One of its key advantages is that the gain in decibels (dB) is directly proportional to the control voltage, making it particularly suitable for time gain compensation (TGC) circuits in ultrasonic instruments. This article provides an overview of the AD604's features, internal structure, and typical applications, with a focus on how it can be used for ultrasonic signal attenuation compensation. Compared to similar devices, the AD604 offers ultra-low noise, high accuracy, and continuously adjustable gain. In medical ultrasound systems, TGC circuits require the gain to increase exponentially with the control voltage, meaning the dB value should be linear with respect to the control voltage. The AD604 perfectly meets this requirement, effectively reducing the dynamic range of signals sent to the ADC, which improves overall system performance. Key features of the AD604 include: - Ultra-low input noise, with both voltage and current noise minimized at maximum gain. - A bandwidth of 40 MHz (-3dB). - Two independent gain channels, each with a dB value proportional to the control voltage. - Programmable gain adjustment: when the preamplifier is set to 14 dB, the gain can vary from 0 to +48 dB; when set to 20 dB, the range is +6 to +54 dB. - Input resistance of 300 kΩ. - Gain range of 20–40 dB/V. - High stability across temperature and supply voltage variations. - Single-ended unipolar gain control. - Automatic power-off when the gain drops below a certain threshold. - Direct drive capability for ADCs. Pin Function Description The AD604 comes in a 24-pin package, available in DIP, SSOP, and SOIC configurations. The pinout is shown in Figure 1. Here’s a breakdown of each pin’s function: - **PAI1/PAI2**: Preamplifier positive input. - **PAO1/PAO2**: Preamplifier output. - **FBK1/FBK2**: Preamplifier feedback terminal. - **COM1/COM2**: Signal ground. When connected to the positive supply, the preamplifier channel is disabled. - **-DSX1/-DSX2**: Differential attenuator signal input negative terminal. - **+DSX1/+DSX2**: Differential attenuator signal input positive terminal. - **VGN1/VGN2**: Gain control input and power-down terminal. Grounding these turns off the channel; otherwise, increasing the positive voltage increases the gain. - **VREF**: Gain control reference for both channels. At +2.5 V, the gain slope is 20 dB/V; at +1.67 V, it is 30 dB/V. - **VOCM**: Common-mode voltage control for the output signal, used to set the DC midpoint of the signal. - **OUT1/OUT2**: Signal output terminals. - **VPOS/VNEG**: Positive/negative power supply connections. - **GND1/GND2**: Ground pins. Internal Structure and Working Principle The AD604 is a dual-channel variable gain amplifier. Each channel includes a low-noise preamplifier and a variable gain amplifier (XAMP). The XAMP consists of a precision-controlled differential attenuator, a gain control unit, a fixed-gain feedback amplifier, and a VOCM common-mode voltage control circuit using resistors R3 and R4. This design allows each channel to provide a variable gain range of up to 48 dB. Preamplifier Each channel of the AD604 includes a high-performance preamplifier that can be adjusted between +14 dB and +20 dB via an external resistor in the feedback loop. The internal circuit of the preamplifier is shown in Figure 3, where R5, R6, and R7 are the gain-setting resistors. The actual gain is determined by the resistance between FBK1 and PAO1.

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