TI’s low-noise analog front ends (AFEs) have a time-gaincontrol (TGC) feature that supports ultrasound applications because they can alter the gain of the receiver as a function of time. The ultrasound signal incident on the receiver decreases in amplitude as a function of the time elapsed since transmission, and the TGC helps achieve the best possible signal-to-noise ratio (SNR), even with the decreasing signal amplitude. This article describes the specifications and design considerations of a circuit used for generating a time-varying VCNTL to drive multiple AFE receiver chips in a parallel configuration.
The figure shows the signal chain of the AFE5818 analog front end. The TGC is implemented using an attenuator that can be controlled with a control voltage, VCNTL. External circuitry comprised of a digital-to-analog converter (DAC) and operational amplifier (op amp) generates the control signal. The input signal for the DAC is a time-varying digital control from a field-programmable gate array (FPGA) that could also handle the beam forming operation required in an ultrasound application.