Ultrasound systems are among the most advanced signal processing tools currently in widespread use. As with any complex device, there are numerous trade-offs to consider due to performance, physical constraints, and cost limitations. Understanding system-level concepts is essential for fully grasping the functionality and performance of key front-end ICs such as low noise amplifiers (LNAs), time gain compensation amplifiers (TGCs), and analog-to-digital converters (ADCs). These components play a critical role in defining the overall performance of the system. The characteristics of the front-end ICs set the boundaries of what the system can achieve, and once noise or distortion is introduced, it is nearly impossible to eliminate.
In all ultrasound systems, a multi-sensor array is connected via a long cable (typically around 2 meters) containing between 48 to 256 micro-coaxial cores. Some arrays use high-voltage multiplexers or demultiplexers to simplify transmit and receive hardware, but this comes at the cost of reduced flexibility. The most flexible systems employ phased array digital beamformers, which allow full electronic control of all channels—though they tend to be more expensive due to the complexity involved. However, modern front-end ICs like the AD8332 dual VGA, AD8335 quad VGA, and AD9229 12-bit quad ADC are helping reduce per-channel costs and power consumption, making full electronic control more accessible even in lower-cost systems.
At the transmit side, the beamformer determines the delay mode to focus the transmit signal, then amplifies the output using a high-voltage transmitter amplifier that drives the sensor. On the receive side, a T/R switch isolates high-voltage transmit pulses, typically using a diode rectifier bridge, followed by an LNA and one or more VGAs.
**Figure 1: Basic block diagram of the DBF system**
After amplification, either analog beamforming (ABF) or digital beamforming (DBF) is performed. In addition to continuous wave (CW) Doppler processing, the large dynamic range of received signals makes ABF less practical for many applications, leading most systems to adopt DBF. Finally, the received beam is processed to produce grayscale images, 2D color images, and/or color Doppler outputs.
The primary goal of an ultrasound system is to provide accurate internal organ imaging and to assess blood flow through Doppler signal processing. This requires overcoming several technical challenges, including signal attenuation, power consumption, and managing a wide dynamic range. Choosing the right front-end ICs plays a crucial role in addressing these challenges effectively.
**1. Signal Attenuation Problem**
Medical ultrasound operates across a frequency range from 1 MHz to 40 MHz. External imaging typically uses frequencies between 1 MHz and 15 MHz, while venous imaging may go up to 40 MHz. For a given penetration depth, tissue absorption causes signal loss, with an approximate attenuation of 1 dB/cm/MHz. For example, a 10 MHz signal traveling 5 cm into tissue would experience about 100 dB of round-trip attenuation.
This creates a major challenge for systems handling large dynamic range signals. The receiver must have both low noise and high signal handling capability, and fast overload recovery is also essential. Even the T/R switch must prevent large pulses from reaching the receiver, as even small leakage can cause saturation. Poor overload recovery can leave the receiver in a "blind" state until it recovers, directly affecting image quality near the skin surface.
**2. ABF and DBF Systems**
In analog ABF and digital DBF systems, the signals from each channel are delayed or stored based on the desired focal point, then ordered in time and coherently summed. This process provides spatial processing gain because the signal is correlated while the noise is not, resulting in a theoretical gain of 10 log(N), where N is the number of channels.
Images can be formed in two ways: either by simulating delay lines with analog values, summing them, and converting to digital after summation (ABF), or by digitally sampling and storing the analog values from each element before performing digital summation (DBF). The DBF system replaces variable delay lines with ADCs and FIFOs, offering greater flexibility. Both systems require excellent channel-to-channel matching, and while both need VGAs, ABF systems only require high-resolution, relatively slow ADCs, whereas DBF systems demand multiple high-speed, high-resolution ADCs to sample RF bandpass signals.
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