Ultrasound imaging is one of the most important medical imaging methods due to its safety, cost effectiveness, and real-time capability. Conventional ultrasonic imaging systems use frequencies from 2–20 MHz with a resolution of millimeter level. They are widely used in monitoring fetuses, as well as diagnosing diseases of internal organs such as the heart, liver, gallbladder, spleen, pancreas, kidneys, bladder, etc. Current ultrasound systems can visualize both, internal organs with B-mode imaging and blood flow with ultrasound Doppler. A block diagram of a typical ultrasound system is illustrated in Figure 1.

Designing an ultrasound system is as complex as designing every integrated circuit (IC) for this system. The performance of a complex system can be influenced significantly by its analog circuitry. Therefore, every feature in the analog front end (AFE), highlighted in Figure 1, is critical for any ultrasound system design. 

Figure 1. Ultrasound block diagram.

Before any IC design can be initiated, process selection is a key consideration for semiconductor manufacturers. The selection process must balance performance, power consumption, cost, and upgrade feasibility. Complementary metal-oxide-semiconductors (CMOS) and bipolar processes are the most popular for an ultrasound AFE design. Recently, bipolar-CMOS (BiCMOS) process has become more popular than a pure bipolar process since it contains both high-performance bipolar transistors for analog design and CMOS components for digital design.

Bipolar transistors are suitable for amplifier design, considering its ultra-low 1/f noise, wide bandwidth and good power/noise efficiency. The bipolar process also reduces the capacitance of the circuit for obtaining good total harmonic distortion. Thus, amplifiers based on a bipolar or BiCMOS process can achieve the same performance in a much smaller area and lower power than an amplifier based on a CMOS process. Figure 2 shows that a bipolar transistor-based amplifier achieves much lower noise under the same bias current. It also illustrates that the bipolar transistor has ultra-low 1/f noise characteristics, which is critical for ultrasound Doppler with modulation and demodulation circuits. 

Figure 2. CMOS versus Bipolar amplifiers.

When a circuit has more digital content and switching components such as mid-speed analog-to-digital converters (ADCs), a CMOS process is more suitable. Since medical ultrasound signal frequency is in a range of 1~20MHz and its ADC sampling rate is usually below 100 MSPS, most current CMOS processes can handle this easily. With a 0.18 um CMOS process, better integration and power reduction can be achieved in an ADC design. Additionally, a CMOS process usually costs less and realizes a shorter fabrication cycle than a comparable BiCMOS process. All of these indicate that a CMOS process is suitable for ADC design in the ultrasound AFE [1, 2 and 3].

Consequently, the combination of a BiCMOS amplifier and a CMOS ADC can realize a great AFE with <0.8nV/rtHz noise and <150mW/CH power. This combination requires advanced silicon processes as well as a state-of-the-art package technology. Figure 3 shows an AFE solution with two dies in the same package. If needed, more dies or even passive components can be integrated in the same package. 

Figure 3. Multi-chip-module package.

In summary, ultrasound system designers can benefit significantly from advanced semiconductor technologies, realizing lower power and higher imaging quality in a compact size to accelerate development of ultrasound-specific analog ICs.

[1] Rick Jordanger, A Comparison of LinBiCMOS and CMOS Process Technology in LVDS Integrated Circuits, Application Report, SLLA065, Texas Instruments, 2000.
[2] Harish Venkataraman, et al., Medical Imaging receive Chain: CMOS or Bipolar?, Texas Instrument Developer Conference, 2008, Dallas, Texas.
[3] Adam H. Pawlikiewicz and David Hess, Choosing RF CMOS or SiGe BiCMOS in mixed-signal design,, 2006