Abstract:
Ultrasound imaging has a widespread application in industrial and biomedical applications
and in particular, portable/miniaturized ultrasound imaging systems are gaining popularity
in point-of-care diagnostics. However, miniaturizing an ultrasound imaging system is very
challenging because of the multidimensional tradeoffs inherent in the design. A careful
balance of design tradeoffs, such as power consumption, chip area, bandwidth, and cost is
essential. Since modern scanners rely heavily on digital signal processing, it is preferable
to convert the ultrasound signal to a digital domain as early as possible in the system.
However, this requires prior amplification with analog low-noise amplifiers to meet the
dynamic range, bandwidth, and SNR requirements. Hence, a transistor-level design of
Analog Front End (AFE) circuit with 90 nm CMOS technology is presented. The design is
intended to have wide gain variation, small area, low power, low noise, and a wide dynamic
range. The proposed AFE comprises of a low noise variable gain amplifier followed by an
auxiliary programmable gain amplifier and a low pass filter. A common mode feedback
circuit is designed for each of the fully differential amplifier stages to stabilize the output
common mode voltage under variations in bias currents and voltage. To compensate for
exponential signal attenuation as a function of imaging depth, the design adopts a linear in
dB time-gain compensation circuitry. The AFE circuitry is designed and simulated in
OrCAD Capture CIS Lite by integrating BSIM3 SPICE model for 90 nm CMOS process
to MbreakN and MbreakP transistors from breakout library of PSPICE model. The
simulation results show that in addition to providing 46dB gain, the front-end circuitry is
capable of compensating the exponential signal attenuation as a function of tissue imaging
depth with a tunable gain range of 23dB. The pramplifier consumes 0.6 mW power with a
1.2V supply. The measured input referred noise of the preamplifier is 3.6
𝑜𝑤
√𝐼𝑧
and the
dynamic range is 62 dB, significantly better compared to designs in the literature. The
programmable gain amplifier enables selection of three gain values: 16 dB, 20 dB, and 26
dB. Having a benefit of overall performance enhancement and chip area reduction, the
thesis contributes to the research and development efforts of portable ultrasound imaging
systems.
Key-words: AFE, Miniaturization, CMOS, noise Figure, MbreakN, and MbreakP