Beamforming Noise-Shaping SAR ADC for 3-D Ultrasound Imaging
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Abstract
3-D ultrasound imaging using a 2-D array provides much more information about our body than conventional 2-D imaging. However, due to the large number of elements in a 2-D array, channel count reduction is required and the power and area budget for each element is limited. It is quite challenging to design an ultrasound interface circuit with limited power and area budget. In the interface circuit, the ADC usually consumes considerable amount of power and area, which limits the performance of the whole system.
This thesis explores a new ADC architecture: a beamforming noise-shaping SAR ADC (BENSSAR), in an attempt to reduce power and area of the ultrasound receiver and thus improve the performance of 3-D ultrasound imaging. BENSSAR mainly consists of three parts: a subarray beamformer, a charge-sharing SAR ADC and a loop filter. They all operate in the charge domain, which makes the communication between each other efficient and thus reduces power and area.
Previously published works on noise-shaping SAR ADCs are all based on charge-redistribution SAR. In BENSSAR, the noise-shaping function is built based on the charge-sharing SAR. With the help of noise-shaping, the strict requirement on comparator noise, a disadvantage of charge-sharing SAR, is greatly relaxed. Since the error-feedback (EF) structure typically operates in the charge domain, this structure is very suitable for the charge-sharing noise-shaping SAR.
An active charge amplifier is implemented in the EF loop filter to achieve sufficient SNR improvement. The charge amplifier is realized by a current conveyor. We find out that the noise of the basic current conveyor is inherently limited but can be reduced by adding auxiliary amplifiers.
The BENSSAR has been designed in 0.18μm CMOS technology and has a subarray size of 9. The sampling rate is 30MHz. The BENSSAR consumes 1.31mW from a 1.8V supply and occupies an estimated area of 0.096mm2. In a bandwidth ranging from 3.75MHz to 6.25MHz, the BENSSAR achieves a SNR of 63.25dB, an ENOB of 10.21 bits and a Schreier Figure of Merit (FoM) of 156.1dB.