24.8 An analog-digital-hybrid single-chip RX beamformer with non-uniform sampling for 2D-CMUT ultrasound imaging to achieve wide dynamic range of delay and small chip area

Ultrasound imaging is widely used for medical diagnosis, because it is harmless to the human body and has real-time processing capability. Usually the focusing (beamforming) operation is performed for both TX and RX. The RX focusing is performed by an RX beamformer [1-5], which consists of delay elements and adders. Nowadays, digital beamformers (DBF) are mostly used for conventional ultrasound imaging because of high SNR. Recently, 2D ultrasound transducers have been introduced for 3D imaging. Since the 2D transducer has a huge number of transducer elements (e.g., 9216 for a 72×128 array), it cannot use DBF because of the huge number of required ADCs and wires inside the probe cable. Therefore, analog beamforming must be performed, at least at the front stage of the 2D transducer.In this work, where a 2D CMUT array is used, the maximum delay difference among transducer elements is 8μs with a maximum steering angle of 45° and a maximum focal depth of 15cm. The target sampling resolution is 6.25ns (λc / 53.3) with a carrier frequency of 3MHz. An analog-digital-hybrid architecture and a non-uniform sampling scheme are used for the RX beamformer of this work to achieve the wide dynamic range of delay time and small chip-area. The RX beamformer consists of 8 analog beamformers (ABF) followed by a single DBF, as shown in Fig. 24.8.1. An ABF performs the focusing operation for the input signals of the adjacent 8 channels to generate an analog output signal. The 8 analog output signals from the 8 ABFs are applied to the DBF. The DBF converts the 8 analog input signals into the 8 digital signals, and then performs the focusing operation on the 8 digital signals to generate a digital output signal for every focal point.