Integrated Hybrid Sub-Aperture Beamforming and Time-Division Multiplexing for Massive Readout in Ultrasound Imaging.

This paper demonstrates hybrid sub-aperture beamforming (SAB) with time-division multiplexing (TDM) for massive interconnect reduction in ultrasound imaging systems. A single-chip front-end system prototype has been fabricated in 180-nm HV BCD technology that combines 5×1 SAB with 8×1 TDM to efficiently reduce the number of receive signal interconnects by a factor of 40. The system includes on-chip high-voltage (HV) pulsers capable of generating unipolar pulses up to 70 V in transmit (TX) mode. The receiver (RX) chain consists of a T/R switch, a variable-gain low-noise amplifier (VG-LNA) with 4-step gain control (15-32 dB) for time-gain compensation followed by a programmable switched-capacitor analog delay-and-sum beamformer. The proof-of-concept prototype operates at a 200-MHz clock frequency and the SAB provides 32-step fine delays with a maximum delay of 310 ns corresponding to better than λ/20 delay quantization at 5 MHz. With these specifications, the SAB is capable of beam steering from 0 ° to 45 ° for a 5-element subarray with 150-micron pitch ( λ/2), providing a near-ideal phased array imaging performance. The sub-aperture beamformer is followed by the TDM system where each of the 8 channels is sampled at a rate of 25 MS/s after an anti-aliasing bandpass filter. The full functionality of the prototype chip is validated through electrical and acoustic measurements on a 1-D capacitive micromachined ultrasonic transducer (CMUT) array designed for intracardiac echocardiography (ICE).

A Power-Efficient Bridge Readout Circuit for Implantable, Wearable, and IoT Applications.

A power-efficient bridge-to-digital sensing interface is proposed, which also offers immunity against power supply noise. The interface utilizes duty-cycling to reduce the static power consumption of resistive bridge sensors, which are commonly used in implantable, wearable, and internet of things (IoT) applications, such as intracranial pressure (ICP) sensing and blood pressure (BP) monitoring. The proposed interface uses a revised version of the pseudo-pseudo differential (PPD) topology with the ping-pong technique to reduce the complexity of traditional fully-differential counterparts. A proof-of-concept prototype has been fabricated in 0.35-µm CMOS and occupies an active area of 0.48 mm2. It achieves 9.13 effective number of bits (ENOB) at 3.72 kHz sampling rate and improvement of more than 50 dB in the power supply rejection ratio (PSRR) by employing the ping-pong technique. It reduces the power consumption of a 5-kΩ Wheatstone bridge by 99.6% compared to a traditional interface, down to 2.53 µw at 1.8 V supply. The functionality of the system has also been demonstrated in an experimental setup in conjunction with an embedded resistive bridge pressure sensor.

Supply-Inverted Bipolar Pulser and Tx/Rx Switch for CMUTs Above the Process Limit for High Pressure Pulse Generation.

A combined supply-inverted bipolar pulser and a Tx/Rx switch is proposed to drive capacitive micromachined ultrasonic transducers (CMUTs). The supply-inverted bipolar pulser adopts a bootstrap circuit combined with stacked transistors, which guarantees high voltage (HV) operation above the process limit without lowering device reliability. This circuit generates an output signal with a peak-to-peak voltage that is almost twice the supply level. It generates a bipolar pulse with only positive supply voltages. The Tx/Rx switch adopts a diode-bridge structure with the protection scheme dedicated to this proposed pulser. A proof- of-concept ASIC prototype has been implemented in 0.18-µm HV CMOS/DMOS technology with 60 V devices. Measurement results show that the proposed pulser can safely generate a bipolar pulse of -34.6 to 45 V, from a single 45 V supply voltage. The Tx/Rx switch blocks the HV bipolar pulse, resulting in less than 1.6 V at the input of the receiver. Acoustic measurements are performed connecting the pulser to CMUTs with 2 pF capacitance and 8 MHz center frequency. The variation of acoustic output pressures for different pulse shapes were simulated with the large signal CMUT model and compared with the experimental results for transmit pressure optimization. A potential implementation of the methods using MEMS fabrication methods is also described.

A Reduced-Wire ICE Catheter ASIC With Tx Beamforming and Rx Time-Division Multiplexing.

This paper presents a single chip reduced-wire active catheter application-specific integrated circuit (ASIC), equipped with programmable transmit (Tx) beamforming and receive (Rx) time-division multiplexing (TDM). The proposed front-end ASIC is designed for driving a 64-channel one-dimensional transducer array in intracardiac echocardiography (ICE) ultrasound catheters. The ASIC is implemented in 60 V 0.18-µm HV-BCD technology, integrating Tx beamformers with high voltage pulsers and Rx front end in the same chip, which occupies 2.6 × 11 mm2 that can fit in the catheter size of 9 F (<3 mm). The proposed system reduces the number of wires from >64 to only 22 by integrating Tx beamformer that is programmable using a single low-voltage differential signaling data line. In Rx mode, the system uses 8:1 TDM with direct digital demultiplexing providing raw channel data that enables dynamic Rx beamforming using individual array elements. This system has been successfully used for B-mode imaging on standard ultrasound phantom with 401 mW of average power consumption. The ASIC has a compact element pitch-matched layout, which is also compatible with capacitive micromachined ultrasound transducer on CMOS application. This system addresses cable number and dimensional restrictions in catheters to enable ICE imaging under magnetic resonance imaging by reducing radio frequency induced heating.

Supply-Doubled Pulse-Shaping High Voltage Pulser for CMUT Arrays.

A supply-doubled pulse-shaping high voltage (HV) pulser is presented for medical ultrasound imaging applications, particularly those that use capacitive micromachined ultrasonic transducers (CMUT). The pulser employs a bootstrap circuit combined with dynamically-biased stacked transistors, which allow HV operation above process limit without lowering device reliability. The new pulser overcomes supply voltage limitation of conventional unipolar pulsers by generating output signals that are almost twice the supply level. It also can generate three-level pulses to further optimize the transmit pressure signals. A proof-of-concept prototype has been implemented in 0.18-µm HV CMOS/DMOS technology with 60 V devices. Measurement results show that the HV pulser can safely generate controllable three-level pulses with up to 85 Vpp from 45 V supply. Acoustic measurements are conducted connecting the pulser to a CMUT with 2 pF capacitance and 8.3 MHz center frequency. The pulse shape has been adjusted for the CMUT under test to generate maximum pressure output and the results are in good agreement with a large signal CMUT model.