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).

An Adaptive Element-Level Impedance-Matched ASIC With Improved Acoustic Reflectivity for Medical Ultrasound Imaging.

This paper presents an active impedance matching scheme that tries to optimize electrical power transfer and acoustic reflectivity in ultrasound transducers. Leveraging negative capacitance-based impedance matching would potentially improve the bandwidth and electrical power transfer while minimizing acoustic reflection of transducer elements and improve uniformity while reducing acoustic crosstalk of transducer arrays. A 16-element transceiver front-end is designed which employs an element-level active capacitive impedance cancellation scheme using an element-level negative impedance converter. The ASIC fabricated in 180-nm HVBCD technology provides high-voltage pulses up to 60 V consuming 3.6 mW and occupying 2.5 mm2. The front-end ASIC is used with a 1-D capacitive micromachined ultrasonic transducer (CMUT) array and its acoustical reflectivity reduction and imaging capabilities have successfully been demonstrated through pulse-echo measurements and acoustic imaging experiments.

Analysis of Negative Capacitance-Based Broadband Impedance Matching for CMUTs.

Tight integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with integrated circuits can make active impedance matching feasible for practical imaging devices. In this article, negative capacitance-based impedance matching for CMUTs is investigated. Simple equivalent circuit model-based calculations show the potential of negative capacitance matching for improving the bandwidth along with electrical power transfer and acoustic reflectivity, but the model has limitations especially for acoustic reflectivity evaluation. For more realistic results, an experimentally validated CMUT array model is applied to a small 1-D CMUT array operating in the 5-15 MHz range. The results highlight the difference between electrical power transfer and acoustic reflectivity as well as the tradeoffs in signal-to-noise ratio (SNR). According to the results, ideal negative capacitance termination matched to the CMUT capacitance provides the broadest bandwidth and highest SNR if acoustic or electrical reflections are of no concern. On the other hand, negative capacitance and resistance matching to minimize acoustic reflectivity provides both lower reflection and closer to ideal SNR as compared with electrical power matching. It is observed that acoustic matching also reduces acoustic crosstalk and improves array uniformity. While several challenges for integrated circuit implementation are present, negative capacitance-based impedance matching can be a viable broadband active impedance matching method for CMUTs operating in conventional and collapsed mode as well as other ultrasound transducers with mainly capacitive impedance.

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.

Highly Integrated Guidewire Ultrasound Imaging System-on-a-Chip.

In this article, we present a highly integrated guidewire ultrasound (US) imaging system-on-a-chip (GUISoC) for vascular imaging. The SoC consists of a 16-channel US transmitter (Tx) and receiver (Rx) electronics, on-chip power management IC (PMIC), and quadrature sampler. Using a synthetic aperture imaging algorithm, a Tx/Rx pair, connected to capacitive micromachined ultrasound transducers (CMUTs), can be activated at any time. The Tx generates acoustic waves by driving the CMUT, while the Rx picks up the echo signal and amplify it to be delivered through an interconnect that is driven by a buffer. On-chip logic controls the pulsers that generate the high-voltage (HV)-pulse for Tx. An on-chip PMIC provides 1.8-, 5-, 39-, and 44-V supplies and a clock signal from the two interconnects besides GND. A quadrature sampler down-converts the Rx echo signal to baseband, reducing its bandwidth requirement for the output interconnect. The system design, including transimpedance amplifier (TIA) optimization, based on the equivalent circuit of a specific CMUT is presented. The SoC was fabricated by a 0.18-µm HV CMOS process, occupying 1.5-mm2 active area and consuming 25.2 and 44 mW from 1.8 to 44 V supplies, respectively. The US Tx and Rx show bandwidths of 32-42 and 32.7-37.5 MHz, respectively. The input-referred noise of the system was measured as 9.66 nA in band with 2-m-long 52 American Wire Gauge (AWG) wire interconnects. The functionality of the GUISoC was verified in vitro by imaging wire targets.

An Impulse Radio PWM-Based Wireless Data Acquisition Sensor Interface.

A sensor interface circuit based on impulse radio pulse width modulation (IR-PWM) is presented for low power and high throughput wireless data acquisition systems (wDAQ) with extreme size and power constraints. Two triple-slope analog-to-time converters (ATC) convert two analog signals, each up to 5 MHz in bandwidth, into PWM signals, and an impulse radio (IR) transmitted (Tx) with an all-digital power amplifier (PA) combines them while preserving the timing information by transmitting impulses at the PWM rising and falling edges. On the receiver (Rx) side, an RF-LNA followed by an envelope detector recovers the incoming impulses, and a T-flipflop reverts the impulse sequence back to PWM to be digitized by a time-to-digital converter (TDC). Detailed analysis and design guideline on ATC was introduced, and a proof-of-concept prototype was fabricated for a capacitive micromachined ultrasound transducer (CMUT) imaging system in a 0.18-µm HV CMOS process, occupying 0.18 mm2 active area and consuming 3.94 mW from a 1.8 V supply. The proposed TDC in this prototype yielded 7-bit resolution, while the entire wDAQ achieved 5.8 effective number of bits (ENOB) at 2 × 10 MS/s.