Zu-yao Chang

Front-end receiver electronics for a matrix transducer for 3-D transesophageal echocardiography

There is a clear clinical need for creating 3-D images of the heart. One promising technique is the use of transesophageal echocardiography (TEE). To enable 3-D TEE, we are developing a miniature ultrasound probe containing a matrix piezoelectric transducer with more than 2000 elements. Because a gastroscopic tube cannot accommodate the cables needed to connect all transducer elements directly to an imaging system, a major challenge is to locally reduce the number of channels, while maintaining a sufficient signal-to-noise ratio. This can be achieved by using front-end receiver electronics bonded to the transducers to provide appropriate signal conditioning in the tip of the probe. This paper presents the design of such electronics, realizing time-gain compensation (TGC) and micro-beamforming using simple, low-power circuits. Prototypes of TGC amplifiers and micro-beamforming cells have been fabricated in 0.35-μm CMOS technology. These prototype chips have been combined on a printed circuit board (PCB) to form an ultrasound-receiver system capable of reading and combining the signals of three transducer elements. Experimental results show that this design is a suitable candidate for 3-D TEE.

A Comparison of Two- and Four-Electrode Techniques to Characterize Blood Impedance for the Frequency Range of 100 Hz to 100 MHz

Measurement setups that characterize the impedance of suspensions of blood over the wide frequency range of 100 Hz to 100 MHz are presented in this paper. The performance of the two- and four-electrode techniques have been compared and evaluated. By applying a combination of the two measurement techniques the best result is achieved when taking into account the main nonidealities, such as electrode polarization impedance and parasitic capacitances. It has been found that the conventional three-element model for the impedance of blood can be used for frequencies up to 1 MHz. For frequencies exceeding 1 MHz, an extended model is introduced where a constant phase angle element is used for modeling the cell membrane and a capacitor Cliq is added for modeling the electrical capacitance of water in blood.

A Prototype PZT Matrix Transducer With Low-Power Integrated Receive ASIC for 3-D Transesophageal Echocardiography

This paper presents the design, fabrication, and experimental evaluation of a prototype lead zirconium titanate (PZT) matrix transducer with an integrated receive ASIC, as a proof of concept for a miniature three-dimensional (3-D) transesophageal echocardiography (TEE) probe. It consists of an array of 9 × 12 piezoelectric elements mounted on the ASIC via an integration scheme that involves direct electrical connections between a bond-pad array on the ASIC and the transducer elements. The ASIC addresses the critical challenge of reducing cable count, and includes front-end amplifiers with adjustable gains and microbeamformer circuits that locally process and combine echo signals received by the elements of each 3 × 3 subarray. Thus, an order-of-magnitude reduction in the number of receive channels is achieved. Dedicated circuit techniques are employed to meet the strict space and power constraints of TEE probes. The ASIC has been fabricated in a standard 0.18-μm CMOS process and consumes only 0.44 mW/channel. The prototype has been acoustically characterized in a water tank. The ASIC allows the array to be presteered across ±37° while achieving an overall dynamic range of 77 dB. Both the measured characteristics of the individual transducer elements and the performance of the ASIC are in good agreement with expectations, demonstrating the effectiveness of the proposed techniques.

Extending the Limits of a Capacitive Soil-Water-Content Measurement

The capacitive component of soil impedance is a good measure for the water content. For long time people use rod-shaped pairs of electrodes to measure electrically the water content in natural- and artificial soil. Especially in high-conductive mediums, with conductivity as high as 15 mS/cm, this technique is not able to acquire the capacitive component of the impedance reliably. Because of the necessary usage of high measurement frequencies, physical effects, such as skin effect limit the applicability of long rod-shaped electrode pairs. Having analyzed this problem a new electrodes structure has been built and tested to reduce this unwanted physical effect thus enabling reliably water-content measurements

27.5 A 30ppm <80nJ ring-down-based readout circuit for resonant sensors

Resonant sensors are a promising candidates for energy-constrained applications. For instance, the resonance frequency shift of polymer-coated MEMS resonators has been used to realize electronic nose systems for personalized health and environmental sensing [1]. Oscillator-based readout circuits for such sensors have been successfully implemented [2,3], but are relatively power-hungry, difficult to design in the presence of parasitic capacitance, and only provide information about the resonance frequency, fres, while the quality factor, Q, often includes additional valuable information.

A mixed-signal multiplexing system for cable-count reduction in ultrasound probes

This paper presents an approach to time-multiplexing multiple receive signals in a miniature ultrasound probe onto a single micro-coaxial cable. The resulting reduction in the number of receive cables alleviates the design of high-element-count endoscope- or catheter-based ultrasound probes. A prototype multiplexing system is presented that employs a custom multiplexing chip that uses current-mode drivers to combine four receive channels, sampled at 25 MHz each, on a single 3-m micro-coaxial cable. On the system-side of the cable, a transimpedance amplifier turns the multiplexed signal back into a voltage, after which it is digitized and equalized to correct for channel-to-channel crosstalk due to non-idealities of the cable. The chip has been implemented in a 0.18 μm CMOS process and consumes less than 1 mW per input channel. Experimental results show that the system can successfully convey 6 MHz Gaussian-shaped pulses applied to the four input channels of the multiplexing chip to the system with a channel-to-channel crosstalk below -31 dB.

A compact 0.135-mW/channel LNA array for piezoelectric ultrasound transducers.

This paper presents a power- and area-efficient 9-channel LNA array for piezoelectric ultrasound transducers to enable real-time 3D imaging with miniature endoscopic and catheter-based probes. In view of the relatively low impedance of piezoelectric transducers, the LNA is implemented as a capacitive feedback voltage amplifier, rather than a trans-impedance amplifier, to achieve a better noise-power trade-off. The use of a current-efficient inverter-based OTA with optimized bias scheme and dual-rail regulation further improves the power efficiency of the LNA while keeping the area compact: 0.01 mm2 per channel. Electrical and acoustic measurement results show that the proposed LNA achieves a 0.6 mPa/√Hz input-referred noise at 4 MHz while consuming only 0.135 mW, which represents a noise efficiency 2.5 × better than the state-of-the-art.

A 30 ppm < 80 nJ Ring-Down-Based Readout Circuit for Resonant Sensors

This paper presents an energy-efficient readout circuit for micro-machined resonant sensors. It operates by briefly exciting the sensor at a frequency close to its resonance frequency, after which resonance frequency and quality factor are determined from a single ring-down transient. The circuit employs an inverter-based trans-impedance amplifier to sense the ring-down current, with a programmable feedback network to enable the readout of different resonant sensors. An inverter-based comparator with dynamically-adjusted threshold levels tracks the ring-down envelope to measure quality factor, and detects zero crossings to measure resonance frequency. The excitation frequency is dynamically adjusted to accommodate large resonance frequency shifts. Experimental results obtained with a prototype fabricated in 0.35 μm standard CMOS technology and three different SiN resonators are in good agreement with conventional impedance analysis. The prototype achieves a frequency resolution better than 30 ppm while consuming less than 80 nJ/meas from a 1.8 V supply, which is 7.8x less than the state-of-the-art.

Low-power receive electronics for a miniature real-time 3D ultrasound probe

Transesophageal echocardiography (TEE) involves the use of an ultrasound transducer mounted at the tip of an endoscope to make ultrasonic images of the human heart from the esophagus. In conventional TEE probes, each transducer element is wired-out by a micro-coaxial cable to an external imaging system. To obtain real-time three-dimensional images, however, a two-dimensional transducer array with more than 1000 elements is required. Direct wiring of so many elements through an endoscope is not feasible, so channel reduction should be performed locally. In this paper, we present an application-specific integrated circuit (ASIC) that includes low-noise amplifiers, programmable-gain amplifiers and micro-beamformer circuits that locally process and combine the signals received by sub-groups of the transducer array. Thus, an order-of-magnitude channel reduction is achieved. The acoustic characterization of the prototype ASIC with a co-integrated transducer array will be presented.

An eddy-current displacement-to-digital converter based on a ratio-metric delta-sigma ADC

This paper describes a smart Eddy-current displacement sensor for use in precision industrial applications. A novel readout scheme based on ratio-metric delta-sigma analog-to-digital conversion is proposed. The system employs two sensing coils incorporated in a low-power front-end oscillator. This produces two anti-phase outputs whose amplitudes are proportional to the inductances of the coils, and are thus a differential function of displacement. After synchronous down-conversion, these signals are fed into a second-order continuous-time delta-sigma modulator that directly produces a digital output that is a ratio-metric function of the coil inductances. This approach eliminates the need for a stable voltage reference, suppresses the oscillators multiplicative noise contributions, and effectively filters the ripple associated with the down-conversion. The sensors are excited at 15 MHz, which reduces the eddy-current penetration depth to only a few tens of μm. The interface has been realized in a 0.35 μm BiCMOS technology and consumes 18 mW from a 3.3 V supply. In a measurement time of 1 ms, it digitizes the inductance ratio with a resolution of 15 bits, and thus achieves a displacement resolution of 135 nm on a range of 3 mm.