M. Wasequr Rashid

Reverse Nested Miller Compensation Using Current Buffers in a Three-Stage LDO

A novel frequency compensation scheme called reverse nested Miller compensation using current buffers (RNMCCB) for three-stage amplifiers is proposed. As opposed to previous reverse nested schemes, our work uses inverting gain stages for both the second and third stages. The outer compensation loop utilizes a current mirror as an inverting current buffer (CB), and the inner loop uses a common-gate amplifier as a CB, creating two left-half-plane (LHP) zeros. We introduce a simple and effective method of placing a resistor in series with a CB for accurate placement of LHP zeros. As a design example of the RNMCCB scheme, we propose a three-stage low dropout voltage regulator (LDO) in a 0.5-¿m CMOS process to supply 1.21 V to a load ranging from 1 ¿A to 100 mA. Our design goals were to simultaneously achieve very high current efficiency and very low transient output voltage variation. As such, we achieved a 99.95% current efficiency and a maximum load transient output voltage variation of ±48 mV with an output capacitor of 100 nF. Experimental results, in good agreement with theoretical analysis, validate the novel RNMCCB frequency compensation scheme.

Single Miller compensation using inverting current buffer for multi-stage amplifiers

A novel single Miller frequency compensation topology utilizing a current mirror as an inverting current buffer (SMCICB) for multi-stage amplifiers is proposed. An effective method for accurate placement of the Left-Half-Plane (LHP) zero, introduced by the current buffer is detailed. The SMCICB network effectively cancels the pole at output node, resulting in a single-pole system. This topology does not introduce additional transistors and static power dissipation. As a design example, we simulated a four-stage amplifier driving a 25kΩ//125-pF load, achieving 8.2MHz gain-bandwidth product, with 80μA of quiescent current and ±1.5Vpower supplies in a 0.5μm ON Semi 2P3M process CMOS. Simulation results, in good agreement with theoretical analysis, validate the proposed SMCICB scheme.

Time-division multiplexing for cable reduction in ultrasound imaging catheters

In ultrasound imaging catheter applications, gathering the data from multi-element transducer arrays is difficult as there is a restriction on cable count due to the diameter of the catheter. In such applications, CMUT-on-CMOS technology allows for 2D arrays with many elements to be designed and bonded directly onto CMOS circuitry. This allows for complex electronics to be placed at the tip of the catheter which leads to the possibility to include electronic multiplexing techniques to greatly reduce the cable count required for a large element array. Current approaches to cable reduction tend to rely on area and power hungry circuits to function, making them unsuitable for use in catheters. Furthermore the length requirement for catheters and lack of power available to on-chip cable drivers leads to limited signal strength at the receiver end. In this paper an alternative approach using Analogue Time Division Multiplexing (TDM) is presented, which addresses the cable restrictions of the catheter and, using a novel digital demultiplexing technique, allows for a reduction in the number of analogue signal processing stages required.

Direct Digital Demultiplexing of Analog TDM Signals for Cable Reduction in Ultrasound Imaging Catheters

In real-time catheter-based 3-D ultrasound imaging applications, gathering data from the transducer arrays is difficult, as there is a restriction on cable count due to the diameter of the catheter. Although area and power hungry multiplexing circuits integrated at the catheter tip are used in some applications, these are unsuitable for use in small sized catheters for applications, such as intracardiac imaging. Furthermore, the length requirement for catheters and limited power available to on-chip cable drivers leads to limited signal strength at the receiver end. In this paper, an alternative approach using analog time-division multiplexing (TDM) is presented, which addresses the cable restrictions of ultrasound catheters. A novel digital demultiplexing technique is also described, which allows for a reduction in the number of analog signal processing stages required. The TDM and digital demultiplexing schemes are demonstrated for an intracardiac imaging system that would operate in the 4- to 11-MHz range. A TDM integrated circuit (IC) with an 8:1 multiplexer is interfaced with a fast analog-to-digital converter (ADC) through a microcoaxial catheter cable bundle, and processed with a field-programmable gate array register-transfer level simulation. Input signals to the TDM IC are recovered with -40-dB crosstalk between the channels on the same microcoax, showing the feasibility of this system for ultrasound imaging applications.

Capacitive micromachined ultrasonic transducer arrays as tunable acoustic metamaterials.

Capacitive Micromachined Ultrasonic Transducers (CMUTs) operating in immersion support dispersive evanescent waves due to the subwavelength periodic structure of electrostatically actuated membranes in the array. Evanescent wave characteristics also depend on the membrane resonance which is modified by the externally applied bias voltage, offering a mechanism to tune the CMUT array as an acoustic metamaterial. The dispersion and tunability characteristics are examined using a computationally efficient, mutual radiation impedance based approach to model a finite-size array and realistic parameters of variation. The simulations are verified, and tunability is demonstrated by experiments on a linear CMUT array operating in 2-12 MHz range.

Integrated CMOS Sensor Array for Optical Heterodyne Phase Sensing

A heterodyne interferometric CMOS 8 × 8 phase sensor array was developed to measure the spatial phase distribution of an optical wavefront. This sensor is suitable for measuring rapidly changing surface profiles and characterizing fast turbulence. Using an acousto-optic modulation frequency of 80 MHz and beat frequency of 10 kHz, the system calculates 8-bit phase data at each location in the array at a rate equal to the beat frequency. The phase computation is performed locally, digitized, and stored in 8-bit SRAM. Implemented in a 0.5-μm 2P3M CMOS process, the measured RMS phase error is 1.49° (1 LSB) and mismatch has σ = 4.76° (3.4 LSB). Experimental results, in agreement with theory, validate the proposed approach.

Design of frequency-division multiplexing front-end receiver electronics for CMUT-on-CMOS based intracardiac echocardiography

Designing Intracardiac Echocardiography (ICE) imaging catheters using 2D arrays with large number of elements is extremely challenging with standard interconnect and transducer technology. Using microbeamformers inside the catheter reduces the number of cables; however it requires complex front-end electronics which consumes considerable amount of real estate. By using the CMUT-on-CMOS technology, one can incorporate on-chip transmit electronics and efficiently optimize the 2D transducer array to use less number of active elements to reduce the number of cables by combining multiple channels on a single output cable. Reducing the number of cables with integrated electronics may provide a flexible catheter which can be used for 3D ICE imaging under MRI as well as X-Ray. In this study, we explore the design of the CMOS receiver circuit which implements a Frequency Division Multiplexing (FDM) scheme to achieve reduced number of output channels. We discuss design details of an 8×1 multiplexing chip designed in 0.35μm CMOS process. We also present post layout simulation results from the chip showing the viability of the approach.

Investigation of slow evanescent waves at the surface of immersed micromachined membrane arrays

Capacitive micromachined ultrasonic transducer (CMUT) arrays are made up of microscale (10-100μm wide) membranes with embedded electrodes for electrostatic excitation and detection of acoustic waves. The main application of CMUTs has been in medical imaging where advantages of miniaturization and electronics integration are significant. In addition to generating bulk waves in the far-field imaging medium, CMUT arrays also support dispersive evanescent surface waves. These surface waves derive their dispersive properties not only from the periodic structure of the array, but also from the membrane resonance. The CMUTs can tune the surface wave by changing the applied bias voltage to the membranes, which in effect changes the membrane stiffness. This tunability allows the possibility of CMUTs to exploit these slowly propagating evanescent waves as a means for creating subwavelength resolution fields for high-resolution ultrasound imaging and sensing in the near field. The dispersive behavior of these evanescent surface waves propagating along a CMUT array is quantified using a computationally efficient, boundary element method based model capable of adding parameter variation enabling more realistic modeling. The model is validated with experimental data obtained from a 1×16 CMUT array with a membrane resonance tunable between 5 and 6.5MHz. The effect of random variation of the CMUT properties on the surface wave characteristics is investigated. Analysis was done on transient signals from simulations and experiments in addition to using a time-frequency method to track the group velocity that varied from 1500m/s to 400m/s.

Nested Miller compensation using current buffers for multi-stage amplifiers

Firstly, a literature review of existing nested Miller compensation (NMC) schemes for low-voltage multi-stage amplifiers is made. Secondly, a novel nested Miller compensation using two current buffers (NMCCB) in the feedback network is introduced. As opposed to previous NMC schemes, this scheme uses a non-inverting gain stage for the first stage and inverting gain stages for both the second and third stages. An inverting current buffer is utilized for the first time whereas previous NMC feedback networks utilized a positive current buffer. In conventional NMC, the major limitation is bandwidth since inner compensation capacitance loads the output node. In the proposed scheme, the two current buffers in the feedback network create two LHP zeros, cancelling one of the non-dominant poles and extending the bandwidth. Detailed small-signal modeling and analysis of the NMCCB compensation network is presented, demonstrating the effectiveness of this strategy.

Experimental study of dual-ring CMUT array optimization for forward-looking IVUS

Forward-looking (FL) catheters have guiding and volumetric imaging capacities which are highly desirable for IVUS applications. Large channel and firing counts have to be reduced to enable 3-D real-time imaging and simplify front-end electronics. Recently, we have proposed an optimization procedure for dual ring FL arrays which is based on finding an optimal coarray set using the simulated annealing algorithm. The presented algorithm is based on finding a predefined number of optimal firing set which results in elimination of redundant spatial frequencies in the coarray. In this study, we present the experimental demonstration of the proposed method with fabricated single chip CMUT on CMOS system based FL dual ring arrays. The dual ring CMUT arrays were monolithically fabricated on top of CMOS chips which have 25-V pulsers and low-noise transimpedance amplifiers for each transmit and receive array elements. The fabricated CMUT arrays have 56 transmit and 48 receive elements operating at 12 MHz with a 1.4 mm outer diameter. To test the imaging performance of the optimal reduced set, we obtained a 512-element coarray set from the full 2688-element set. In the experiment, we used a phantom of 100-μm aluminium wires immersed in oil tank. We have reconstructed both 2-D PSFs and B-scan images of wire targets. Experimental results demonstrate that the simulated annealing based optimal firing set achieves acceptable lateral and contrast resolution performances with 1/5 of the full set.