Mohammad Rafiqul Haider

Low-Power Low-Voltage Current Readout Circuit for Inductively Powered Implant System

Low voltage and low power are two key requirements for on-chip realization of wireless power and data telemetry for applications in biomedical sensor instrumentation. Batteryless operation and wireless telemetry facilitate robust, reliable, and longer lifetime of the implant unit. As an ongoing research work, this paper demonstrates a low-power low-voltage sensor readout circuit which could be easily powered up with an inductive link. This paper presents two versions of readout circuits that have been designed and fabricated in bulk complementary metal-oxide semiconductor (CMOS) processes. Either version can detect a sensor current in the range of 0.2 μA to 2 μA and generate square-wave data signal whose frequency is proportional to the sensor current. The first version of the circuit is fabricated in a 0.35-μ m CMOS process and it can generate an amplitude-shift-keying (ASK) signal while consuming 400 μ W of power with a 1.5-V power supply. Measurement results indicate that the ASK chip generates 76 Hz to 500 Hz frequency of a square-wave data signal for the specified sensor current range. The second version of the readout circuit is fabricated in a 0.5-μ m CMOS process and produces a frequency-shift-keying (FSK) signal while consuming 1.675 mW of power with a 2.5-V power supply. The generated data frequency from the FSK chip is 1 kHz and 9 kHz for the lowest and the highest sensor currents, respectively. Measurement results confirm the functionalities of both prototype schemes. The prototype circuit has potential applications in the monitoring of blood glucose level, lactate in the bloodstream, and pH or oxygen in a physiological system/environment.

Flexible capacitive sensors for high resolution pressure measurement

Thin, flexible, robust capacitive pressure sensors have been the subject of research in many fields where axial strain sensing with high spatial resolution and pressure resolution is desirable for small loads, such as tactile robotics and biomechanics. Simple capacitive pressure sensors have been designed and implemented on flexible substrates in general agreement with performance predicted by an analytical model. Two designs are demonstrated for comparison. The first design uses standard flex circuit technology, and the second design uses photolithography techniques to fabricate capacitive sensors with higher spatial and higher pressure resolution. Sensor arrays of varying sensor size and spacing are tested with applied loads from 0 to 1 MPa. Pressure resolution and linearity of the sensors are significantly improved with the miniaturized, custom fabricated sensor array compared to standard flexible circuit technology.

Power-oscillator based high efficiency inductive power-link for transcutaneous power transmission

Transcutaneous power transmission is a critical issue for long term reliable operation of implantable systems. This paper reports a power-oscillator based inductive power link to power up any implantable unit inside the human body. Instead of using power amplifier which requires high drive requirement, two power-oscillator based inductive powering schemes have been presented to achieve high link efficiency. The first scheme utilizes a class-E power oscillator whereas the second scheme uses a differential cross-coupled power oscillator to drive the inductive link. Resonant inductive link has been used to achieve better link efficiency. Simulation results indicate that for a coupling coefficient of 0.45, the class-E power-oscillator based scheme shows a link efficiency of 66% and the differential cross-coupled power-oscillator based scheme shows more than 90% link efficiency. The system has been designed using 0.5-µm standard CMOS process and both of the systems can handle more than 10 mW of power which is adequate for safe operation of biomedical implants.

A low power sensor signal processing circuit for implantable biosensor applications

A low power sensor read-out circuit has been implemented in 0.35 µm CMOS technology that consumes only 400 µW of power and occupies an area of 0.66 mm2. The circuit is capable of converting the current signal from any generic biosensor into an amplitude shift keying (ASK) signal. The on-chip potentiostat biases the chemical sensor electrodes to create the sensor current which is then integrated and buffered to generate a square wave with a frequency proportional to the sensor current level. A programmable frequency divider is incorporated to fix the ASK envelope frequency to be inbetween 20 Hz and 20 kHz, which is within the audible range of human hearing. The entire transmitter block operates with a supply voltage as low as 1.5 V, and it can be easily powered up by an external RF source. Test results emulate the simulation results with good agreement and corroborate the efficacy of the designed system.

An Inductive Link-Based Wireless Power Transfer System for Biomedical Applications

A wireless power transfer system using an inductive link has been demonstrated for implantable sensor applications. The system is composed of two primary blocks: an inductive power transfer unit and a backward data communication unit. The inductive link performs two functions: coupling the required power from a wireless power supply system enabling battery-less, long-term implant operation and providing a backward data transmission path. The backward data communication unit transmits the data to an outside reader using FSK modulation scheme via the inductive link. To demonstrate the operation of the inductive link, a board-level design has been implemented with high link efficiency. Test results from a fabricated sensor system, composed of a hybrid implementation of custom-integrated circuits and board-level discrete components, are presented demonstrating power transmission of 125 mW with a 12.5% power link transmission efficiency. Simultaneous backward data communication involving a digital pulse rate of up to 10 kbps was also observed.

A low-power capacitance measurement circuit with high resolution and high degree of linearity

This paper has presented a low-power capacitance read-out circuit which could be used for biomedical sensor applications. The differential structure of the system eliminates even order distortion. The entire system manifests two current sense amplifiers, two diode rectifiers and one instrumentation amplifier. The circuit has been realized using TSMC 0.35 mum bulk CMOS process. The circuit operates with a 3 V power supply and consumes 5.384 mW of power. Simulation results show that the circuit has a sensitivity of 1.32 mV for 1 fF capacitance change. Measurement results for different capacitance variations demonstrate a change of 10.8 mV for 8.8 fF variation.

A high efficiency inductive power link and backward telemetry for biomedical applications

High-performance wireless power transmission working as a continuous power source of implantable devices can prevent biohazard from leakage of buttery fluid or skin infection from transcutaneous cable. This work reports a high efficiency inductive powering and backward telemetry for implantable medical devices. A new differential class-E type power oscillator with high efficiency excites the external unit, the resonant link coils transfer the power to the internal unit, a modified rectifier circuit rectifies and boosts the recovered signal, and a Zener diode based voltage regulator regulates the output DC voltage. The recovered power is then used to run a ring oscillator-based sensor readout circuit to generate data signal based on sensor current variation and finally a load-shift-keying (LSK) is used to back transit the data to the external unit. The system has been designed using 0.5-µm standard CMOS process with off-chip zener diode, opamp and inductors. The over all power-efficiency of the system is found to be 74.0%.

Wideband injection-locked frequency divider based on a process and temperature compensated ring oscillator

We describe a divide-by-4 injection-locked frequency divider (ILFD) based on a novel process and temperature compensation technique. The ILFD possesses a wide locking range over process corners and a wide temperature range due to the proposed compensation technique. The core of the ILFD consists of ring oscillator based on a modified version of symmetric load (Maneatis) delay elements. A calibration circuitry can be used to further enhance the locking range. Measurement results show that the proposed ILFD functions as a divide-by-4 circuit for an input frequency range from 1.8 GHz to 3.2 GHz for a power level as low as -3 dBm. The worst-case power consumption was approx. 2 mW from a 1.8 V power supply. The proposed ILFD can be used as a low-power prescaler for multi-band applications.

Enhanced class associative generalized fringe-adjusted joint transform correlation for multiple target detection

A modified class associative fringe-adjusted filter-based technique is proposed for multiple target detection, where the number of processing steps remains fixed irrespective of the number of objects in the class. An enhanced version of generalized fringe-adjusted filters is developed for correlation improvement. Again, a shifted phase encoding technique is employed for generation of a single correlation peak per target object. The effectiveness of the proposed technique is verified by computer simulation for binary as well as gray-level images both with and without noise.

A low-loss rectifier unit for inductive-powering of biomedical implants

Biomedical implants have been developed in the recent years with a focus for continuous and real-time monitoring of physiological parameters. Battery-less operation of the implanted unit requires energy harvesting from an inductive link or from the neighboring environment. For efficient conversion of harvested energy to a usable DC level, a rectifier block is employed. However conventional CMOS full bridge rectifier incurs a significant amount of power loss and lowers the overall efficiency of the powering system. In this work a cross-coupled MOSFET based LC oscillator structure has been presented as a modified rectifier circuit. Cross-coupled structure minimizes the loss of the MOS switches and LC tank circuit boosts up the output DC level. The rectifier unit has been designed and simulated using 0.5-µm standard CMOS process. For simulation purposes, different biomedical frequency bands are used to validate the effectiveness of the proposed circuit. Simulation results show that the proposed rectifier circuit can achieve 75% PCE compared to the conventional full bridge CMOS rectifier of only 3% PCE.