Mohanasankar Sivaprakasam

An optimal design methodology for inductive power link with class-E amplifier

This paper presents a design methodology of a highly efficient power link based on Class-E driven, inductively coupled coil pair. An optimal power link design for retinal prosthesis and/or other implants must take into consideration the allowable safety limits of magnetic fields, which in turn govern the inductances of the primary and secondary coils. In retinal prosthesis, the optimal coil inductances have to deal with the constraints of the coil sizes, the tradeoffs between the losses, H-field limitation and dc supply voltage required by the Class-E driver. Our design procedure starts with the formation of equivalent circuits, followed by the analysis of the loss of the rectifier and coils and the H-field for induced voltage and current. Both linear and nonlinear models for the analysis are presented. Based on the procedure, an experimental power link is implemented with an overall efficiency of 67% at the optimal distance of 7 mm between the coils. In addition to the coil design methodology, we are also presenting a closed-loop control of Class-E amplifier for any duty cycle and any value of the systemQ.

A variable range bi-phasic current stimulus driver circuitry for an implantable retinal prosthetic device

This paper reports a driver circuitry to generate bi-phasic (anodic and cathodic) current pulses for stimulating the retinal layer through electrodes which is part of a retinal prosthetic device for implants in blind patients affected by retinitis pigmentosa (RP) and age-related macular degeneration (AMD). Dual voltage architecture is used to halve the number of interface leads from the chip to the stimulation sites compared to a single voltage supply. The driver circuitry is designed to deliver currents with six bit resolution for a wide range of full scale currents up to 600 /spl mu/A. To cater to the varying stimulus requirements among patients and different regions of the retina, variable gain architecture is used to achieve fine resolution even for a narrow range of stimulus. 1:8 demultiplexing feature is embedded within the output stage thus allowing one DAC for eight outputs. A novel charge cancellation circuitry with current limiting capability is implemented to discharge the electrodes for medical safety. Measurement results of a prototype chip fabricated in 1.5-/spl mu/m CMOS technology are presented.

A 128-Channel 6mW Wireless Neural Recording IC with On-the-Fly Spike Sorting and UWB Tansmitter

The chip is composed of eight 16-channel front-end blocks, data serializing circuits, a DSP for on-chip spike sorting, digital MUX, encoder, UWB TX, and bias generators. The chip operates in one of the two modes. In sorting mode, a selected channel is connected to the on-the-fly spike sorting block and the extracted features of the spikes are transmitted for off-chip classification. In streaming mode, all the sampled data from the 128 channels are recorded and transmitted without any additional processing.

Design Optimization for Integrated Neural Recording Systems

Power and chip area are the most important parameters in designing a neural recording system in vivo. This paper reports a design methodology for an optimized integrated neural recording system. Electrode noise is considered in determining the ADCs resolution to prevent over-design of the ADC, which leads to unnecessary power consumption and chip area. The optimal transconductance and gain of the pre-amplifiers, which minimizes the power-area product of the amplifier, are mathematically derived. A numerical example using actual circuit parameters is shown to demonstrate the design methodology. A tradeoff between the power consumption of the system and the chip area in terms of the multiplexing ratio is investigated and the optimal number of channels per ADC is selected to achieve the minimum power-area product for the entire system. Following the proposed design methodology, a chip has been designed in 0.35 mum CMOS process, with the multiplexing ratio of 16:1, resulting in total chip area of 2.5 mm times 2.0 mm and power consumption of 5.3 mW from plusmn1.65 V.

A Dual Band Wireless Power and Data Telemetry for Retinal Prosthesis

Inductive coupling is commonly used for wireless power and data transfer in biomedical telemetry systems. The increasing demand on the performance of medical devices requires high data rate and high power efficiency at the same time. If only one radio frequency carrier is used, it is difficult to achieve both high data rate and high power efficiency due to the competing requirements on carrier frequency and system-Q of the power and data transmission. We propose a dual band telemetry system to implement power and data transmission using different frequencies by allocating lower frequency for power transmission and higher frequency for data transmission. However, the magnetic coupling between the power carrier and data carrier will affect the operation of both links. In this paper, this interference is analyzed and design equations are derived, which are used to design coils to maximize the data signal level received at the implant side. A prototype of dual band telemetry for a retinal prosthetic device has been built and experimental results show that both power and data can be transmitted and high data rate can be achieved without compromising the power transmission efficiency

Implantable biomimetic microelectronic systems design

In this article, design examples will be presented for a biomimetic microelectronic system for a retinal prosthesis that electrically stimulates the retinal neurons. The system replaces the functionality of vision in blind patients affected by retinitis pigmentosa and age-related macular degeneration. The components and signal processing needed for a cortical prosthesis are described. Integration of all the components of a wireless biomimetic microelectronic system, such as input signal conditioning, power telemetry, data telemetry, stimulation amplifier and control circuitry (microstimulator), and a neural recording and processing device, into a single chip or a package is a tremendous challenge, requiring innovative approaches at both circuit and system levels and consideration of the multiple trade-offs between size, power consumption, flexibility in functionality, and reliability of the microelectronics. The chips described in this paper are prototypes for testing their implemented functionalities. The die sizes do not reflect the actual size of the implant. When the microelectronics are finally integrated, the circuits will be optimized to minimize the area. The use of submicron CMOS technology will also help reduce the die area. It should be noted that the biocompatible package encapsulating the electronics will increase the implant size.

A smart bi-directional telemetry unit for retinal prosthetic device

This paper describes a smart bi-directional telemetry unit for an implantable retinal prosthetic device. The system is designed to deliver 250 mW to the intra-ocular unit via an inductive link and provide wireless communication capability. A novel dual frequency carrier approach is used to optimize the power link for maximum transfer efficiency and the forward telemetry for high data rates. The power link uses a smart feedback system via reverse telemetry to compensate for instantaneous changes in power level due to coil misalignment and load variations. The system is optimized to minimize the power losses in the analog front-end of the intraocular unit while ensuring proper device operation. Preliminary results are shown in AMI 1.6 /spl mu/m bulk CMOS process for the reverse telemetry and power feedback system.

A closed loop transcutaneous power transfer system for implantable devices with enhanced stability

This paper describes a closed-loop wireless inductive power transfer system for an implantable retinal prosthetic device. The proposed system is designed to ensure optimal power transfer to the implanted unit despite coil displacements and changes in load current while minimizing the sensitivity to component and process variation. Based on the system modeling, stability constraints are identified and applied to the feedback control system. The model is crucial in determining component values, circuit topology and number of transmitted bits per sampling period required to ensure system stability. In addition, the model significantly reduces design iterations compounded by lengthy circuit simulation. The model is verified by Matlab and SPICE level simulations. The critical analog circuits of the control system have been designed and fabricated through AMI 1.6 /spl mu/m process.

Image processing and interface for retinal visual prostheses

Controlled electrical stimulation of the retina can result in visual perception in blind patients. In contrast to the over 100,000,000 photoreceptors in a healthy retina, even hundreds of pixels/electrodes of a retinal implant may restore low-resolution vision for unaided mobility and large print reading. We describe the real-time application of image processing techniques, such as contrast and brightness enhancement, grayscale histogram equalization, edge detection, and grayscale reduction, to enhance the visual perception provided by a retinal implant. We discuss schemes for reducing the amount of data transmitted wirelessly to the implant, as well as the interface between the external imaging unit and retinal implant.

Power supply topologies for biphasic stimulation in inductively powered implants

Biphasic stimulation is commonly used for electrical stimulation of tissue in many implants. A dual voltage scheme is often used for the operation of the stimulus circuitry. Due to the advantages of wireless power transmission, an inductive link is used to recover the transmitted power and generate these voltages. This paper identifies the problem of additional power dissipation in the implant when the traditional rectifier topology with one diode is used. A dual diode topology is proposed which reduces the power dissipated in the implant by half. A detailed analysis for both topologies is done through derivation of expressions for the power losses on the primary and secondary coils. A comparison of single diode and dual diode topologies is presented under different conditions and inferences are drawn that can be used as design guidelines.