Mohamad Rahal

Comparison of a new integrated current source with the modified Howland circuit for EIT applications

Multi-frequency electrical impedance tomography (MF-EIT) systems require current sources that are accurate over a wide frequency range (1 MHz) and with large load impedance variations. The most commonly employed current source design in EIT systems is the modified Howland circuit (MHC). The MHC requires tight matching of resistors to achieve high output impedance and may suffer from instability over a wide frequency range in an integrated solution. In this paper, we introduce a new integrated current source design in CMOS technology and compare its performance with the MHC. The new integrated design has advantages over the MHC in terms of power consumption and area. The output current and the output impedance of both circuits were determined through simulations and measurements over the frequency range of 10 kHz to 1 MHz. For frequencies up to 1 MHz, the measured maximum variation of the output current for the integrated current source is 0.8% whereas for the MHC the corresponding value is 1.5%. Although the integrated current source has an output impedance greater than 1 MOmega up to 1 MHz in simulations, in practice, the impedance is greater than 160 kOmega up to 1 MHz due to the presence of stray capacitance.

An ASIC Front End for Planar High-Frequency Contactless Inductive Position Sensors

We describe an application-specific integrated circuit (ASIC) front end for readout and control of planar high-frequency contactless inductive position sensors that contain transmitter and receiver coils on a fixed printed circuit board and a moving passive resonant target. Such an inductive position sensor suffers from transmitter-to-receiver signal coupling, which can result in a phase-sensitive offset; hence, an error in the position measurement occurs. For the receiver front end, we consider two analog synchronous mixer demodulators, which we call mixer-1 and mixer-2, and analyze their ability to reject phase-sensitive offsets due to transmitter signal breakthrough. The mathematical analysis is validated with measured results from the fabricated ASIC in a 0.35-mum CMOS process technology. The ASIC front end contains the transmitter driver, the two receiver mixer variants, a frequency divider/shifter, and an amplifier low-pass filter. Measurements from five ASIC samples connected to the sensor show that, with a system gain of 320, the average output offset variation with phase difference from -99 to +117deg is more than 237 mV with mixer-1 compared to less than 7 mV with mixer-2.

Floating voltage-controlled current sources for electrical impedance tomography

The design of a current source for wideband electrical impedance tomography medical instrumentation is a challenging task. This paper describes two simple OTA-based floating voltage-controlled current sources (VCCSs) for this application. Both designs are suitable for VLSI implementation and overcome the drawbacks of existing opamp-based discrete VCCS designs. The floating VCCSs are designed to drive loads of 100 Omega to 2 kOmega with current amplitudes of up to 500 muA. The working frequency band is between 100 Hz to 1 MHz. Simulated results using a 0.35-mum CMOS process technology are presented to show the operation of the circuits.

A Synchronous Chopping Demodulator and Implementation for High-Frequency Inductive Position Sensors

We describe a new method for high-frequency precision sensing. The method combines synchronous detection with chopping in a fully differential architecture that includes an instrumentation amplifier. An integrated circuit implementation of the proposed synchronous chopping demodulator front end was designed and fabricated in a 0.35-mum CMOS process technology and tested with high-frequency inductive position sensors. The measured results show that the new technique offers considerable advantages in terms of offset reduction compared to traditional techniques for these sensors, which rely on a microcontroller to measure the offset before each position measurement is taken. The measured average input-referred offset for the 20 fabricated chip samples is 87 muV at a chopping frequency of 500 kHz when the resonant target is off and synchronous demodulation and transmitter excitation are both applied at 1 MHz. The technique, in addition to improving system resolution and immunity to common-mode interference, allows these high-frequency position sensors to work with multiple targets, thus increasing speed and functionality.

A programmable ENG amplifier with passive EMG neutralization for FES applications

An integrated amplifier for electroneurogram (ENG) recordings from tripolar cuff electrodes is described. The amplifier is dedicated to urinary incontinence and other functional electrical stimulation (FES) applications. To remove myoelectric (EMG) interference a parallel RC network is used to balance the electrode impedances in the quasi-tripole amplifier configuration. The various ENG amplifier settings, such as resistance and capacitance trimming for the neutralization RC network, amplifier gain and filter cut-off frequencies, are controlled by an external microcontroller which communicates with the embedded SPI (Serial Port Interface) block in the amplifier. By this topology control of the ENG amplifier is executed in software allowing for the system parameters to be re-configured after implantation. The analog system blocks and details of the SPI logic are described. The amplifier was fabricated in a 3-V 0.35-mum BiCMOS process technology and preliminary measured results are reported. Input signals as low as 1muV can be reliably detected. The amplifier occupies an area of 1.5mm2 and consumes about 1.4mW when configured to detect sub-microvolt neural signals.

An integrated signal conditioner for high-frequency inductive position sensors

This paper describes the design, implementation and evaluation of a signal conditioner application-specific integrated circuit (ASIC) for high-frequency inductive non-contact position sensors. These sensors employ a radio frequency technology based on an antenna planar arrangement and a resonant target, have a high inherent resolution (0.1% of antenna length) and can measure target position over a wide distance range ( 10 m). However, due to the relatively high-frequency excitation (1 MHz typically) and to the specific layouts of these sensors, there is unwanted capacitive coupling between the transmitter and receiver coils; this type of distortion reduces linearity and resolution. The ASIC, which is the first generation of its kind for this type of sensor, employs a differential mixer topology which suppresses the capacitive coupling offsets. The system architecture and circuit details are presented. The ASIC was fabricated in a 0.6 µm high-voltage CMOS technology occupying an area of 8 mm2. It dissipates about 30 mA from a 24 V power supply. The ASIC was tested with a high-frequency inductive position sensor (with an antenna length of 10.8 cm). The measured input-referred offset due to transmitter crosstalk is on average about 22 µV over a wide phase difference variation (−99° to +117°) between the transmitter and demodulating signals.

An integrated common-mode feedback topology for multi-frequency bioimpedance imaging

One of the key limitations in medical impedance imaging and bio-impedance measurements is common-mode errors. We present an integrated common-mode feedback topology which reduces these errors for use in a bio-imaging system for in-vivo monitoring of neonate lung function (10–200 kHz current injection frequency). A frequency-selective feedback network is described which reduces the common-mode voltage due to electrode impedance mismatch at the input of the differential amplifier. The theory and key design blocks are presented. The circuit was implemented in a 5-V 0.35-µm CMOS technology, occupying an area of 0.75 mm2 and dissipating about 20 mW. Experiments were conducted using, an RC model of the electrodes, and ECG electrodes on the forearm to demonstrate the working of the integrated circuit. Measured results show that the common-mode signal is reduced by 85%, 75%, 70% and 65% at 10kHz, 50kHz, 100 kHz and 200 kHz, respectively.

Recent advances in the design of implantable stimulator output stages

As the most important part of an implanted stimulation device, the stimulator output stage is in direct contact with the biological tissue, and is responsible for triggering the action potential in the stimulated nerves. The design of the stimulator output stage is governed by both biomedical and IC design constraints. In this paper, we present an overview of some recent advances in the design of implantable stimulator output stages for functional electrical stimulation applications. We also discuss a stimulator output stage structure which is suitable for large reduction of the physical size of the entire stimulator circuit.

An ENG Amplifier with Passive EMG Neutralization

We describe an implantable electroneurogram (ENG) amplifier dedicated to incontinence and other functional electrical stimulation applications. The amplifier features a novel simple passive technique for removing myoelectric (EMG) interference in neural recording systems using tripolar cuff electrodes. The system blocks of the front- end (instrumentation amplifier, post-filter, rectifier and programmable gain block) are described. The amplifier was fabricated in a 3-V 0.35-mum BiCMOS process technology and preliminary measured results are presented. Input signals aslow as 1 muV can be reliably detected.

A readout system for inductive position sensors

The readout electronics for contact-less inductive position sensors are currently based on discrete solutions. To enhance overall system performance, the implementation of the readout electronics into a single ASIC is desirable. This paper discusses the key system blocks of the analog front-end. Two variants of the mixer architecture for the synchronous detection are examined in terms of their ability to reject phase-sensitive offsets. Simulated results using a 0.35-mum CMOS process technology are presented.