Ioannis Pachnis

Passive Neutralization of Myoelectric Interference From Neural Recording Tripoles

In this paper, we present a simple passive technique for removing myoelectric interference in neural recording systems using tripolar electrodes. Imbalance is simply unavoidable with the conventional quasi-tripole (QT) amplifier and this technakshnique is based on a modified version of the QT, which sees the tripole as a bridge that can be balanced by adding impedance to one of the outer electrodes. We show that parallel resistance and capacitance is better than a series combination for use at all frequencies, and that with a tripole that was intentionally made imbalanced, by an amount that we measure as 3%, the interference can be reduced about 10-fold. It is important to null the interference at low frequencies, in the electromyography (EMG) band, to get the best improvement. Common-mode interference can also be reduced by appropriate trimming if necessary

An Integrated Amplifier With Passive Neutralization of Myoelectric Interference From Neural Recording Tripoles

This paper describes an integrated amplifier for neural recording from tripolar electrode books connected in the quasi-tripole arrangement. The same tripole is used in the implant for neural stimulation. To remove myoelectric interference from neural recordings, a resistor-capacitor network is used to balance the electrode impedances in the tripole. The amplifier is programmable (recording mode, passband, gain, and trimming impedance) via a serial peripheral interface. Its front-end instrumentation amplifier employs current feedback to achieve high common-mode rejection ratio. The circuit was implemented in 0.35- μm CMOS technology and occupies an area of ~1.2 mm2. In the very-low noise mode, the analog front-end features a maximum of 0.68 μVrms input-referred noise in the passband and consumes ~310 μA from a 3 V supply. The measured common-mode rejection ratio of the instrumentation amplifier (CMRRIA) approaches 100 dB. The effect of component mismatch on the CMRRIA and the noise performance of the instrumentation amplifier are examined. In addition, the recording tripole is analyzed to derive the overall common-mode rejection ratio that is shown to exceed 80 dB. The ability of the neural amplifier to reduce myoelectric interference is demonstrated. The amplifier is also tested with a tripole immersed in saline in the stimulate-record mode.

Design of an implant for preventing incontinence after spinal cord injury.

An implanted device is being designed and tested which has the main function of suppressing hyperreflexic bladder contractions by stimulating the pudendal afferent pathway. The concept is that the contractions will be detected by recording natural nerve signals. This is challenging because the changes in neural signal are very small (sub-microvolt), and the device must run 24 h per day, which means that for convenience it must be battery-powered. The energy budget is therefore tight. Furthermore, because the patient must be able to intervene to occasionally empty the bladder, a radio link is needed to the device. Within the EU project Healthy Aims, most aspects of the design have been made and tested. This includes the battery, battery charger, neural amplifier, and the package incorporating the Medical Implant Communication System (MICS) antenna, which are briefly described here. This article is a progress report.

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.

Comparison of Transconductance Reduction Techniques for the Design of a Very Large Time-Constant CMOS Integrator

This paper compares three transconductance (gm) reduction techniques in terms of analog mismatch and total achievable amount of gm reduction. The techniques investigated are current division, current cancellation, and cascade of gm-1/gm stages. Each of these is applied to the design of a very long time-constant integrator for use in a neural recording bladder control implant. Extensive Monte-Carlo simulations in a 0.35mum CMOS process showed that for the target gm of about 50 pA/V, the current division technique is the best option as it is insensitive to analog mismatch when used in closed-loop configuration. The achievable gm with the current cancellation technique is limited to about 65 nA/V, whereas the cascade of gm-1/gm stages is extremely sensitive to DC offsets.

Towards an adaptive modified quasi-tripole amplifier configuration for EMG neutralization in neural recording tripoles

Recording neural information using implanted cuff electrodes in a tripolar arrangement is affected by myoelectric interference present at the implantation site. The performance of the recently proposed mQT was suboptimal in that tuning was required to compensate for such interference. In this work we present an improvement where interference in the mQT is removed by modeling the impedance profile of our electrodes. Simulation data show that by employing this “adaptive” approach, a 33% imbalance in the axial resistances when the interference is of 10 mV at 500 Hz and 10 kHz, results in an interference breakthrough that is about 7230 and 100 times less, respectively, at the amplifiers output.

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.

Interference reduction techniques in neural recording tripoles: An overview

Since the introduction of the quasi-tripole amplifier configuration in the mid-70s for the purpose of long-term recording of neural signals from peripheral nerves, research efforts are now being focused in using peripheral nerve signals as control inputs to neural prostheses. This paper gives an overview of the most important neural recording tripolar configurations, and explores the idea of modeling the polarization impedance of electrodes using branched ladder networks to achieve adaptive passive neutralization of myoeletric interference.

Adaptive EMG neutralization using the modified QT

In this paper we present a passive impedance network that offers, to conventional QT configuration, the ability to null EMG interference without requiring tuning. The network is based on the Randles circuit model which fits the electrode- electrolyte impedance profile of our platinum electrode book, throughout the ENG band. The calculated RMS noise input voltage of the system was found to be 0.73 uV, thus making the solution plausible in terms of noise performance.

Myoelectric and Common-Mode Interference Rejection in a Quasi-Tripole Amplifier Configuration

In this paper we present a simple technique for removing myoelectric interference in neural recording tripoles. Cuff imbalance is simply unavoidable with the conventional quasi-tripole (QT) configuration and this technique is based on a modified version of the QT, which is capable of compensating for cuff imbalance that is causing electromyogram interference to be present at the output of an amplifier. By carrying out in-vitro experiments we show that, with a tripole that was intentionally made imbalanced, the interference can be reduced about 10-fold.