Development of an ultra low noise, miniature signal conditioning device for vestibular evoked response recordings

Chathura L Kumaragamage,Brian J Lithgow,Zahra Moussavi

doi:10.1186/1475-925x-13-6

Abstract

BackgroundInner ear evoked potentials are small amplitude (<1 μVpk) signals that require a low noise signal acquisition protocol for successful extraction; an existing such technique is Electrocochleography (ECOG). A novel variant of ECOG called Electrovestibulography (EVestG) is currently investigated by our group, which captures vestibular responses to a whole body tilt. The objective is to design and implement a bio-signal amplifier optimized for ECOG and EVestG, which will be superior in noise performance compared to low noise, general purpose devices available commercially.MethodA high gain configuration is required (>85 dB) for such small signal recordings; thus, background power line interference (PLI) can have adverse effects. Active electrode shielding and driven-right-leg circuitry optimized for EVestG/ECOG recordings were investigated for PLI suppression. A parallel pre-amplifier design approach was investigated to realize low voltage, and current noise figures for the bio-signal amplifier.ResultsIn comparison to the currently used device, PLI is significantly suppressed by the designed prototype (by >20 dB in specific test scenarios), and the prototype amplifier generated noise was measured to be 4.8 nV/Hz @ 1 kHz (0.45 μVRMS with bandwidth 10 Hz-10 kHz), which is lower than the currently used device generated noise of 7.8 nV/Hz @ 1 kHz (0.76 μVRMS). A low noise (<1 nV/Hz) radio frequency interference filter was realized to minimize noise contribution from the pre-amplifier, while maintaining the required bandwidth in high impedance measurements. Validation of the prototype device was conducted for actual ECOG recordings on humans that showed an increase (p < 0.05) of ~5 dB in Signal-to-Noise ratio (SNR), and for EVestG recordings using a synthetic ear model that showed a ~4% improvement (p < 0.01) over the currently used amplifier.ConclusionThis paper presents the design and evaluation of an ultra-low noise and miniaturized bio-signal amplifier tailored for EVestG and ECOG. The increase in SNR for the implemented amplifier will reduce variability associated with bio-features extracted from such recordings; hence sensitivity and specificity measures associated with disease classification are expected to increase. Furthermore, immunity to PLI has enabled EVestG and ECOG recordings to be carried out in a non-shielded clinical environment.

Highlights

Development of low noise bio-signal acquisition devices have evolved substantially since the inception of the amplified electrocardiogram in the late 1940’s [1], due to the demand for high resolution electrophysiological measurements and increased patients’ safety [2]
In comparison to the currently used device, power line interference (PLI) is significantly suppressed by the designed prototype, and the prototype amplifier generated noise was measured to be 4.8 nV= Hz @ 1 kHz (0.45 μVRMS with bandwidth 1p0ffiffiHffiffiffizffi -10 kHz), which is lower than the currentlypusffieffiffiffidffi device generated noise of 7.8 nV=
Validation of the prototype device was conducted for actual ECOG recordings on humans that showed an increase (p < 0.05) of ~5 dB in Signal-to-Noise ratio (SNR), and for EVestG recordings using a synthetic ear model that showed a ~4% improvement (p < 0.01) over the currently used amplifier

Summary

Introduction

Development of low noise bio-signal acquisition devices have evolved substantially since the inception of the amplified electrocardiogram in the late 1940’s [1], due to the demand for high resolution electrophysiological measurements and increased patients’ safety [2]. One such method that demands a high resolution signal acquisition protocol is Electrocochleography (ECOG), a specialized technique that captures electrical activity from the cochlea, where the signal of interest can range from 0.1-2 μV for the extratympanic approach [3]. EVestG recordings are currently obtained using a commercially available, bench-top, low noise bio-signal amplifier in a sound attenuated electrically shielded chamber to reduce background interference. The algorithm’s FP detection accuracy is found to be largely dependent on three main types of interference present in the recording [15]: 1) biological signal interference within the bandwidth 40-500 Hz that is comprised predominantly of muscle activity [5]), 2) power line interference (PLI) (predominantly odd harmonics), and 3) system generated noise (where effects are significant for frequencies above 500 Hz)

Results

Discussion

Conclusion