A front-end for sensing the stimulation and response of auditory nerve cells

Abstract

At the moment, about 30 million people worldwide are suffering from hearing loss. Most people are helped with a simple hearing aid that just amplifies sounds coming to the ear. However, if the hearing loss is too severe, these aids will not help. More than 100.000 people worldwide need a more complex device in order to let them hear again. A cochlear implant is such a device and is partially implanted inside the human body. Today’s implants are not able to let people hear sounds as people hear them with normal hearing. The main cause can be found in the stimulation accuracy of the auditory nerve fibres. To improve this accuracy, research should be performed in order to improve stimulation algorithms, increase the amount of electrodes and focus the stimulation field of the electrodes. Researchers now reached the limitations of existing neural response readout systems needed for reading out the evoked compound action potential for their research. These limitations urge the need for a new neural response readout system having a dynamic range of 126dB, that is small, power efficient, has noise levels under 10µV and can handle input signals exceeding the supply voltage. The readout system will be able to read out 256 electrodes and is implemented in a MEMS cochlear implant. Existing techniques do not offer solutions to meet the above specifications. An overall readout system design is proposed based on compensation containing an input system, multiplexer, compensation circuit, amplifier and an analog to digital converter (ADC). Because the overall readout system is too big to design in one thesis, only the input system is handled. Existing techniques do not offer solutions to handle input signals above supply voltage being compact, power efficient, have high dynamic range and have a low noise contribution. A new technique based on additive instantaneous companding in order to record the evoked compound action potentials from the stimulated auditory nerve is proposed. Three alternative circuit implementations are proposed, two voltage domain implementations and a charge domain implementation. The proposed input systems are designed to be implemented in AMIS I3T25 (high voltage) CMOS technology. The charge domain circuit implementation is very promising only using 70 components, having a bandwidth of 10kHz, and a noise level of 1.1pV. It is capable to readout input signals which are bigger than the supply voltage while the signal information is preserved. For checking the correct working of the circuit, a signal reconstruction is performed. In addition, this charge domain implementation is never reported before in literature.