Radios for the brain? A practical micropower sensing and algorithm architecture for neurostimulators

The monitoring of neuronal activity could potentially expand the diagnostic and therapeutic capabilities of neuroprosthesis. The challenge of designing sensing and control systems is two-fold: first, the signal input must be robust for chronic recording; second, the circuit architecture must be capable of achieving signal processing, algorithm control, and telemetry with a limited power budget. The first requirement should be met by measuring field potentials, which represent ensemble behavior in a neural network and can be measured chronically. For the second requirement, architecting an effective solution requires identification of the key information of interest and partitioning the signal chain to play to the strengths of analog vs. digital processing. For many neurological states of interest, information 'biomarkers' are encoded as low frequency power fluctuations within well-defined frequency bands of field potentials, similar to the amplitude modulation found in an AM radio. Recognizing this similarity, the feasibility prototype adapts a chopper-stabilized instrumentation amplifier to act as a superheterodyning AM receiver for brain signals. Since the physiological power fluctuations are generally orders of magnitude slower than the frequency at which they are encoded, the use of efficient analog preprocessing greatly reduces the overall energy requirements for implementing a complete mixed-signal system. Since the science of field potentials is rapidly evolving, the superheterodyning chopper is advantageous given its flexibility and immunity to process, temperature, and mismatch variations. This paper will discuss the design of a complete system prototype for a neurostimulator research tool; the design has a noise floor of under 2μVrms and a total system current of 25μW/processing channel (1.8V supply) while performing biomarker extraction, algorithmic processing and control, and data loop recording.