Redundant Crossfire: A Technique to Achieve Super-Resolution in Neurostimulator Design by Exploiting Transistor Mismatch

A high-resolution neurostimulator is the essential component of many bidirectional neural interfaces. In practice, the effective resolution of fully integrated neurostimulator designs is often hindered by the transistor mismatch, especially in submicrometer CMOS processes. In this article, we present a new circuit technique called redundant crossfire (RXF) to address this challenge. It is derived from our redundant sensing (RS) framework, which aims at engineering information redundancy into the system architecture to enhance its effective resolution. RXF involves combining (i.e., crossfiring) the outputs of two or more current drivers to form a redundant structure that, when properly configured, can produce accurate current pulses with an effective super-resolution beyond the limitation commonly permitted by the physical constraints. Unlike any previous works, the proposed technique achieves high-accuracy by directly exploiting the random transistor mismatch with an excessively large mismatch ratio of 10%-20%. The effectiveness of RXF is verified through both Monte Carlo simulations and measurement results of a fully integrated neurostimulator chip. Equipped with a 5-bit current digital-to-analog converter (IDAC) and two 4-bit current multipliers, the stimulator achieves an effective resolution of 9.75 bits in a 1.1-mA full range. An application of the fabricated chip is to deliver neuro-feedback to a human amputee through peripheral nerves where the amplitude of stimulation pulses is accurately controlled to encode the tactile response's intensity.