DC Current Measurement Systems for Neurostimulator Implants

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Copyright: Moghe, Yashodhan
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Abstract
This research explores methods of accurately measuring DC current flowing in a circuit sub-block or an entire chip. The major application demonstrated is a novel mixed-signal system to monitor DC current imbalance on the supply/ground lines of the stimulation circuitry in a neurostimulator implant chip, which indicates a fault that may lead to dangerous levels of DC current leaking into tissue. The system can detect all known failure modes that cause leakage current including ESD damage, fluid leaks and electrode/PCB/component shorts. Furthermore, the system is agnostic of the inner workings of the stimulation circuitry it monitors, and can be applied to highly complex implants containing any number of electrodes, without having to scale circuit area and power. The system is sensitive to DC current imbalances 50-56 dB smaller than the full-scale current, programmable in the range 0.25-8mA, yielding an absolute detection sensitivity of 0.75-12uA; sufficient to make it useful in current and future implants. Static and dynamic power consumption of the system in a typical implant scenario with 10V stimulation supply are 0.22-0.49mW and 0.64-1.17mW respectively, while the supply/ground current sensors consume a total of up to 1V of stimulation voltage headroom. The trade-off between sensitivity and headroom loss can be programmed digitally, as can all other aspects of the system. Practical guidelines on integrating the system into real implant systems are given. The system was developed and tested on a mainstream 0.35um CMOS process, with care taken to ensure mass-producibility. An essential part of this was the development of 2-3ns latency HV digital level-shifters that outperform all prior designs. The supply/ground current sensor itself performed up to 22 dB better than comparable prior art. The level-shifter and the sensor have general applicability to many systems beyond neurostimulator implants.
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Author(s)
Moghe, Yashodhan
Supervisor(s)
Lehmann, Torsten
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Publication Year
2014
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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download public version.pdf 2.76 MB Adobe Portable Document Format
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