Analog to interval encoder with active use of gate leakage for an implanted blood-sugar sensor

We have developed an ultra low power integrated circuit control module that will be incorporated into a micro machined pill-sized medical implant that continuously monitors blood-sugar levels for patients with Diabetes mellitus. The circuit converts a piezoresistive sensor signal to an inter-pulse interval suited for digital transmission through a wire-less inductive link. Instead of a full analog-to-digital conversion, this analog-to-analog conversion is much simpler and more power conservative. The circuit is entirely asynchronous, requiring no energy consuming clock and operates on sub-threshold currents. A first prototype, produced with the STM 90 nm CMOS process, consumes 1.7muW. A compact on-chip resistive element is employed in a feedback loop to cancel 1/f-noise and offsets in both the sensor and the initial amplification stage. The resistive element is implemented using the quantum effect of gate-leakage, achieving an equivalent resistance of several GOmega with minimal consumption of layout space. The effectiveness of this noise reduction has been asserted in a 62 hour recording with fixed input. The measured noise spectrum appears completely white down to the minimal frequency of the recording, i.e. 4.5muHz. The standard deviation of single pulse intervals (dynamic range from 4.3ms to 15.4 ms) restricts the reconstruction of the sensor value to an accuracy equivalent to 4.41 bits. Averaging over the samples during 1 second increases this accuracy to 7.84 bits. Longer averaging will further improve that figure at the cost of longer periods of active power consumption of the implant, which will be woken up only once every 5 minutes.