A 1.3-μW, 0.6-μm CMOS Current–Frequency Analog–Digital Converter for Implantable Blood-Glucose Monitors

Blood-glucose monitors, like many biomedical implants, must operate autonomously, integrate into small spaces, and remain inconspicuous to the body. The problem is that, while including a battery large enough to sustain a system through its entire life impedes integration, under-sizing its energy reservoir to fit into a miniaturized platform shortens operational life. Fortunately, harvesting energy saves space because the environment (not the device) stores the energy a system requires. Harvesters, however, generate little power per unit volume so implantable sensors must operate under stringent power constraints. For this reason, this paper presents a 1.3-μW, 0.6-μm CMOS current–frequency (I–F) analog–digital converter (ADC). The differential, hysteretic-based ADC proposed uses nA-range input currents to set and compare voltage oscillations against a self-produced reference to resolve the input level an amperometric glucose sensor generates. The prototyped ADC ultimately draws 1.1 A from a 1.2-V supply to resolve 0–32 nA with 4.25 bits of accuracy at a sampling rate of 225 Hz, which relatively simple and well-understood circuit and layout modifications can improve accuracy to over five bits.