A Frequency Counter Based Analog-to-Digital Converter for a Low-Power RFID Biomedical Telemetry System

This paper describes a method for determining a digital representation of a remote sensing element using a novel and lower power method of analog to digital conversion [1]. This conversion process is most effective for low-frequency and very low current Radio Frequency Identification (RFID) sensing systems where the sensing element tags are powered by an inductively coupled carrier signal of fixed frequency. This method eliminates the need for a traditional, large and power-hungry Analog-to-Digital Converter (ADC). This approach is being developed for an orthopedic application that measures the invivo strain on titanium rods to help surgeons better understand the progress of fusion in spinal fusion surgery [2]. Previous work has been shown using the difference of two clocks for sending digital data from the reader to the tag [3], whereas this approach is optimized for sending digital data in the other direction, from tag to the reader. The sensor element may be a resistive or capacitive device integrated into an oscillator of variable frequency. This variable oscillation signal is then divided down and used as the time base to a frequency counter clocked by the recovered carrier signal. In recovering and using the carrier signal as an internal clock, an additional on chip oscillator is not necessary. The resultant value then undergoes additional post processing to add a unique identification string, a CRC check word, Manchester encoding, and Frequency-Shift Key (FSK) encoding for load modulation transmission [3,4].