A frequency counter based analog-to-digital converter for a RFID telemetry system

This paper describes a method for determining a digital representation of a value for a remote sensing element using a novel and lower power method of analog to digital conversion. This conversion process is most effective for low-frequency, radio frequency identification (RFID) sensing systems where the sensing element tags are powered by an inductively coupled carrier signal of fixed frequency. The method uses a specially gated frequency counter to create a digital value that is related to the system´s carrier frequency and the frequency of variable oscillator used with the sensing element. This conversion method is highly scalable while maintaining a low current consumption. One configuration yields four bits of resolution with a conversion time of 1.34 ms and a current consumption of 30 muA. The ASIC was fabricated on a 1.5 mum CMOS process from AMI Semiconductor.