Measurement and wireless transmission of embedded capacitive microsensor's output using Sigma Delta conversion and radio frequency identification (RFID) technology

This article concerns the design and post-fabrication testing of a CMOS integrated circuit (IC) for the Remote-Queried Embedded Microsensor (RQEM) system. The IC may be coupled to capacitive microsensors to measure the output of the sensors, to digitize this measured output, and to condition and encode the digital data. Wireless transmission of the code to a commercial Radio Frequency Identification (RFID) system reader is implemented using Differential Phase Shift Keying (DPSK) of a low-frequency signal, which inductively couples the RQEM antenna coil to the receiving antenna of the RFID reader. The IC extracts its own operating power and digital clock signal from the interrogating signal, which is transmitted by the RFID reader. The IC uses switched-capacitor techniques for acquisition and for A/D conversion of data, A first-order Sigma-Delta (Sigma Delta) A/D converter is used with an output transconductance amplifier (OTA) in the balancing integrator and the comparator. The same OTA is also used in the acquisition circuit, which is a sample-and-hold offset-free circuit. Several fabricated chips were rested with on-chip lest capacitors, used to calibrate the IC's output.