Ultra-low Power Readout Circuit Design and Simulation using Dynamic Wheatstone Bridge Technique for Implantable Body Temperature Sensor Application

An ultra-low power consumption body temperature circuit for electroceuticals is proposed. Electroceuticals implanted in the body for nerve signal measurement and nerve stimulation can cause short-term or long-term infections, so it is necessary to monitor temperature increases due to inflammation. A Wheatstone bridge is used to read the resistance value of the thermistor mounted on the neural electrode interface. To reduce the current consumption, a dynamic Wheatstone bridge technique is proposed. The current is supplied only for a short time to obtain the temperature information, and the temperature information is obtained once from the bridge in the order of the external thermistor and the built-in resistor, and the temperature information is obtained again by flipping the bridge in the order of the built-in resistor and the external thermistor. Temperature information sampled over a short period of time is stored in the S&H block for use by the VCO in the rear end. The following VCO and TDC both operate at 0.6 V for low power consumption. The post-layout simulation results showed a sensitivity of 2230 digits/degree and a nonlinear error of −0.05/+0.08℃ in the 34 to 42℃ range, satisfying the ASTM thermometer standards. Power consumption was 112 nW, which is small enough to be incorporated into implantable devices.