This thesis describes the design of an ultra-low power temperature to digital converter. It is intended to monitor the excess heat produced during the wireless charging of implantable medical devices such as pacemakers. The TDC is designed to achieve an accuracy of ±0.1 °C (3 sigma) from 27 °C to 47 °C, and ±0.3 °C (3 sigma) from -40 °C to 85 °C after a 1-point trim. It also achieves a resolution of 0.01 °C at 10 Sa/s. The low power consumption (155 nW) is made possible by the implementation of a self-biasing BJT-core. Its low power consumption, accuracy, and resolution make it ideally suited for clinical temperature monitoring.