A PSK-demodulator for a bi-directional data transmission in passive telemetric microsystems is presented in this work. These systems, which use the telemetry link for energy and data transmission, are based on identification systems, which use ASK-modulation for both data transfer directions. This leads to problems with a bi-directional data transmission in sensor applications, where the power consumption is significantly higher than in identification systems. A system that uses a PSK to transfer data into the microsystem to improve the energy transfer during data transmission is presented. The key component of this system is a novel PSK-demodulator, which works without an internal oscillator and therefore no PLL is needed. The major advantages of the presented system are the self-adaptation to the carrier frequency and the independence of the demodulator of parameter drifts.