Totally passive wireless biopotential measurement sensor by utilizing inductively coupled resonance circuits

A measurement method to measure biopotentials with a passive LC resonator is presented. The sensor consists of an LC resonance circuit where the capacitive component is a varactor and the biopotential electrodes are connected over the varactor ends. This induces a change in the varactor capacitance which can be measured over an inductive link as modified reflected impedance. The sensor itself dissipates virtually no energy at all but of course, some energy is lost in form of resistive losses in the sensor wires and components. The sensor is inexpensive to manufacture and since it is a totally passive device, it is very suitable for implantable applications.Two different measurement methods to sense the reflected impedance are also presented: a slope detector and a phase locked detector. Measurement results from human ECG are discussed that are measured with the both suggested measurement methods.The sensor is proven to be suitable for biopotential measurements with some limitations regarding the movements between the sensor and the receiver device. The properties and performance of the two different measurement devices are discussed. The measurement range, i.e. distance between the sensor and the measurement coils was approximately 5 cm in air for the both measurement devices. However, it seems that the phase locked measurement device suits better to the measurements than the slope detector since its signal-to-noise ratio remains almost constant throughout the measurement range (distance).