Frequency sensing of medical signals using low-voltage piezoelectric sensors

Piezoelectric sensors are used to pick up low level vibrations in automotive, biomedical and industrial fields and are subject to errors due to the input current and voltage noise of amplifiers, when connected with a large bias resistor. The noise levels, especially from the bias resistor, degrade the signal to noise ratio of the signal which leads to less reliable feature detection. We report the experimental investigation of a frequency sensing front-end in terms of its linearity, noise and gain in order to acquire medical signals at low frequencies and voltages, e.g. arterial pulse signal, Ballistocardiogram and heart sounds. An LC oscillator is used with a commercial phase locked loop for this purpose. We show that the noise levels in frequency sensing can be reduced by increasing the oscillation frequency, while maintaining 1% non-linearity. Overall, the readout runs at 5 V and has a mean noise floor of 1.43 μV/Hz and a non-linearity of 1% when subject to a sine wave between 20 mV and 200 mV peak to peak amplitude. The results of this work are expected to contribute towards low noise analog front end designs for piezoelectric sensors using frequency sensing as an alternate architecture.