A precision relaxation oscillator with a self-clocked offset-cancellation scheme for implantable biomedical SoCs

Abstract

Recently, there has been an increasing demand for SoCs in the biomedical field. In implantable applications, SoCs are designed under very stringent power and area constraints. The analog and mixed-signal circuits as well as digital circuits in those SoCs require a clock source, because clock-based signal-processing techniques, such as sampling and chopper stabilization, are often used. The primary clock source for such SoCs needs to provide good accuracy and long-term stability of the oscillation frequency foscto minimize variations and drifts of the system characteristics. A fairly pure clock signal is required to avoid signal distortion when sampling or chopping techniques are applied. Considering such a source is typically a free-running oscillator, and biomedical signals of typical interest reside at low frequencies, the close-in phase noise is important. For implantable biomedical sensor SoCs, due to high power consumption of quartz crystal oscillators and their bulky size, monolithic oscillators are preferred as a primary clock source. Relaxation oscillators can operate with low power and set their oscillation period Tosc in well-defined manner while consuming small silicon area. However, their performance is limited by process variations, noisy current sources, and noise and offset voltages of the comparators. Geraedts et al. presented an idea of subtracting a fixed charge packet and filtering out the comparator noise to improve the phase noise particularly at large offset frequencies. In this work, a self-clocked offset-cancellation scheme is presented for comparators in the relaxation oscillator, resulting in smaller frequency drifts and lower close-in phase fluctuations, which are more relevant parameters for primary clock sources in implantable biomedical SoCs.