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Design Notes: A low power, high stability relaxation oscillator for implantable biomedical applications

The designers of a self-clocked offset cancellation technique for comparators within a relaxation oscillator share their findings and explain why the resulting improvements in frequency drift and close-in phase fluctuations are relevant to industrial, scientific or medical applications.

Recently, there has been a growing trend towards adopting system-on-chips (SoCs) in implantable biomedical applications, where power consumption and device footprint are key design considerations. The digital, analog and mixed-signal circuits in these SoCs require a clock source for commonly used signal processing techniques, such as sampling and chopper stabilisation. The primary clock source for such SoCs needs to provide good accuracy, as well as long-term stability in its oscillation frequency. An accurate clock signal avoids signal distortion when sampling or chopping techniques are applied. Meanwhile, stability is important because in biomedical applications, variation and drift in the system characteristics would present certain risks. In addition, close-in phase noise is important for biomedical signals that typically reside at low frequencies.

Monolithic oscillators are generally preferred as a primary clock source for implantable biomedical sensor SoCs. A relaxation oscillator can operate with low power and has a much smaller form factor than quartz crystal oscillators. However, most relaxation oscillator performance is limited by process variations, noisy current sources, and noise and offset voltage within the comparators.

Hence, IME researchers invented a self-clocked offset cancellation technique for comparators in the relaxation oscillator. In their relaxation oscillator design, only one of the two comparators in the oscillator performs a comparison operation at a certain point in time, while the other is idle. The comparators alternately execute the offset cancellation operation in their idle time. This cancellation operation removes the offset voltage as well as any non-idealities caused by low-frequency noise from the comparators. Self-generated, non-overlapping clocks control the switches to configure capacitor networks in the oscillator according to the comparator operation phases – comparison and offset-storing phases.

Figure 1: Die micrograph of the test chip in a 0.13um CMOS process.

A novel 38uW 3.2MHz relaxation oscillator with self-clocked offset-canceling comparators has been fabricated in a 0.13um CMOS process (see Figure 1). While consuming just 5uW more than the conventional design without offset cancellation scheme, it exhibits 4dB lower Allan variance floor, with no random walk noise effect until 10ms of gate-time. The close-in phase noise is significantly reduced with corner frequency lowered from 50 to 20KHz.

Figure 2: Allan deviation of oscillators with and without offset cancellation scheme. The inset is PSD of fractional frequency fluctuation multiplied by the offset frequency.

Figure 3: Measured output PSD of oscillators with and without offset cancellation. The inset is oscillator output power measured with 10kHz RBW over a wider frequency span.

The oscillation period changes due to supply and temperature are within 0.4% and 0.25% over 1.4 – 1.6V and 20 – 60oC respectively. As for periodic rms jitter, 455ps and 524ps are obtained with and without offset cancellation respectively. In addition, the offset-cancelled oscillator exhibits a 0.6% duty cycle error compared to 3.1% with the conventional design.

Researchers at IME have applied the self-clocked offset cancellation technique for comparators in the relaxation oscillator to small form factor implantable and invasive biomedical devices such as cardiac wall motion-sensing and catheter guiding systems. Other applications for high stability relaxation oscillator are in the automotive and industrial control applications where a stable time reference is needed

Author profile: Minkyu Je, Kunil Choe, Olivier Daniel Bernal and Charles Lee are based at the Institute of Microelectronics (www.ime.a-star.edu.sg), part of Singapore's Agency for Science, Technology and Research (ASTR). ASTR is a member of A*STAR, a government organisation dedicated to promoting Singapore's Science and Technology activities. IME's key research areas are in integrated circuits design, advanced packaging, bioelectronics, MEMS, nanoelectronics and photonics.

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