A digital crystal-less clock generation scheme for wireless biomedical implants

A miniature implantable medical device (IMD) needs a quality clock source for its communication and control circuits. Crystal device is hardly suitable for the in-body clock generator because of the device size limitation. In this paper, a new digital crystal-less clock generation scheme is presented. It uses a digitally controlled oscillator with coarse-fine-tuning structure as the clock source. As the current IMDs are commonly inductively-powered, a strong inductive power carrier is transferred into the in-body system. We propose to leverage the power carrier as the reference clock. We introduce a new all-digital frequency locked loop to track the power carrier frequency. This loop improves the frequency accuracy and stability of the in-body clock source against voltage variation and process variation. Results from post-layout simulation show that the frequency accuracy of the proposed clock source is 0.025%. The RMS and peak-to-peak period jitters of the output clock are 11.3 ps and 60 ps at a frequency of 2 MHz, respectively. The clock generator consumes 0.51 mW with a supply voltage of 1.8 V (required by analog circuits). The core silicon area of the clock generator is 0.036 μm
²
in the 0.18-μm CMOS process.