Topic: A Low Noise Temperature-to-Digital Converter for a High Stability MEMS-Based Oscillator

Abstract: This talk presents a MEMS-based programmable oscillator which achieves better than +/-0.5ppm frequency stability from -40 degrees C to 85 degrees C and less than 1ps (rms) integrated phase noise (12kHz to 20MHz). We focus on the key component of this system, which is a thermistor-based temperature-to-digital converter (TDC) that enables accurate and low noise compensation of temperature-induced variation of the MEMS resonant frequency. The TDC utilizes several circuit techniques including a high resolution, tunable reference resistor based on a switched capacitor network and fractional-N frequency division, a switched resistor measurement approach which allows a pulsed bias technique for reduced noise, and a VCO-based quantizer for digitization of the temperature signal. To verify the effectiveness of these techniques, measured performance of a 180nm CMOS IC with co-packaged MEMS die is presented which demonstrates 0.1mK (rms) resolution for the TDC within a 5Hz bandwidth and power consumption of 3.97mA with 3.3V supply.