A Micro-Power Temperature-to-Digital Converter for Use in a MEMS-Based 32 kHz Oscillator

Abstract

This paper describes the design of a low-power energy-efficient temperature-to-digital converter (TDC) intended for the temperature compensation of a 32 kHz MEMS-based oscillator (TCXO). The compensation scheme enables a frequency stability of ±3 ppm over temperatures ranging from −40 to 85 °C. The TDC consists of an NPN-based temperature sensing element and a 15-bit second order ΔΣ modulator. A novel dynamic element matching (DEM) scheme ensures that DEM tones do not inter-modulate with the modulator’s bit-stream, thus improving the TDC’s accuracy without impacting its resolution. The TDC occupies 0.085 mm2 in a 180 nm CMOS process, draws less than 4.5 μA from a 1.5 to 3.3 V supply, and achieves a resolution of 25 mK in a conversion time of 6 ms. This corresponds to a figure of merit of 24 pJ°C2.