Interface Electronics for a CMOS Electrothermal Frequency-Locked-Loop

This paper describes a new implementation of a CMOS electrothermal frequency-locked-loop (FLL), whose output frequency is determined by the temperature-dependent phase shift of an electrothermal filter (ETF). The FLL maintains a constant phase shift in the ETF, and as a result drives it with a signal whose frequency is a well-defined function of temperature. Compared to a previous implementation, the FLL described here has significantly more loop gain, less electrical phase-spread, and is more suitable for full integration. Measurements on 16 samples (from one batch) show that the temperature dependence of the FLL´s output frequency agrees very well with the known thermal properties of bulk silicon. The untrimmed spread of this frequency is less than plusmn0.45% (3sigma) from -40degC to 100degC, which corresponds to a temperature-sensing inaccuracy of less than plusmn0.7degC (3sigma).