A digitally-assisted electrothermal frequency-locked loop

A digitally-assisted electrothermal frequency-locked loop (FLL) is presented, whose output frequency is determined by the temperature-dependent thermal diffusivity of bulk silicon. In contrast to previous work, its noise bandwidth is defined by a digital, rather than an analog, filter. This obviates the need for external capacitors, thus enabling full CMOS integration. Without trimming, an implementation in a 0.7 mum CMOS process achieves an output frequency spread of about plusmn0.3% (3sigma) from -55degC to 125degC. This corresponds to a temperature sensing inaccuracy of about plusmn0.7degC (3sigma).