A temperature-to-digital converter based on an optimized electrothermal filter

The design of a CMOS temperature-to-digital converter (TDC) is presented. It operates by measuring the phase shift of an electrothermal filter (ETF), which is a function of the temperature-dependent thermal diffusivity of bulk silicon. Compared to previous work, this TDC employs an improved ETF, whose layout has been optimized to minimize the phase spread caused by lithographic inaccuracy. Furthermore, the TDCpsilas front-end consists of a gain-boosted transconductor, whose wide bandwidth minimizes electrical phase spread. The resulting current is then digitized by a phase-domain SigmaDelta modulator. The phase-subtracting node of the modulator is realized by a chopper demodulator, whose switching action, however, will give rise to a residual offset current. This is minimized by locating the demodulator at the virtual grounds of the transconductorpsilas gain boosting amplifiers. Any residual offset is then eliminated by chopping the entire front-end. Measurements on 16 samples show that the TDC has an untrimmed inaccuracy of less than plusmn0.4degC (3sigma) over the military range (-55degC to 125degC).