A Thermal-Diffusivity-Based Frequency Reference in Standard CMOS With an Absolute Inaccuracy of $\pm $0.1% From $-$55 $^{\circ}$C to 125 $^{\circ}$C

An on-chip frequency reference exploiting the well-defined thermal-diffusivity (TD) of IC-grade silicon has been realized in a standard 0.7
$\mu{\hbox{m}}$
CMOS process. A frequency-locked loop (FLL) locks the frequency of a digitally controlled oscillator (DCO) to the process-insensitive phase shift of an electrothermal filter (ETF). The ETF's phase shift is determined by its geometry and by the thermal diffusivity of bulk silicon (
$D$
). The temperature dependence of
$D$
is compensated for with the help of die-temperature information obtained by an on-chip band-gap temperature sensor. The resulting TD frequency reference has a nominal output frequency of 1.6 MHz and dissipates 7.8 mW from a 5 V supply. Measurements on 16 devices show that it has an absolute inaccuracy of
$\pm $
0.1% (
$\sigma= \pm $
0.05%) over the military temperature range (
$-$
55
$^{\circ}{\hbox{C}}$
to 125
$^{\circ}{\hbox{C}}$
), with a worst case temperature coefficient of
$\pm \hbox{11.2~ppm}/^{\circ}{\hbox{C}}$
.