The comparator circuits are the overflow path of a kill-zero
carry-lookahead adder, as shown in Fig. 4. The comparator
indicates if a sample Tt is greater than or equal to its previous
output Tt−1. If the temperature change is too small to affect both
sensor outputs, the comparators indicate Tt ≥ Tt−1, regardless
of the actual I − T dependence. To account for this possibility,
the Valid signal in Fig. 1 is used to tell the system if the
current temperature dependence reading is meaningful. For a
sample to be valid, both the delay-tracking temperature sensor
and the reference temperature sensor must record a change in
temperature. For example, if the temperature sensor resolution
is 0.29 ◦C and the temperature only changes by 0.1 ◦C between
readings, there may not be a change in both Out vectors, and the
reading will be invalid; thus, the sampling rate is constrained
by the sensor resolution and rate of temperature change. As the
purpose of our sensor system is to determine the temperature
dependence at each new VOP, the actual sensor sampling rate
will depend on the frequency at which VOP is adjusted. (Switching
supply voltages can take hundreds of microseconds [18], so
the maximum required sampling rate is expected to be in the
kilohertz range).