Measurement of Sensor System Functionality
System functionality is shown over a wide voltage range
in Fig. 6. Positive temperature dependence occurs at voltages
below ∼1.4 V in 0.35-μm technology, as shown by simulation
in Fig. 6(a); thus, the measurements in Fig. 6(b) show that the
sensor system correctly detects the transition between positive
and negative I − T slopes. Further measurements of the impact
of supply voltage on temperature variation are shown in Fig. 7,
showing the importance of detecting positive I − T slopes at
low voltages; at 0.5 V, temperature affects fosc by over an
order of magnitude over the measured temperatures. Fig. 8
shows the change in temperature ΔT needed to ensure a valid
reading at each supply voltage. (Enable period En affects the
sensor resolution [16].) Lower voltages require larger ΔTs or
Linearity measurements of fosc are provided in Fig. 9
for six separate chips; linearity offsets of as low as 0.2%
(corresponding to an accuracy of 1.3 ◦C, including both sensor
inaccuracies and measurement inaccuracies) were achieved
over the temperature range of 5 ◦C–80 ◦C, although interdie
process variation resulted in fosc shifts of up to 6.5%. The
sensor system output uses the difference between readings of
the reference and delay-tracking sensors; thus, if the sensors
are placed in close proximity, the impact of process and voltage
variations on the overall temperature dependence sensor output
will be reduced. The sensors can be calibrated with the process
compensation unit, as shown in Fig. 10 to provide a coarse
calibration of up to +14%/ − 10% (at the cost of