Comparator_A Operation
17-8 Comparator_A
The thermistor measurement is based on a ratiometric conversion principle.
The ratio of two capacitor discharge times is calculated as shown in
Figure 176.
Figure 176. Timing for Temperature Measurement Systems
VC
VCC
0.25 × VCC
Phase I:
Charge Phase II:
Discharge Phase III:
Charge
tref
Phase IV:
Discharge
tmeas
t
Rmeas
Rref
The VCC voltage and the capacitor value should remain constant during the
conversion, but are not critical since they cancel in the ratio:
Nmeas
Nref
+
Rmeas C ln Vref
VCC
Rref C ln Vref
VCC
Nmeas
Nref
+Rmeas
Rref
Rmeas +Rref Nmeas
Nref