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Comparator_A Operation
The thermistor measurement is based on a ratiometric conversion principle.
The ratio of two capacitor discharge times is calculated as shown in
Figure 16−6.
Figure 16−6. Timing for Temperature Measurement Systems
V
C
V
CC
0.25 × V
CC
Phase I:
Charge
The V
conversion, but are not critical since they cancel in the ratio:
N
meas
N
N
meas
N
R
meas
16-8
Comparator_A
Phase II:
Discharge
t ref
and the capacitor value should remain constant during the
voltage
CC
–R
C
ln
meas
V
+
V
ref
–R
C
ln
ref
V
R
meas
+
R
ref
ref
N
meas
+ R
ref
N
ref
Phase III:
Phase IV:
Charge
Discharge
t meas
V
ref
CC
ref
CC
R
meas
R
ref
t
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