Texas Instruments MSP430x1xx - Page 354

Comparator_A Operation
16-8
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 16−6.
Figure 16−6. Timing for Temperature Measurement Systems
V
C
V
CC
0.25 × V
CC
Phase I:
Charge
Phase II:
Discharge
Phase III:
Charge
t
ref
Phase IV:
Discharge
t
meas
t
R
meas
R
ref
The V
CC
voltage and the capacitor value should remain constant during the
conversion, but are not critical since they cancel in the ratio:
N
meas
N
ref
+
–R
meas
C ln
V
ref
V
CC
–R
ref
C ln
V
ref
V
CC
N
meas
N
ref
+
R
meas
R
ref
R
meas
+ R
ref
N
meas
N
ref