
S1C62740 TECHNICAL HARDWARE
EPSON
I-91
CHAPTER 4: PERIPHERAL CIRCUITS AND OPERATION (A/D Converter)
Fig. 4.12.15
Circuit diagram at the time of resistance measurement
+
–
BUF
+
–
+
–
CAZ
CI
BF
INT
CMP
GND
S1
RI
S2
S3
CO
AI2(AI3)
VR, -VR
generation
circuit
-VR
VR
VIN
GND
VR = -VRref
GND
VRref
Thermistor
etc.
R
AI4
VR1
To A/D converter
control circuit
Rref
Resistance measurement mode
At the time of resistance measurement, the non-inverted input of
the integral AMP is set to the GND level.
As shown in Figure 4.12.15, a voltage drop of the reference resist-
ance is obtained as the reference voltage at the time of resistance
measurement by impressing a VR1 voltage from the AI4 terminal
onto the reference resistance connected between the AI4–AI3 (or
AI2) terminals. You can obtain an A/D conversion value according
to the resistance value by A/D conversion of the voltage generated
by the measured resistance connected between AI3 (or AI2) and
GND, using the reference voltage generated by the reference resist-
ance, VR.
For this reason, even when the resistance value of the measured
resistance has been changed to the maximum/minimum, you
should adjust the resistance, such that the voltage that is input
into the A/D converter does not exceed ±320 mV (GND reference).
When using an internally generated VR1, a resistance should be
used such that the resistance variation range is within a
maximum:minimum of 4:1 and this condition is met by setting the
reference resistance at 1/2 of the resistance variation range of the
measured resistance.
However, you should configure the circuit such that the reference
resistance becomes 1 k
to 1 M. Also be careful of these condi-
tions when externally impressing VR1.
When the measured resistance has been made R and the reference
resistance has been made Rref, the voltage VIN input into the A/D
converter and the reference voltage VR are expressed by the follow-
ing expressions.
VIN = VR1 * R / (R + Rref)
(Expression 4.12.5)
VR = VR1 * Rref / (R + Rref)
(Expression 4.12.6)
According to the Expressions 4.12.4, 4.12.5 and 4.12.6, it becomes
R = n * Rref / N
(Expression 4.12.7)