R01UH0136EJ0210 Rev.2.10
Page 643 of 800
Jul 31, 2012
M16C/64A Group
27. A/D Converter
27.5
External Sensor
To perform A/D conversion accurately, charging the internal capacitor C shown in Figure 27.16 must be
completed within a specified period of time.
T: Specified period of time (sampling time)
R0: Output impedance of sensor equivalent circuit
R: Internal resistance of the MCU
X: Precision (error) of the A/D converter
Y: Resolution of the A/D converter be Y (Y is 1024)
Generally,
When t = T,
Therefore,
Figure 27.16 shows Analog Input Pin and External Sensor Equivalent Circuit. Impedance R0 by which
voltage VC between pins of the capacitor C changes from 0 to VIN - (0.1/1024)VIN in time T when the
difference between VIN and VC is 0.1LSB is obtained. (0.1/1024) means that A/D precision drop due to
insufficient capacitor charge is kept to 0.1LSB in A/D conversion. However, the actual error is the value of
absolute accuracy added to 0.1LSB.
When
φ
AD is 20 MHz, T is 0.75
μ
s. Output impedance R0 for charging capacitor C sufficiently within time
T is obtained as follows.
T = 0.75 µs, R = 10 k
Ω
, C = 6.0 pF, X = 0.1, and Y = 1024. Therefore,
Thus, the output impedance R0 of the sensor equivalent circuit, making the A/D converter precision
(error) 0.1LSB or less, is up to 3.5 k
Ω
.
Figure 27.16 Analog Input Pin and External Sensor Equivalent Circuit
VC
VIN
1
e
1
C R
0
R
+
(
)
--------------------------
t
–
–
⎩
⎭
⎨
⎬
⎧
⎫
=
VC
VIN
X
Y
---
VIN
VIN
1
X
Y
---
–
⎝
⎠
⎛
⎞
=
–
=
e
1
C R
0
R
+
(
)
---------------------
T
–
X
Y
---
=
1
C R
0
R
+
(
)
−−−−−−−−−−−−−−−
–
T
X
Y
---
ln
=
R
0
T
C
X
Y
---
ln
•
−−−−−−−−−−−
–
R
–
=
R
0
0.75
10
6
–
×
6.0
10
12
–
0.1
1024
------------
ln
•
×
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
–
10
10
3
3.5
10
3
×
≈
×
–
=
MCU
Sensor equivalent circuit
R (10 k
Ω
)
C (6.0 pF)
R0
VIN
VC
Содержание M16C/60 Series
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