r5f21346mnfp Renesas Electronics Corporation., r5f21346mnfp Datasheet - Page 633

• Manufacturer Part Number: r5f21346mnfp
• Description: Single-chip Mcus Incorporates The R8c Cpu Core
• Manufacturer: Renesas Electronics Corporation.

R8C/34M Group
R01UH0131EJ0100 Rev.1.00
Jun 21, 2011
28.9
Figure 28.10
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 28.10 has to be completed
within a specified period of time. T (sampling time) as the specified time. Let output impedance of sensor
equivalent circuit be R0, internal resistance of microcomputer be R, precision (error) of the A/D converter be X,
and the resolution of A/D converter be Y (Y is 1024 in the 10-bit mode, and 256 in the 8-bit mode).
Figure 28.10 shows the Analog Input Pin and External Sensor Equivalent Circuit. When the difference between
VIN and VC becomes 0.1LSB, we find impedance R0 when voltage between pins VC changes from 0 to VIN-
(0.1/1024) VIN in time T. (0.1/1024) means that A/D precision drop due to insufficient capacitor charge is held to
0.1LSB at time of A/D conversion in the 10-bit mode. Actual error however is the value of absolute precision
added to 0.1LSB.
T = 0.8 µ s when φ AD = 20 MHz. Output impedance R0 for sufficiently charging capacitor C within time T is
determined as follows.
Thus, the allowable output impedance of the sensor equivalent circuit, making the precision (error) 0.1LSB or less,
is approximately 4.4 k Ω . maximum.
Output Impedance of Sensor under A/D Conversion
VC is generally
And when t = T,
Hence,
T = 0.8 µ s, R = 10 k Ω , C = 6.0 pF, X = 0.1, and Y = 1024. Hence,
R0
Analog Input Pin and External Sensor Equivalent Circuit
=
R0
–
-------------------------------------------------- -
6.0
=
×
–
0.8
VC
10
-------------------
C
Sensor equivalent
circuit
VC
•
–
VIN
–
e
T
×
12
=
ln
–
--------------------------
C R0
10
•
VIN 1 e
X
--- -
Y
=
--------------------------T
C R0
(
6 –
ln
(
–
VIN
1
----------- -
1024
R



0.1
1
+
+
R0
R
–
–
R
)
–
X
--- - VIN
Y
T
10
)
–
=
× 10
--------------------------
C R0
=
ln
(
X
--- -
Y
X
--- -
Y
3
1
=
≈
+
4.4
MCU
VIN 1
R
C (6.0 pF)
× 10
)
R (10 kΩ)
t





3
–
X
--- -
Y


VC
28. A/D Converter
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