r5f21368sdfp Renesas Electronics Corporation., r5f21368sdfp Datasheet - Page 588

• Manufacturer Part Number: r5f21368sdfp
• Description: Renesas Mcu R8c Family / R8c/3xt-a Series
• Manufacturer: Renesas Electronics Corporation.

Under development
R8C/36T-A Group
R01UH0240EJ0001 Rev.0.01
Apr 28, 2011
23.9
Figure 23.11
To carry out A/D conversion properly, charging the internal capacitor C shown in Figure 23.11 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 23.11 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.
When  AD = TBD MHz, T = TBD  s. Output impedance R0 for sufficiently charging capacitor C within time T is
determined as follows.
T = TBD  s, R = TBD k  , C = TBD pF, X = 0.1, and Y = 1024.
Thus, the allowable output impedance of the sensor equivalent circuit, making the precision (error) 0.1 LSB or less,
is approximately TBD k  . maximum.
VC is generally
And when t = T,
Hence,
Hence,
Output Impedance of Sensor during A/D Conversion
R0
R0
Preliminary document
Specifications in this document are tentative and subject to change.
Analog Input Pin and External Sensor Equivalent Circuit
=
=
–
–
                     
TBD
VC
-------------------
C
VC
e
–
–

--------------------------
C R0
T
ln
TBD

--------------------------T
C R0
=
=


X
--- -
Y
ln
VIN 1 e
1
VIN
–
+
----------- -
1024
Sensor equivalent
circuit
1
0.1
+
R
R
VIN




R
–
T

–
X
--- - VIN
Y
–
=
TBD
–
=
ln
--------------------------
C R0
X
--- -
Y
X
--- -
Y
R0

=
•
=
•
1
VIN 1
TBD
+
R



t
–



MCU
R (TBD k)
C (TBD pF)
X
--- -
Y


VC
23. A/D Converter
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