R8C/38T-A Group
23. A/D Converter
Under development Preliminary document
Specifications in this document are tentative and subject to change.
R01UH0241EJ0010 Rev.0.10
Page 564 of 730
Aug 05, 2011
23.9
Output Impedance of Sensor during A/D Conversion
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).
VC is generally
And when t = T,
Hence,
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. The 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.
Hence,
Thus, the allowable output impedance of the sensor equivalent circuit, making the precision (error) 0.1 LSB or less,
is approximately TBD k
. maximum.
Figure 23.11
Analog Input Pin and External Sensor Equivalent Circuit
VC
VIN 1
e
1
CR0
R
+
--------------------------
–
t
–
=
VC
VIN
X
Y
---- VIN
VIN 1
X
Y
----
–
=
–
=
e
1
CR0
R
+
--------------------------T
–
X
Y
----
=
1
CR0
R
+
--------------------------
–T
X
Y
----
ln
=
R0
T
C
X
Y
----
ln
-------------------
–R
–
=
R0
TBD
0.1
1024
------------
ln
–TBD
==
R0
R (TBD k
)
C (TBD pF)
VIN
MCU
Sensor equivalent
circuit
VC