External Sensor - Renesas M16C/64C User Manual

M16C/64C Group
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,
VC = VIN 1 e
When t = T,
VC =
–
e
− − − − − − − − − − − − − − −
–
C R0
=
Therefore, R0 –
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,
0.75
=
− − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
R0 –
×
6.0
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
R01UH0092EJ0110 Rev.1.10
Jul 31, 2012
1
⎧ ⎫
– ------------------------- - t
( )
⎨ C R0 + R ⎬
–
⎩ ⎭
⎛
X
VIN – -- -VIN = VIN 1 –
⎝
Y
1
-------------------- -
T
X
( )
C R0 + R
= -- -
Y
1
X
T = ln -- -
( )
+ R
Y
T
− − − − − − − − − − −
– R
X
•
C ln -- -
Y
× 6 –
10
×
– 10 10
0.1
– 12 •
10 ln ----------- -
1024
Sensor equivalent circuit
R0
VIN
⎞
X
-- -
⎠
Y
3 ≈ × 3
3.5 10
MCU
R (10 k Ω )
C (6.0 pF)
27. A/D Converter
VC
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