Section 13 A/D Converter (ADC)
Rev. 3.00 Sep. 27, 2007 Page 500 of 758
REJ09B0243-0300
13.7.6
Notes on Noise Countermeasures
A protection circuit should be connected in order to prevent damage due to abnormal voltage, such
as an excessive surge at the analog input pins (AN0 to AN7), between AVcc and AVss, as shown
in figure 13.8. Also, the bypass capacitors connected to AVcc and the filter capacitor connected to
AN0 to AN7 must be connected to AVss.
If a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN7) are
averaged, and so an error may arise. Also, when A/D conversion is performed frequently, as in
scan mode, if the current charged and discharged by the capacitance of the sample-and-hold circuit
in the A/D converter exceeds the current input via the input impedance (R
in
), an error will arise in
the analog input pin voltage. Careful consideration is therefore required when deciding circuit
constants.
AV
cc
*
1
AN0 to AN7
AV
ss
R
in
*
2
100
Ω
0.1 µF
0.01 µF
10 µF
Notes: Values are reference values.
1.
2. Rin: Input impedance
Figure 13.8 Example of Analog Input Protection Circuit
Table 13.7 Analog Pin Specifications
Item Min.
Max.
Unit
Condition
Analog input capacitance
20 pF
Permissible signal source impedance
3 k
Ω
Conversion
time
≥
4.0
µ
s
1 k
Ω
Conversion
time
<
4.0
µ
s
Содержание SH7124 R5F7124
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