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CHAPTER 13 10-BIT A/D CONVERTER (
µ
PD780354, 780354Y SUBSERIES)
User’s Manual U15798EJ2V0UD
(15) Internal equivalent circuit of ANI0 to ANI7 pins and permissible signal source impedance
To complete sampling within the sampling time with sufficient A/D conversion accuracy, the impedance of the
signal source such as a sensor must be sufficiently low. Figure 13-22 shows the internal equivalent circuit of
the ANI0 to ANI7 pins.
If the impedance of the signal source is high, connect capacitors with a high capacitance to the ANI0 to ANI7
pins. An example of this is shown in Figure 13-23. In this case, however, the microcontroller cannot follow an
analog signal with a high differential coefficient because a lowpass filter is created.
To convert a high-speed analog signal or to convert an analog signal in scan mode, insert a low-impedance buffer.
Figure 13-22. Internal Equivalent Circuit of ANI0 to ANI7 Pins
Remark
n = 0 to 7
Table 13-3. Resistances and Capacitances of Equivalent Circuit (Reference Values)
AV
DD
R1
R2
C1
C2
C3
2.7 V
12 k
Ω
8 k
Ω
8 pF
3 pF
2 pF
4.5 V
4 k
Ω
2.7 k
Ω
8 pF
1.4 pF
2 pF
Caution The resistances and capacitances in Table 13-3 are not guaranteed values.
Figure 13-23. Example of Connection if Signal Source Impedance Is High
Remark
n = 0 to 7
C3
C1
R1
ANIn
R2
C2
C0
ANIn
C0
≤
0.1 F
<Sensor internal circuit>
R0
Output impedance
of sensor
<Microcontroller internal circuit>
R1
R2
C1
C2
C3
Lowpass
filter is created.
µ
