243
CHAPTER 12 8-BIT A/D CONVERTER (
µ
PD780344, 780344Y 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 12-18 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 12-19. 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 the scan mode, insert a low-impedance
buffer.
Figure 12-18. Internal Equivalent Circuit of ANI0 to ANI7 Pins
Remark
n = 0 to 7
Table 12-3. 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 12-3 are not guaranteed values.
Figure 12-19. Example of Connection if Signal Source Impedance Is High
Remark
n = 0 to 7
C3
C2
R2
R1
C1
ANIn
C3
C2
R2
R1
<Sensor internal circuit>
<Microcontroller internal circuit>
R0
C0
≤
0.1 F
ANIn
C1
C0
Lowpass filter
is created.
Output impedance
of sensor
µ
