191
CHAPTER 11 8-BIT A/D CONVERTER (
µ
PD780024AS SUBSERIES)
Preliminary User’s Manual U16035EJ1V0UM
(14) Internal equivalent circuit of ANI0 to ANI3 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 11-22 shows the internal equivalent circuit of
the ANI0 to ANI3 pins.
If the impedance of the signal source is high, connect capacitors with a high capacitance to the pins ANI0 to ANI3.
An example of this is shown in Figure 11-23. 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 the scan mode, insert a low-impedance
buffer.
Figure 11-22. Internal Equivalent Circuit of Pins ANI0 to ANI3
C3
C2
R2
R1
C1
ANIn
Remark
n = 0 to 3
Table 11-2. Resistance and Capacitance of Equivalent Circuit (Reference Values)
AV
REF
R1
R2
C1
C2
C3
1.8 V
75 k
Ω
30 k
Ω
8 pF
4 pF
2 pF
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 resistance and capacitance in Table 11-2 are not guaranteed values.
