229
CHAPTER 12 A/D CONVERTER
User
’
s Manual U14701EJ3V0UD
(12) V
DD1
pin and AV
REF0
pin
Connect a capacitor to the V
DD1
and AV
REF0
pins to minimize conversion errors due to noise. If an A/D conversion
operation has been stopped and then started, the voltage applied to the V
DD1
and AV
REF0
pins becomes unstable,
causing the accuracy of the A/D conversion to drop. To prevent this, also connect a capacitor to the V
DD1
and
AV
REF0
pins.
Figure 12-20 shows an example of connecting capacitors.
Figure 12-20. Example of Connecting Capacitor to V
DD1
and AV
REF0
Pins
Remark
C1, C2: 4.7
μ
F to 10
μ
F (reference value)
C3, C4: 0.01
μ
F to 0.1
μ
F (reference value)
Connect C3 and C4 as close to the pin as possible.
(13) Internal equivalent circuit of ANI0 to ANI9 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 12-21 shows the internal equivalent circuit of
the ANI0 to ANI9 pins.
If the impedance of the signal source is high, connect capacitors with a high capacitance to the ANI0 to ANI9
pins. An example of this is shown in Figure 12-22. 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.
C1
C2
C3
V
DD1
AV
REF0
AV
SS0
C4