
246
CHAPTER 13
A/D CONVERTER
User’s Manual U12697EJ4V1UD
(12) Cautions on board design
In order to avoid negative effects from digital circuit noise on the board, analog circuits must be placed as far
away as possible from digital circuits. It is particularly important to prevent analog and digital signal lines from
crossing or coming into close proximity, as A/D conversion characteristics are vulnerable to degradation from
the induction of noise or other such factors.
Connect AVSS and VSS1 to a single stable location on the board.
(13) VDD0 and AVDD pins
The AVDD pin functions as the analog circuit power supply pin and the A/D converter’s reference voltage input
pin, and also supplies power to the input circuits of ANI0/P10 to ANI7/P17.
Connect a capacitor between the VDD0 and AVDD pins to minimize conversion error caused by noise. Note also
that the voltage applied to the VDD0 and AVDD pins following the resumption of A/D conversion after it was stopped
may be unstable, causing a degradation in the accuracy of the A/D conversion. Be sure to connect a capacitor
between the VDD0 and AVDD pins in this case also. An example of capacitor connection is shown in Figure 13-
20 below.
Figure 13-20. Example of Capacitor Connection Between VDD0 and AVDD
Remark
C1, C2: 4.7
F to 10 F (reference values)
C3, C4: 0.01
F to 0.1 F (reference values)
Connect C3 and C4 as close to the pins as possible.
(14) Internal equivalence circuit and allowable signal source impedance of ANI0 to ANI7
In order to complete sampling within the sampling time and obtain a high enough A/D conversion accuracy, it
is necessary to sufficiently reduce the impedance of the sensor and other signal sources. Figure 13-21 shows
the internal equivalence circuit of the ANI0 to ANI7 pins in the microcontroller.
If the impedance of the signal source is high, it can be made to seem smaller by connecting a large capacitance
to the ANI0 to ANI7 pins. A circuit example is shown in Figure 13-22. In this case, because a low pass filter
is configured in the circuit, impedance will no longer be able to follow analog signals with large differential
coefficients.
When converting high-speed analog signals or performing conversion in scan mode, be sure to insert a low-
impedance buffer.
C1
C2
C3
C4
VDD0
AVDD
AVSS