
215
CHAPTER 11
A/D CONVERTER
User’s Manual U12790EJ2V0UD
(4) Noise countermeasures
In order to maintain 8-bit resolution, attention must be paid to noise on the AVDD and ANI0 to ANI7 pins. Since
the effect increases in proportion to the output impedance of the analog input source, it is recommended that
a capacitor be connected externally as shown in Figure 11-11 in order to reduce noise.
Figure 11-11. Analog Input Pin Handling
(5) ANI0/P10 to ANI7/P17 pins
The analog input pins ANI0 to ANI7 also function as I/O port pins (port 1). When A/D conversion is performed
with any of pins ANI0 to ANI7 selected, be sure not to execute an instruction that inputs data to port 1 while
conversion is in progress, as this may reduce the conversion resolution.
Also, if digital pulses are applied to a pin adjacent to the pin in the process of A/D conversion, the expected
A/D conversion value may not be obtainable due to coupling noise. Therefore, avoid applying pulses to pins
adjacent to the pin undergoing A/D conversion.
(6) Input impedance of ANI0 to ANI7 pins
In this A/D converter, the internal sampling capacitor is charged and sampling is performed for approx. one
tenth of the conversion time.
Since only the leakage current flows other than during sampling and the current for charging the capacitor
also flows during sampling, the input impedance fluctuates and has no meaning.
To perform sufficient sampling, however, it is recommended to make the output impedance of the analog input
source 10 k
or lower, or attach a capacitor of around 100 pF to the ANI0 to ANI7 pins (see Figure 11-11).
ANI0 to ANI7
AVDD
Analog
voltage input
C = 100 to 1,000 pF
If there is possibility that noise whose
level is AVDD or higher or AVSS or lower may enter,
clamp with a diode with a small VF (0.3 V or less).
VDD
AVSS
VSS