UPD70F3218 Datasheet, PDF (446/884 Pages) NEC – 32-Bit Single-Chip Microcontrollers

CHAPTER 14 A/D CONVERTER
(4) Measures against noise
To keep a resolution of 10 bits, be aware of noise on the AVREF0 and ANI0 to ANI15 pins. The higher the
output impedance of the analog input source, the greater the effect of noise. Therefore, it is recommended to
connect external capacitors as shown in Figure 14-6 to reduce noise.
Figure 14-6. Handling of Analog Input Pins
Reference voltage input
If noise of AVREF0 or higher or AVSS or lower could be
generated, clamp with a diode with a small VF (0.3 V or lower).
AVREF0
C = 100 to 1000 pF
ANI0 to ANI15
AVSS
VSS
(5) ANI0/P70 to ANI15/P715 pins
The analog input pins (ANI0 to ANI15) function alternately as input port pins (P70 to P715).
When performing A/D conversion by selecting any of the ANI0 to ANI15 pins, do not execute an input
instruction to port 7 during conversion. This may decrease the conversion resolution.
If digital pulses are applied to the pin adjacent to the pin subject to A/D conversion, the value of the A/D
conversion may differ from the expected value because of coupling noise. Therefore, do not apply pulses to
the pin adjacent to the pin subject to A/D conversion.
(6) Input impedance of AVREF0 pin
A series resistor string of tens of kΩ is connected between the AVREF0 pin and AVSS pin.
Therefore, if the output impedance of the reference voltage source is high, this will result in a series
connection to the series resistor string between the AVREF0 pin and AVSS pin, resulting in a large reference
voltage error.
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User’s Manual U16889EJ1V0UD