Renesas RL78/G1P Hardware User Manual page 315

RL78/G1P
Reference voltage input
C = 10 pF to 0.1 F
(5) Analog input (ANIn) pins
<1> ANI0 to ANI7 pins are also used as P20 to P27 pins.
When A/D conversion is performed with any of the ANI0 to ANI7 pins selected, do not change the output value
P20 to P27 while conversion is in progress; otherwise the conversion accuracy may be degraded.
<2> If a pin adjacent to the pin whose value is being A/D converted is used as a digital I/O port pin, the A/D
conversion value might differ from the expected value due to coupling noise. To prevent coupling noise, make
sure that pulses whose voltage suddenly change, such as digital pulses, are not input or output to a pin
adjacent to the pin whose value is being A/D converted.
(6) Input impedance of analog input (ANIn) pins
This A/D converter charges a sampling capacitor for sampling during sampling time.
Therefore, only a leakage current flows when sampling is not in progress, and a current that charges the capacitor
flows during sampling. Consequently, the input impedance fluctuates depending on whether sampling is in progress,
and on the other states.
To make sure that sampling is effective, however, we recommend using the converter with analog input sources that
have output impedances no greater than 1 k. If a source has a higher output impedance, lengthen the sampling time
or connect a larger capacitor (with a value of about 0.1 F) to the pin from among ANI0 to ANI7 and ANI16 to which
the source is connected (see Figure 9-44). The sampling capacitor may be being charged while the setting of the
ADCS bit is 0 and immediately after sampling is restarted and so is not defined at these times. Accordingly, the state
of conversion is undefined after charging starts in the next round of conversion after the value of the ADCS bit has
been 1 or when conversion is repeated. Thus, to secure full charging regardless of the size of fluctuations in the
analog signal, ensure that the output impedances of the sources of analog inputs are low or secure sufficient time for
the completion of conversion.
R01UH0895EJ0100 Rev.1.00
Nov 29, 2019
Figure 9-44. Analog Input Pin Connection
If there is a possibility that noise equal to or higher than
AV
REFP
may enter, clamp with a diode with a small V
AV
REFP
ANI0 to ANI7, ANI16
CHAPTER 9 A/D CONVERTER
and V , or equal to or lower than AV
DD
or V
DD
and V ,
REFM SS
value (0.3 V or lower).
F
296