Datasheet

78K0/Kx2-L CHAPTER 12 A/D CONVERTER
R01UH0028EJ0400 Rev.4.00 431
Sep 27, 2010
Figure 12-22. Analog Input Pin Connection
Reference
voltage
input
C = 100 to 1,000 pF
If there is a possibility that noise equal to or higher than AV
REF or
equal to or lower than AV
SS may enter, clamp with a diode with a
small V
F value (0.3 V or lower).
AV
REF
AVSS
ANI0 to ANI10
V
SS
(5) ANI0/P20 to ANI7/P27, ANI8/P10 to ANI10/P12, and ANI8/P70 to ANI10/P72
<1> The analog input pins (ANI0 to ANI7 and ANI8 to ANI10) are also used as digital I/O port pins (P20 to P27 and
P10 to P12). When A/D conversion is performed with any of ANI0 to ANI7 and ANI8 to ANI10 selected, do not
access P20 to P27 while conversion is in progress; otherwise the conversion resolution may be degraded.
<2> To use the ANI0/P20 to ANI7/P27 and ANI8/P10 to ANI10/P12 pins for digital I/O port, it is recommended to use
starting with the furthest pin from AV
REF (for example, the ANI0/P20 pin in the 78K0/KC2-L). To use these pins
as analog input, it is recommended to use starting with the closest pin to AV
SS (for example, the ANI7/P27 pin in
the 78K0/KC2-L (44-pin and 48-pin products)).
<3> If a digital pulse is applied to the pins adjacent to the pins currently used for A/D conversion, the expected value
of the A/D conversion may not be obtained due to coupling noise. Therefore, do not apply a pulse to the pins
adjacent to the pin undergoing A/D conversion.
(6) Input impedance of ANI0 to ANI10 pins
This A/D converter charges a sampling capacitor for sampling during sampling time.
Therefore, only a leakage current flow 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, it is recommended to keep the output impedance of the analog
input source to within 10 kΩ, and to connect a capacitor of about 100 pF to the ANI0 to ANI10 pins (refer to Figure
12-22).
(7) AV
REF pin input impedance
A series resistor string of several tens of kΩ is connected between the AV
REF and AVSS pins.
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 AV
REF and AVSS pins, resulting in a large reference voltage error.