Datasheet

CHAPTER 12 10-BIT SUCCESSIVE APPROXIMATION TYPE A/D CONVERTER (
μ
PD78F041x only)
User’s Manual U18698EJ1V0UD
310
Figure 12-20. Analog Input Pin Connection
R
eference
voltage
input
C
= 100 to 1,000 pF
If there is a possibility that noise equal to or higher than AV
REF
o
r
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
AV
SS
V
SS
ANI0 to ANI5
(5) ANI0/SEG21/P20 to ANI5/SEG16/P25 pins
<1> The analog input pins (ANI0 to ANI5) are also used as I/O port pins (P20 to P25).
When A/D conversion is performed with any of ANI0 to ANI5 selected, do not access P20 to P25 while
conversion is in progress; otherwise the conversion resolution may be degraded. It is recommended to any
pin of P20 to P25 used as digital I/O port starting with the ANI0/P20 that is the furthest from AV
REF.
<2> If a digital pulse is input or output, or segment-output 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 input or output a pulse, or segment-output to the pins adjacent to the pin undergoing A/D
conversion.
(6) Input impedance of ANI0 to ANI5 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, 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 ANI5 pins (see Figure 12-
20).
(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.