MB89F202 Datasheet, PDF (34/44 Pages) Fujitsu Component Limited. – 8-Bit Proprietary Microcontroller

MB89202 Series
(3) Notes on Using A/D Converter
• About the external impedance of analog input and its sampling time
• A/D converter with sample and hold circuit. If the external impedance is too high to keep sufficient sampling
time, the analog voltage charged to the internal sample and hold capacitor is insufficient, adversely affecting
A/D conversion precision.
• Analog input circuit model
Analog input
R
↑
C
During sampling : ON
Note : The values are reference values.
Comparator
MB89202
MB89F202
R
2.2 kΩ (Max)
2.0 kΩ (Max)
C
45 pF (Max)
16 pF (Max)
• To satisfy the A/D conversion precision standard, consider the relationship between the external impedance
and minimum sampling time and either adjust the operating frequency or decrease the
external impedance so that the sampling time is longer than the minimum value.
• The relationship between the external impedance and minimum sampling time
[External impedance = 0 kΩ to 100 kΩ]
100
90
80
70
60
50
40
30
20
10
0
0
MB89F202
MB89202
5 10 15 20 25 30 35
Minimum sampling time (µs)
[External impedance = 0 kΩ to 20 kΩ]
MB89F202
20
MB89202
18
16
14
12
10
8
6
4
2
0
01 2 3456 78
Minimum sampling time (µs)
• If the sampling time cannot be sufficient, connect a capacitor of about 0.1 µF to the analog input pin.
• About errors
As |VCC − AVSS| becomes smaller, values of relative errors grow larger.
34