MB95120MB Series
(2) 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. Therefore to satisfy the A/D conversion precision standard, consider the relationship
between the external impedance and minimum sampling time and either adjust the register value and operating
frequency or decrease the external impedance so that the sampling time is longer than the minimum value. Also,
if the sampling time cannot be sufficient, connect a capacitor of about 0.1 μF to the analog input pin.
• Analog input equivalent circuit
R
Analog input
Comparator
C
During sampling : ON
R
C
4.5 V ≤ AVcc ≤ 5.5 V 2.0 kΩ (Max)
4.0 V ≤ AVcc < 4.5 V 8.2 kΩ (Max)
16 pF (Max)
16 pF (Max)
Note : The values are reference values.
• The relationship between external impedance and minimum sampling time
(External impedance = 0 kΩ to 20 kΩ)
(External impedance = 0 kΩ to 100 kΩ)
100
20
18
90
AVCC ≥ 4.5 V
80
16
AVCC ≥ 4.5 V
70
14
60
12
AVCC ≥ 4.0 V
AVCC ≥ 4.0 V
50
10
40
30
20
10
0
8
6
4
2
0
8
14
0
2
4
6
10
12
0
1
2
3
4
Minimum sampling time [μs]
Minimum sampling time [μs]
• About errors
As |AVCC − AVSS| becomes smaller, values of relative errors grow larger.
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