
MB90945 Series
59
7.
About the external impedance of the 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.
Notes on A/D Converter Section
To satisfy the A/D conversion precision standard, consider the relationship between the external impedance
and minimum sampling time and either adjust the resistor value and operating frequency or decrease the
external impedance so that the sampling time is longer than the minimum value.
If the sampling time cannot be sufficient, connect a capacitor of about 0.1
μ
F to the analog input pin.
About the error
The accuracy gets worse as
|
AVRH
AVRL
|
becomes smaller.
R
C
R
C
MB90F946A/947A/
F947/F947A/F949/
F949A
2.4 k
(Max) 36.4 pF (Max)
Analog input circuit model
Note : The values are reference values.
Analog input
Comparator
During sampling : ON
0
5
10
15
20
25
30
35
100
90
80
70
60
50
40
30
20
10
0
MB90F947
MB90F949
0
1
2
3
5
6
7
8
20
18
16
14
12
10
8
6
4
2
0
MB90F947
MB90F949
4
(External impedance
=
0 k
to 100 k
)
(External impedance
=
0 k
to 20 k
)
E
]
Minimum sampling time [
μ
s]
E
]
Minimum sampling time [
μ
s]
The relationship between the external impedance and minimum sampling time