Hardware manual

Microcomputer Technical Q&A
129
A/D converter equivalent circuit
C
AD
≈ 20 pF
R
AD
≈ 10 kΩ
R
out
Low-pass
filter C
Permissible signal
source impedance
Sensor input
AN0 to AN7
Example of Analog Input Circuit
• Low-pass filter
Adding a large capacitance (up to 0.1 µF or so) to the input pins makes sensor output
impedance R
out
essentially negligible in single-mode conversion.
When scan mode is used, however, the internal capacitance must be charged in a short time,
and so error may arise.
Therefore, if permissible signal source impedance R
out
exceeds 5 k and a low-pass filter is
added, use of single mode is recommended, with the following times set for conversion cycle
T
int
.
T
int
> 2 ms (100 k R
out
> 5 k )
T
int
> 4 ms (200 k R
out
> 100 k )
Applicable Products
Applicability Series Applicability Series Applicability Series
Yes Entire H8S Series H8S/2655 H8S/2350
H8S/2355 H8S/2357 H8S/2345
H8S/2245 H8S/2148 H8S/2144
H8S/2138 H8S/2134 H8S/2128
H8S/2124 — — — —