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18.7 Usage
Notes
18.7.1
Module Stop Mode Setting
Operation of the A/D converter can be disabled or enabled using the module stop control register.
The initial setting is for operation of the A/D converter to be halted. Register access is enabled by
clearing module stop mode. For details, refer to section 22, Power-Down Modes.
18.7.2
Permissible Signal Source Impedance
This LSI’s analog input is designed so that conversion accuracy is guaranteed for an input signal
for which the signal source impedance is 5 k
Ω
or less. This specification is provided to enable the
input capacitance of the A/D converter’s sample-and-hold circuit to be charged within the
sampling time; if the sensor output impedance exceeds 5 k
Ω
, charging may be insufficient and it
may not be possible to guarantee the A/D conversion accuracy. However, if a large capacitance is
provided externally for conversion in single mode, the input load will essentially comprise only
the internal input resistance of 10 k
Ω
, and the signal source impedance is ignored. However, since
a low-pass filter effect is obtained in this case, it may not be possible to follow an analog signal
with a large differential coefficient (e.g., 5 mV/
µ
s or greater) (see figure 18.6). When converting a
high-speed analog signal or performing conversion in scan mode, a low-impedance buffer should
be inserted.
Equivalent circuit of A/D converter
This LSI
20 pF
Cin =
15 pF
10 k
Ω
Up to 10 k
Ω
Low-pass
filter
C to 0.1
µ
F
Sensor output
impedance
Sensor input
Figure 18.6 Example of Analog Input Circuit
Summary of Contents for H8S/2437
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