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2. Allowable Signal-Source Impedance: The analog inputs of the H8/3664 Series are designed to
assure accurate conversion of input signals with a signal-source impedance not exceeding 10
kΩ. The reason for this rating is that it enables the input capacitor in the sample-and-hold
circuit in the A/D converter to charge within the sampling time. If the sensor output impedance
exceeds 10 kΩ, charging may be inadequate and the accuracy of A/D conversion cannot be
guaranteed.
If a large external capacitor is provided in single mode, then the internal 10-kΩ input resistance
becomes the only significant load on the input. In this case the impedance of the signal source
is not a problem.
A large external capacitor, however, acts as a low-pass filter. This may make it impossible to
track analog signals with high dv/dt (e.g. a variation of 5 mV/µs) (figure 16.9). To convert
high-speed analog signals or to use scan mode, insert a low-impedance buffer.
3. Effect on Absolute Accuracy: Attaching an external capacitor creates a coupling with ground,
so if there is noise on the ground line, it may degrade absolute accuracy. The capacitor must be
connected to an electrically stable ground.
If a filter circuit is used, be careful of interference with digital signals on the same board, and
make sure the circuit does not act as an antenna.
Sensor output impedance
Up to 10 kΩ
Sensor
input
Low-pass
filter
Up to 0.1 µF
358
H8/3664 Series
Figure 16.9 Analog Input Circuit (Example)
Cin =
15 pF
Equivalent circuit of
A/D converter
10 kΩ
20 pF
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