HD6433044 Datasheet, PDF (553/867 Pages) Hitachi Semiconductor – Hitachi Single-Chip Microcomputer

Digital
output
Ideal A/D
conversion
characteristic
Full-scale
error
Nonlinearity
error
Actual A/D conversion
characteristic
Offset error
FS
Analog input
voltage
Figure 15-10 A/D Converter Accuracy Definitions (2)
8. Allowable Signal-Source Impedance: The analog inputs of the H8/3048 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 scan 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 15-11). To convert
high-speed analog signals or to use scan mode, insert a low-impedance buffer.
9. 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, such as AV_SS.
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.
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