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7. Allowable Signal-Source Impedance
The analog inputs of the H8/3062 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 14.11). To convert
high-speed analog signals or to use scan mode, insert a low-impedance buffer.
8. 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
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.
Digital
output
Ideal A/D
conversion
characteristic
Offset error
Figure 14.10 A/D Converter Accuracy Definitions (2)
Full-scale
error
Nonlinearity
error
Actual A/D conversion
characteristic
Analog input
voltage
.
SS
FS
461
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