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Section 15 A/D Converter
Figure 15.10 A/D Converter Accuracy Definitions (2)
7. Allowable Signal-Source Impedance: The analog inputs of the H8/3067 Group 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 15.11). 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
Rev. 4.00 Jan 26, 2006 page 590 of 938
REJ09B0276-0400
Digital
output
Ideal A/D
conversion
characteristic
Actual A/D conversion
characteristic
Offset error
Full-scale
error
Nonlinearity
error
FS
Analog input
voltage
.
SS
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