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Figure 16-10 A/D Conversion Precision Definitions (2)
Permissible Signal Source Impedance: H8S/2646 Series analog input is designed so that
conversion precision is guaranteed for an input signal for which the signal source impedance is 10
kΩ or less. This specification is provided to enable the A/D converter's sample-and-hold circuit
input capacitance to be charged within the sampling time; if the sensor output impedance exceeds
10 kΩ, charging may be insufficient and it may not be possible to guarantee the A/D conversion
precision.
However, if a large capacitance is provided externally, the input load will essentially comprise
only the internal input resistance of 10 kΩ, 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/µs or greater).
When converting a high-speed analog signal, a low-impedance buffer should be inserted.
608
Digital output
Ideal A/D conversion
characteristic
Actual A/D conversion
characteristic
Offset error
Full-scale error
Nonlinearity
error
FS
Analog
input voltage
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