Hitachi F-ZTAT H8/3039 Series Hardware Manual page 440

Permissible Signal Source Impedance: H8/3039 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.
When converting in the single mode, 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., voltage regulation 5 mV/µs or greater).
When converting a high-speed analog signal and when performing conversion in the scan mode, a
low-impedance buffer should be inserted.
Digital output
Ideal A/D conversion
characteristic
Actual A/D conversion
characteristic
Offset error
Figure 13-10 A/D Conversion Precision Definitions (2)
Full-scale error
Nonlinearity
error
FS
Analog
input voltage
431