HD64F2149 Datasheet, PDF (685/1035 Pages) Renesas Technology Corp – Hitachi 16-Bit Single-Chip Microcomputer

Permissible Signal Source Impedance: H8S/2169 or H8S/2149 analog input is designed so that
conversion precision is guaranteed for an input signal for which the signal source impedance is 5
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
5 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.
But 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/µsec or greater).
When converting a high-speed analog signal, a low-impedance buffer should be inserted.
Influences on Absolute Precision: Adding capacitance results in coupling with GND, and
therefore noise in GND may adversely affect absolute precision. Be sure to make the connection to
an electrically stable GND such as AVSS.
Care is also required to insure that filter circuits do not communicate with digital signals on the
mounting board, so acting as antennas.
Sensor output
impedance,
up to 5 kΩ
Sensor input
Low-pass
filter
C to 0.1 µF
The chip
C_in =
15 pF
A/D converter
equivalent circuit
10 kΩ
20 pF
Figure 20.11 Example of Analog Input Circuit
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