Datasheet

Section 16 A/D Converter
REJ09B0140-0900 Rev. 9.00 Page 615 of 846
Sep 16, 2010
H8S/2215 Group
16.8 Usage Notes
16.8.1 Permissible Signal Source Impedance
This LSI's analog input is designed such 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 A/D conversion precision. However, for A/D conversion in single mode with
a large capacitance provided externally, the input load will essentially comprise only the internal
input resistance of 10 kΩ, and the signal source impedance is ignored. However, as 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/Ωs or greater) (see figure 16.9). When converting a high-speed
analog signal, a low-impedance buffer should be inserted.
16.8.2 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 (i.e., acting as antennas).
20 pF
10 kΩ
C
in
=
15 pF
Sensor output
impedance
to 5 kΩ
This LSI
Low-pass
filter C
to 0.1 μF
Sensor input
A/D converter
equivalent circuit
Figure 16.9 Example of Analog Input Circuit