Usage Notes; Permissible Signal Source Impedance; Influences On Absolute Accuracy; Figure 19.6 Example Of Analog Input Circuit - Renesas H8S Series Hardware Manual

19.7

Usage Notes

19.7.1

Permissible Signal Source Impedance

This LSI's analog input is designed so that the conversion accuracy 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 accuracy. However, if a large capacitance is
provided externally in single mode, 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 fluctuation ratio of 5 mV/µs or greater) (see figure 19.6).
When converting a high-speed analog signal or converting in scan mode, a low-impedance buffer
should be inserted.
19.7.2

Influences on Absolute Accuracy

Adding capacitance results in coupling with GND, and therefore noise in GND may adversely
affect the absolute accuracy. 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 interfere with digital signals on the
mounting board, so acting as antennas.
Sensor input
Sensor output
impedance
up to 5 kΩ
Low-pass
filter C
up to 0.1 µF

Figure 19.6 Example of Analog Input Circuit

This LSI
A/D converter equivalent circuit
10 kΩ
C =
in
15 pF
Rev. 3.00 Jul. 14, 2005 Page 729 of 986
Section 19 A/D Converter
20 pF
REJ09B0098-0300