Usage Notes; Module Stop Mode Setting; Permissible Signal Source Impedance; Influences On Absolute Precision - Renesas H8S Series Hardware Manual

Section 16 A/D Converter
16.7

Usage Notes

16.7.1

Module Stop Mode Setting

Operation of the A/D converter can be disabled or enabled using the module stop control register.
The initial setting is for operation of the A/D converter to be halted. Register access is enabled by
clearing module stop mode. For details, refer to section 20, Power-Down Modes.
16.7.2

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/µs or greater) (see figure 16.6). When converting a high-speed
analog signal, a low-impedance buffer should be inserted.
16.7.3

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).
Sensor input
Rev. 6.00 Mar 15, 2006 page 450 of 570
REJ09B0211-0600
Sensor output
impedance
to 5 kΩ
Low-pass
filter C
to 0.1 µF

Figure 16.6 Example of Analog Input Circuit

This LSI
A/D converter
equivalent circuit
10 kΩ
C =
in
15 pF
20 pF