Datasheet

Section 23 A/D Converter
Rev. 2.00 Aug. 20, 2008 Page 909 of 1198
REJ09B0403-0200
23.7 Usage Notes
23.7.1 Setting of Module Stop Mode
Operation of the A/D converter can be enabled or disabled by setting the module stop control
register. By default, the A/D converter is stopped. Registers of the A/D converter only become
accessible when it is released from module stop mode. See section 28, Power-Down Modes, for
details.
23.7.2 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 (voltage fluctuation ratio of 5 mV/µs or greater for example) (see figure
23.8). When converting a high-speed analog signal or converting in scan mode, a low-impedance
buffer should be inserted.
A/D converter equivalent circuit
This LSI
20 pF
C
in
=
15 pF
10 kΩ
up to 5 kΩ
Low-pass
filter C
up to 0.1 µF
Sensor output
impedance
Sensor input
Figure 23.8 Example of Analog Input Circuit