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22.8
Usage Notes
22.8.1
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 10 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 22.6).
When converting a high-speed analog signal or converting in scan mode, a low-impedance buffer
should be inserted.
22.8.2
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 communicate with digital signals on the
mounting board, so acting as antennas.
Sensor input
Sensor output
impedance
to 5 kΩ
Low-pass
filter C
Up to 0.1 µF
Figure 22.6 Example of Analog Input Circuit
This LSI
A/D converter equivalent circuit
10 kΩ
C
=
in
15 pF
Rev. 3.00 Jan 25, 2006 page 711 of 872
Section 22 A/D Converter
20 pF
REJ09B0286-0300
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