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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).
Sensor input
16.8.3
Range of Analog Power Supply and Other Pin Settings
If the conditions below are not met, the reliability of the device may be adversely affected.
• Analog input voltage range
The voltage applied to analog input pin ANn during A/D conversion should be in the range
AVSS ≤ ANn ≤ Vref.
Sensor output
impedance
to 5 k
Low-pass
filter
C to 0.1 F
Figure 16.9 Example of Analog Input Circuit
This LSI
A/D converter
equivalent circuit
10 k
C
=
in
15 pF
Rev. 3.0, 10/02, page 547 of 686
20 pF
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