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Chapter 15 A/D Converter
XV - 17
To maintain high precision of A/D conversion, following instructions on use of A/D con-
verter should be strictly kept.
1. Input impedance R of A/D input pin should be under 500 kΩ
*1
. And connect the
external capacitor C (over 1000 pF, under 1 µF)
*1
between Vss and the A/D input
pin.
2. Set the A/D conversion frequency depending on the time constant of R and C.
3. Changing the output level of the microcontroller or switching ON/OFF of the periph-
eral added circuit while the A/D conversion is in progress may lower the precision of
A/D conversion, for these may fluctuate the values of the analog input pin and the
power pin. Check the waveform of the analog input pin before the system evaluation.
Equivalent circuit block that
outputs analog signal
1 µF≥C≥1000 pF *1
as R≤500 kΩ
microcontroller
R
C
Vss
A/D input pin
*1 : That value is for reference.
Recommended Connection with A/D Converter