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Chapter 44 A/D Converter
7.Caution
7. Caution
Tips on using the A/D converter are summarized as follows:
• Power-on sequence
Be sure to turn on the MCU power (Vdd*) before turning on the power to the A/D converter (AVcc, AVRH)
and applying a voltage to the analog input.
• Input impedance of the analog input pin
The A/D converter has a built-in sample hold circuit to receive the voltage present on the analog input pin in
the sample hold capacitor after the activation of an A/D conversion. Therefore, if the analog input external
circuit has a high output impedance, it may happen that analog input voltage fails to get stabilized within the
sampling cycle. For this reason, keep the output impedance of the external circuit sufficiently low.
If the output impedance of the external circuit cannot be kept sufficiently low, lengthen the sampling time
fully.
• As AVRH-AV
SS
decreases the error grows in proportion.