Datasheet

Section 16 A/D Converter
Page 616 of 846 REJ09B0140-0900 Rev. 9.00
Sep 16, 2010
H8S/2215 Group
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.
• Relationship between AVcc, AVss and Vcc, Vss
Set AVss = Vss as the relationship between AVcc, AVss and Vcc, Vss. If the A/D converter is
not used, the AVcc and AVss pins must not be left open. In addition, AN0 and AN1 can be
used only when Vcc = AVcc.
• Vref input range
The analog reference voltage input at the Vref pin set is the range Vref ≤ AVcc.
16.8.4 Notes on Board Design
In board design, digital circuitry and analog circuitry should be as mutually isolated as possible,
and layout in which digital circuit signal lines and analog circuit signal lines cross or are in close
proximity should be avoided as far as possible. Failure to do so may result in incorrect operation
of the analog circuitry due to inductance, adversely affecting A/D conversion values.
Also, digital circuitry must be isolated from the analog input signals (AN0 to AN3 or AN14 to
AN15), analog reference voltage pin (Vref), and analog power supply (AVcc) by the analog
ground (AVss). Also, the analog ground (AVss) should be connected at one point to a stable
digital ground (Vss) on the board.
16.8.5 Notes on Noise Countermeasures
A protection circuit should be connected in order to prevent damage due to abnormal voltage, such
as an excessive surge at the analog input pins (AN0 to AN3 or AN14 to AN15) and analog
reference voltage pin (Vref), between AVcc and AVss, as shown in figure 16.10. Also, the bypass
capacitors connected to AVcc and the filter capacitor connected to analog input pins (AN0 to AN3
or AN14 to AN15) must be connected to AVss.
If a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN3 or AN14
to AN15) are averaged, and so an error may arise. Also, when A/D conversion is performed
frequently, as in scan mode, if the current charged and discharged by the capacitance of the
sample-and-hold circuit in the A/D converter exceeds the current input via the input impedance
(Rin), an error will arise in the analog input pin voltage. Careful consideration is therefore
required when deciding circuit constants.