SH7144 Datasheet, PDF (518/773 Pages) Renesas Technology Corp – Hitachi SuperH RISC engine

15.7.4 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 (VANn) during A/D conversion should be in the
range AVss ≤ VANn ≤ AVcc.
• Relationship between AVcc, Avss and Vcc, Vss
The Relationship between AVcc, AVss and Vcc, Vss should be AVcc = Vcc ± 0.3V and Avss
= Vss. If the A/D converter is not used, this relationship should be AVcc = Vcc and Avss =
Vss.
• Setting range of AVref input voltage (only for SH7145)
Set the AVref pin input voltage as AVref ≤ AVcc. If the A/D converter is not used, set the
AVref pin as AVref = AVcc.
15.7.5 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 AN7), 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.
15.7.6 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 AN7), to AVcc and AVss, as shown in
figure 15.7. Also, the bypass capacitors connected to AVcc and the filter capacitor connected to
AN0 to AN7 must be connected to AVss.
If a filter capacitor is connected, the input currents at the analog input pins (AN0 to AN7) 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 (R_in), an error will arise in
the analog input pin voltage. Careful consideration is therefore required when deciding circuit
constants.
Rev. 2.0, 09/02, page 478 of 732