HD64F2168 Datasheet, PDF (647/874 Pages) Renesas Technology Corp – Renesas 16-Bit Single-Chip Microcomputer

18.7.3 Setting Range of Analog Power Supply and Other Pins
If conditions shown below are not met, the reliability of this LSI 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 ≤ AVref (n = 0 to 7).
• Relation between AVCC, AVSS and VCC, VSS
For the relationship between AVCC, AVSS and VCC, VSS, set AVSS = VSS, and AVCC =
VCC is not always necessary. If the A/D converter is not used, the AVCC and AVSS pins
must on no account be left open.
• AVref pin reference voltage specification range
The reference voltage of the AVref pin should be in the range AVref ≤ AVCC.
18.7.4 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 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.
18.7.5 Notes on Noise Countermeasures
A protection circuit connected to prevent damage due to an abnormal voltage such as an excessive
surge at the analog input pins (AN0 to AN7) should be connected between AVCC and AVSS as
shown in figure 18.7. Also, the bypass capacitors connected to AVCC and AVref, and the filter
capacitors 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 the circuit
constants.
Rev. 3.00, 03/04, page 607 of 830