H8SSLASH2224 datasheet(791/1071 Pages) RENESAS

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Section 17 A/D Converter

Rev. 5.00 Aug 08, 2006 page 705 of 982

REJ09B0054-0500

17.8.4

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 during A/D conversion should be in the range

AVss

≤ ANn ≤ AVcc.

• 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.

• Vref range

The reference voltage input from the Vref pin should be set to AVcc or less.

17.8.5

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.

17.8.6

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), between AVcc and AVss, as shown

in figure 17.10. 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 (Rin), an error will arise in

the analog input pin voltage. Careful consideration is therefore required when deciding circuit

constants.

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