HD6432351 datasheet(692/1025 Pages) RENESAS

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Section 15 A/D Converter

Rev. 3.00 Sep 15, 2006 page 658 of 988

REJ09B0330-0300

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), analog

reference power supply (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.

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) and analog reference power supply (Vref) should be

connected between AVCC and AVSS as shown in figure 15.7.

Also, the bypass capacitors connected to AVCC and Vref and the filter capacitor connected to AN0

to AN7 must be connected to AVSS.

If a filter capacitor is connected as shown in figure 15.7, 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 the circuit constants.

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