H8S2245 datasheet(562/818 Pages) RENESAS

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Section 14 A/D Converter

Rev.3.00 Mar. 26, 2007 Page 520 of 772

REJ09B0355-0300

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 AN3), analog

reference power supply (V

ref), and analog power supply (AVCC) by the analog ground (AVSS). Also,

the analog ground (AV

SS) 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 AN3) and analog reference power supply (V

ref

) should be

connected between AV

CC and AVSS as shown in figure 14.7.

Also, the bypass capacitors connected to AV

CC and Vref and the filter capacitor connected to AN0

to AN3 must be connected to AV

SS.

If a filter capacitor is connected as shown in figure 14.7, the input currents at the analog input pins

(AN0 to AN3) 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

in), an error will arise in the analog input pin voltage. Careful consideration is therefore required

when deciding the circuit constants.

H8S2245 Datasheet(PDF) 562 Page - Renesas Technology Corp