7477 datasheet(273/354 Pages) RENESAS

273 / 354 page

7470/7471/7477/7478 GROUP USER’S MANUAL

2-38

APPLICATION

2.5 A-D converter

7470

7471

7477

7478

2.5.3 Notes on use

s The analog input internal equivalent circuit is shown in Figure 2.5.3. For correct A-D conversion,

it is necessary that the internal capacitor should be completely charged within the specified time.

The maximum value of output impedance of the analog input source required to terminate capacitor

charging within this time is shown below.

• At f(XIN) = 4 MHz, Approximately 10 k

Ω

• At f(XIN) = 8 MHz, Approximately 2 k

Ω

If the maximum value of output impedance exceeds the above value, take a proper measure, for

example, insert a capacitor (0.1

µF to 1 µF) between analog input pin and VSS.

Fig. 2.5.3 Analog input internal equivalent circuit

s For other notes on use, refer to “1.14 A-D converter.”

VREF

VSS

VCC

SW1

(Note 2)

SW2

R=4 k

Ω ±60%

C2=6 pF

±30%

VSS

C1=10 pF

±50 %

Port P2i/INi

(i=0 to 7)

(Note 1)

Amplifier

Reference voltage

generation circuit

Switch tree

Resistor ladder

VREF switch (Note 3)

Notes 1: This is a parasitic diode of the output transistor.

2: SW1 is turned on only when the analog input pin is selected.

3: The VREF switch is not provided in the 7477/7478 group.

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