M38045F2HKP datasheet(255/387 Pages) RENESAS

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APPLICATION

2.7 A/D converter

Rev.1.00 Jan 14, 2005

REJ09B0212-0100Z

3804 Group (Spec.H)

2.7.4 Notes on A/D converter

(1)

Analog input pin

Make the signal source impedance for analog input low, or equip an analog input pin with an external

capacitor of 0.01

µF to 1 µF. Further, be sure to verify the operation of application products on the

user side.

q Reason

An analog input pin includes the capacitor for analog voltage comparison. Accordingly, when

signals from signal source with high impedance are input to an analog input pin, charge and

discharge noise generates. This may cause the A/D conversion precision to be worse.

(2)

A/D converter power source pin

The AVSS pin is A/D converter power source pins. Regardless of using the A/D conversion function

or not, connect it as following :

• AVSS : Connect to the VSS line

q Reason

If the AVSS pin is opened, the microcomputer may have a failure because of noise or others.

(3)

Clock frequency during A/D conversion

The comparator consists of a capacity coupling, and a charge of the capacity will be lost if the clock

frequency is too low. Thus, make sure the following during an A/D conversion.

• f(XIN) is 500 kHz or more

• Do not execute the STP instruction

(4)

Difference between at 8-bit reading in 10-bit A/D mode and at 8-bit A/D mode

At 8-bit reading in the 10-bit A/D mode, “–1/2 LSB” correction is not performed to the A/D conversion

result.

In the 8-bit A/D mode, the A/D conversion characteristics is the same as 3802 group’s characteristics

because “–1/2 LSB” correction is performed.

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