Datasheet

Section 20 A/D Converter

Rev. 4.00 Sep 27, 2006 page 634 of 1130

REJ09B0327-0400

Permissible Signal Source Impedance

Analog input in this LSI is designed so that conversion precision is guaranteed for an input signal

for which the signal source impedance is 10 kΩ (AVcc = 4.0 to 5.5 V, when φ ≤ 12 MHz) or less.

This specification is provided to enable the A/D converter’s sample-and-hold circuit input

capacitance to be charged within the sampling time; if the sensor output impedance exceeds 10 kΩ

(AVcc = 4.0 to 5.5 V, when φ ≤ 12 MHz), charging may be insufficient and it may not be possible

to guarantee the A/D conversion precision.

However, if a large capacitance is provided externally, the input load will essentially comprise

only the internal input resistance of 10 kΩ, and the signal source impedance is ignored.

But since a low-pass filter effect is obtained in this case, it may not be possible to follow an analog

signal with a large differential coefficient (e.g., 5 mV/µsec or greater).

When converting a high-speed analog signal, a low-impedance buffer should be inserted.

Influences on Absolute Precision

Adding capacitance results in coupling with GND, and therefore noise in GND may adversely

affect absolute precision. Be sure to make the connection to an electrically stable GND such as

Care is also required to insure that filter circuits do not communicate with digital signals on the

mounting board, so acting as antennas.

A/D converter

equivalent circuit

This LSI

20 pF

15 pF

10 kΩ

Low-pass

filter

C to 0.1 µF

Sensor output

impedance,

up to 10 kΩ

Sensor input

Note: Values are reference values.

Figure 20.11 Example of Analog Input Circuit