HD6432351 datasheet(696/1025 Pages) RENESAS

696 / 1025 page

Section 15 A/D Converter

Rev. 3.00 Sep 15, 2006 page 662 of 988

REJ09B0330-0300

Permissible Signal Source Impedance

H8S/2350 Group analog input is designed so that conversion precision is guaranteed for an input

signal for which the signal source impedance is 10 k

Ω 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 k

Ω, 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 k

Ω, and the signal source impedance is ignored.

However, 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/µs 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

AVSS.

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

H8/2350 Group

20 pF

Cin =

15 pF

10 k

Ω

to 10 k

Ω

Low-pass

filter

C to 0.1 µF

Sensor output

impedance

Sensor input

Note: Values are reference values.

Figure 15.11 Example of Analog Input Circuit

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