HD64F2149YV datasheet(710/1075 Pages) RENESAS

710 / 1075 page

Section 20 A/D Converter

Rev. 3.00 Jan 18, 2006 page 682 of 1044

REJ09B0280-0300

Permissible Signal Source Impedance: H8S/2169 or H8S/2149 analog input is designed so that

conversion precision is guaranteed for an input signal for which the signal source impedance is 5

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

5 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.

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 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

The chip

20 pF

Cin =

15 pF

10 k

Ω

Low-pass

filter

C to 0.1 µF

Sensor output

impedance,

up to 5 k

Ω

Sensor input

Figure 20.11 Example of Analog Input Circuit

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