H8SXSLASH1663 datasheet(915/1158 Pages) RENESAS

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Section 18 A/D Converter

Rev.1.00 Jun. 07, 2006 Page 863 of 1102

REJ09B0294-0100

18.7

Usage Notes

18.7.1

Module Stop State Setting

Operation of the A/D converter can be disabled or enabled using the module stop control register.

The initial setting is for operation of the A/D converter to be halted. Register access is enabled by

clearing the module stop state. For details, refer to section 23, Power-Down Modes.

18.7.2

Permissible Signal Source Impedance

This LSI's analog input is designed so that the conversion accuracy 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 accuracy. However, if a large capacitance is

provided externally for conversion in single mode, 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) (see figure 18.8). When converting a

high-speed analog signal or conversion in scan mode, a low-impedance buffer should be inserted.

Equivalent circuit of the A/D converter

This LSI

20 pF

Cin =

15 pF

10 k

Ω

Low-pass

filter

C = 0.1

µF

(recommended)

Sensor output

impedance

≤ 10 kΩ

Sensor input

Figure 18.8 Example of Analog Input Circuit

18.7.3

Influences on Absolute Accuracy

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

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

AVss.

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

and filter circuits do not act as antennas.

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