H8S2245 datasheet(566/818 Pages) RENESAS

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Section 14 A/D Converter

Rev.3.00 Mar. 26, 2007 Page 524 of 772

REJ09B0355-0300

Permissible Signal Source Impedance

H8S/2245 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/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

SS.

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

H8/2245 Group

20 pF

Cin =

15 pF

10 k

Ω

Up to 10 k

Ω

Low-pass

filter

C to 0.1 µF

Sensor output

impedance

Sensor input

Note: Values are reference values.

Figure 14.11 Example of Analog Input Circuit

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