HD6432138SW datasheet(626/1061 Pages) RENESAS

626 / 1061 page

Section 19 A/D Converter

Rev. 4.00 Jun 06, 2006 page 572 of 1004

REJ09B0301-0400

Permissible Signal Source Impedance: H8S/2138 Group and H8S/2134 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 (Vcc = 4.0 to 5.5 V, when φ ≤ 12 MHz or CKS = 0) 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 (Vcc = 4.0 to 5.5 V,

when

φ ≤ 12 MHz or CKS = 0), 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 AV

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

H8S/2138 Group or

H8S/2134 Group

chip

20 pF

Cin =

15 pF

10 k

Ω

Low-pass

filter

C to 0.1 µF

Sensor output

impedance,

up to 10 k

Ω

Sensor input

Note: Values are reference values.

Figure 19.11 Example of Analog Input Circuit

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