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Section 12 A/D Converter
Rev. 6.00 Aug 04, 2006 page 429 of 680
REJ09B0145-0600
•
In active mode or sleep mode, analog power supply current (AI
STOP1
) flows into the ladder
resistance even when the A/D converter is not operating. Therefore, if the A/D converter is not
used, it is recommended that AV
CC
be connected to the system power supply and the
ADCKSTP (A/D converter module standby mode control) bit be cleared to 0 in clock stop
register 1 (CKSTPR1).
12.6.2
Permissible Signal Source Impedance
This LSI’s analog input is designed such 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 A/D conversion precision. However, a large capacitance
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, as 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/
µ
s or greater) (see figure 12.6). When converting a high-speed analog signal, a low-
impedance buffer should be inserted.
12.6.3
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.
Care is also required to ensure that filter circuits do not interfere with digital signals or act as
antennas on the mounting board.
A/D converter
equivalent circuit
This LSI
20 pF
C
in
=
15 pF
10 k
Ω
Up to 10 k
Ω
Low-pass
filter
C to 0.1
µ
F
Sensor output
impedance
Sensor input
Figure 12.6 Analog Input Circuit Example
Содержание H8/38342
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Страница 76: ...Section 1 Overview Rev 6 00 Aug 04 2006 page 38 of 680 REJ09B0145 0600...
Страница 240: ...Section 7 RAM Rev 6 00 Aug 04 2006 page 202 of 680 REJ09B0145 0600...
Страница 468: ...Section 12 A D Converter Rev 6 00 Aug 04 2006 page 430 of 680 REJ09B0145 0600...
Страница 580: ...Section 15 Electrical Characteristics Rev 6 00 Aug 04 2006 page 542 of 680 REJ09B0145 0600...