Renesas H8/38024 Series, Hardware Manual

Page: 430 / 661

Rev. 6.00, 08/04, page 400 of 628
12.7 Application Notes
12.7.1 Application Notes
•
Data in ADRRH and ADRRL should be read only when the A/D start flag (ADSF) in the A/D
start register (ADSR) is cleared to 0.
•
Changing the digital input signal at an adjacent pin during A/D conversion may adversely
affect conversion accuracy.
•
When A/D conversion is started after clearing module standby mode, wait for 10
φ
clock
cycles before starting.
•
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).
12.7.2 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 12.8). When converting a high-speed analog signal, a low-
impedance buffer should be inserted.
12.7.3 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.