hd64338023s Renesas Electronics Corporation., hd64338023s Datasheet - Page 455

• Manufacturer Part Number: hd64338023s
• Description: Renesas 8-bit Single-chip Microcomputer Super Low Power Series
• Manufacturer: Renesas Electronics Corporation.

12.7
12.7.1
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 kΩ 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 kΩ, 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 kΩ, 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/µs or greater) (see figure 12.8). When converting a high-speed analog signal, a low-
impedance buffer should be inserted.
12.7.2
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.
Sensor input
Application Notes
Permissible Signal Source Impedance
Influences on Absolute Precision
Sensor output
impedance
Up to 10 kΩ
Low-pass
filter
C to 0.1 µF
Figure 12.8 Analog Input Circuit Example
This LSI
C
15 pF
in
=
Rev. 7.00 Mar 10, 2005 page 413 of 652
A/D converter
equivalent circuit
10 kΩ
Section 12 A/D Converter
REJ09B0042-0700
20 pF