Section 12 A/D Converter
Rev. 7.00 Mar 10, 2005 page 413 of 652
REJ09B0042-0700
12.7 Application Notes
12.7.1 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 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 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 kΩ
Up to 10 kΩ
Low-pass
filter
C to 0.1 µF
Sensor output
impedance
Sensor input
Figure 12.8 Analog Input Circuit Example
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