Datasheet

Section 14 A/D Converter (8 Analog Input Channel Version)
Rev.7.00 Feb. 14, 2007 page 552 of 1108
REJ09B0089-0700
Permissible Signal Source Impedance: The chip’s analog input is designed so that conversion
precision is guaranteed for an input signal for which the signal source impedance is 5 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 5 kΩ,
charging may be insufficient and it may not be possible to guarantee the A/D conversion
precision.
If a large capacitance is provided externally, the input load will essentially comprise only the
internal input resistance of 10 kΩ, and the signal source impedance is ignored. However, 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/μs 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
Chip
20 pFC
in
= 15 pF
10 kΩ
Low-pass
filter
C to 0.1 μF
Sensor output
impedance
Max. 5 kΩ
Sensor input
Note: Values are reference values.
Figure 14.10 Example of Analog Input Circuit