Datasheet

Section 20 A/D Converter
Rev. 2.00 Jul. 04, 2007 Page 453 of 692
REJ09B0309-0200
20.7 Usage Notes
20.7.1 Permissible Signal Source Impedance
This LSI's analog input is designed such that conversion accuracy 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 accuracy.
As a countermeasure, with 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 20.8).
When converting a high-speed analog signal, a low-impedance buffer should be inserted.
20.7.2 Influences on Absolute Accuracy
Adding capacitance results in coupling with GND, and therefore noise in GND may adversely
affect absolute accuracy. 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.
48 pF
10 kΩ
C
in
=
15 pF
Sensor output
impedance
up to 10 kΩ
This LSI
Low-pass
filter C
up to 0.1 µF
Sensor input
A/D converter
equivalent circuit
Figure 20.8 Example of Analog Input Circuit