Datasheet

Section 14 A/D Converter
Rev.4.00 Nov. 02, 2005 Page 214 of 304
REJ09B0143-0400
14.6 Usage Notes
14.6.1 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 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 A/D conversion accuracy. However, for A/D conversion in single mode 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 14.6). When converting a high-speed
analog signal or converting in scan mode, a low-impedance buffer should be inserted.
14.6.2 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.
20 pF
10 kΩ
C
in
=
15 pF
Sensor output
impedance
to 5 kΩ
This LSI
Low-pass
filter
C to 0.1 µF
Sensor input
A/D converter
equivalent circuit
Figure 14.6 Analog Input Circuit Example