Section 18 A/D Converter
18.6
Usage Notes
18.6.1
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 18.6). When converting a high-speed
analog signal or converting in scan mode, a low-impedance buffer should be inserted.
18.6.2
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.
This LSI
A/D converter
Sensor output
impedance
equivalent circuit
10 kΩ
up to 5 kΩ
Sensor input
Cin
15 pF
=
Low-pass
filter
20 pF
C to 0.1 µF
Figure 18.6 Analog Input Circuit Example
Rev. 3.00 Sep. 10, 2007 Page 378 of 528
REJ09B0216-0300
RENESAS [ RENESAS TECHNOLOGY CORP ]
