HD64F38602R Datasheet, PDF (391/552 Pages) Renesas Technology Corp – Renesas 16-Bit Single-Chip Microcomputer

Section 17 A/D Converter
17.7 Usage Notes
17.7.1
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. However, 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 17.8). When converting a high-speed analog signal, a low-
impedance buffer should be inserted.
17.7.2 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.
Sensor output
impedance
to 10 kΩ
Sensor input
Low-pass
filter
C to 0.1 µF
This LSI
C_in =
15 pF
A/D converter
equivalent circuit
10 kΩ
48 pF
Figure 17.8 Example of Analog Input Circuit
Rev. 3.00 May 15, 2007 Page 359 of 518
REJ09B0152-0300