HD6473847RHV Datasheet, PDF (355/546 Pages) Renesas Technology Corp – Renesas 8-Bit Single-Chip Microcomputer H8 Family / H8/300L Super Low Power Series

12.7 Usage Notes
Section 12 A/D Converter
12.7.1 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, a large capacitance can be provided externally to the analog input pin. This
will cause the actual input resistance to comprise only the internal input resistance of 10 k ,
allowing the signal source impedance to be ignored. This countermeasure has the disadvantage of
creating a low-pass filter from the signal source impedance and capacitance, with the result that it
may not be possible to follow analog signals having a large differential coefficient (e.g., 5 mV/μs
or greater) (see figure 12.7). When converting a high-speed analog signal, a low-impedance buffer
should be inserted.
12.7.2 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
20 pF
Figure 12.7 Example of Analog Input Circuit
Rev. 7.00 Mar. 08, 2010 Page 323 of 510
REJ09B0024-0700