HD64F3039F18 Datasheet, PDF (458/710 Pages) Renesas Technology Corp – Renesas 16-Bit Single-Chip Microcomputer H8 Family / H8/300H Series

Section 13 A/D Converter
Permissible Signal Source Impedance
H8/3039 Group analog input is designed so that conversion precision 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 the A/D conversion precision.
When converting in the single mode, if a large capacitance is provided externally, the input load
will essentially comprise only the internal input resistance of 10 kΩ, and the signal source
impedance is ignored.
However, since 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., voltage regulation 5 mV/µs or greater).
When converting a high-speed analog signal and when performing conversion in the scan mode, a
low-impedance buffer should be inserted.
Influences on Absolute Precision
Adding capacitance results in coupling with GND, and therefore noise in GND may adversely
affect absolute precision. Be sure to make the connection to an electrically stable GND such as
AV_SS.
Care is also required to insure that filter circuits do not communicate with digital signals on the
mounting board, thus acting as antennas.
Sensor output
impedance
Up to 10 kΩ
Sensor input
Low-pass
filter C
Up to 0.1 µF
H8/3039 Group
A/D converter
equivalent circuit
10 kΩ
C_in =
15 pF
20 pF
Note: Values are reference values.
Figure 13.11 Example of Analog Input Circuit
Rev.3.00 Mar. 26, 2007 Page 434 of 682
REJ09B0353-0300