HD6412240 RENESAS [Renesas Technology Corp], HD6412240 Datasheet - Page 566

• Manufacturer Part Number: HD6412240
• Description: Renesas 16-Bit Single-Chip Microcomputer H8S Family / H8S/2200 Series
• Manufacturer: RENESAS [Renesas Technology Corp]

Section 14 A/D Converter
Permissible Signal Source Impedance
H8S/2245 Group analog input is designed so that conversion precision is guaranteed for an input
signal for which the signal source impedance is 10 k 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 k , charging may be insufficient and it
may not be possible to guarantee the A/D conversion precision.
However, if a large capacitance is provided externally, the input load will essentially comprise
only the internal input resistance of 10 k , 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., 5 mV/µsec or greater).
When converting a high-speed analog signal, 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
Care is also required to insure that filter circuits do not communicate with digital signals on the
mounting board, so acting as antennas.
Sensor input
Note: Values are reference values.
Rev.3.00 Mar. 26, 2007 Page 524 of 772
REJ09B0355-0300
SS
.
Sensor output
impedance
Up to 10 k
Low-pass
filter
C to 0.1 µF
Figure 14.11 Example of Analog Input Circuit
H8/2245 Group
C
15 pF
in
=
A/D converter
equivalent circuit
10 k
20 pF