H8S-2350 RENESAS [Renesas Technology Corp], H8S-2350 Datasheet - Page 696

• Manufacturer Part Number: H8S-2350
• Description: 16-Bit Single-Chip Microcomputer
• Manufacturer: RENESAS [Renesas Technology Corp]

Section 15 A/D Converter
Permissible Signal Source Impedance
H8S/2350 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/µs 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.
Rev. 3.00 Sep 15, 2006 page 662 of 988
REJ09B0330-0300
Sensor input
Note: Values are reference values.
SS
.
Sensor output
impedance
to 10 k
Low-pass
filter
C to 0.1 µF
Figure 15.11 Example of Analog Input Circuit
H8/2350 Group
C
15 pF
in
=
A/D converter
equivalent circuit
10 k
20 pF