h8s-2646 Renesas Electronics Corporation., h8s-2646 Datasheet - Page 647

• Manufacturer Part Number: h8s-2646
• Description: Renesas 16-bit Single-chip Microcomputer H8s Family/h8s/2600 Series
• Manufacturer: Renesas Electronics Corporation.

Permissible Signal Source Impedance: H8S/2646 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.
However, 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., 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.
Sensor input
Sensor output
impedance
to 5 k
Low-pass
filter
C to 0.1 µF
Figure 16.11 Example of Analog Input Circuit
SS
.
H8S/2646 Group
C
15 pF
in
=
Rev. 5.00 Sep 22, 2005 page 621 of 1136
A/D converter
equivalent circuit
10 k
Section 16 A/D Converter
REJ09B0257-0500
20 pF