p2125vps20 Renesas Electronics Corporation., p2125vps20 Datasheet - Page 489

• Manufacturer Part Number: p2125vps20
• Description: 16-bit Single-chip Microcomputer H8s Family / H8s/2100 Series
• Manufacturer: Renesas Electronics Corporation.

17.7
17.7.1
This LSI’s analog input is designed so that the conversion accuracy is guaranteed for an input
signal for which the signal source impedance is 5 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 5 kΩ, charging may be insufficient and it
may not be possible to guarantee the A/D conversion accuracy. However, if a large capacitance is
provided externally in single mode, 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 fluctuation ratio of 5 mV/µs or greater) (see figure 17.6).
When converting a high-speed analog signal or converting in scan mode, a low-impedance buffer
should be inserted.
17.7.2
Adding capacitance results in coupling with GND, and therefore noise in GND may adversely
affect the absolute accuracy. Be sure to make the connection to an electrically stable GND such as
AVSS.
Care is also required to insure that filter circuits do not interfere with digital signals on the
mounting board, so acting as antennas.
Usage Notes
Permissible Signal Source Impedance
Influences on Absolute Accuracy
Sensor input
Sensor output
impedance
up to 5 kΩ
Low-pass
filter C
up to 0.1 µF
Figure 17.6 Example of Analog Input Circuit
This LSI
C
15 pF
in
=
A/D converter equivalent circuit
Rev. 1.00 Sep. 21, 2006 Page 451 of 658
10 kΩ
Section 17 A/D Converter
20 pF
REJ09B0310-0100