r4f2426 Renesas Electronics Corporation., r4f2426 Datasheet - Page 1013

• Manufacturer Part Number: r4f2426
• Description: 16-bit Single-chip Microcomputer H8s Family / H8s/2400 Series
• Manufacturer: Renesas Electronics Corporation.

17.7.4
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 for conversion in single mode, the input load will essentially comprise only
the internal input resistance of 5 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) (see figure 17.11). When converting a
high-speed analog signal or conversion in scan mode, a low-impedance buffer should be inserted.
Sensor input
Permissible Signal Source Impedance
Sensor output
impedance
R to 5 kΩ
Low-pass
filter
C to 0.1 µF
Figure 17.11 Example of Analog Input Circuit
This LSI
Cin =
15 pF
Equivalent circuit of the A/D converter
Rev. 1.00 Sep. 19, 2008 Page 985 of 1270
10 kΩ
Section 17 A/D Converter
7 pF
REJ09B0466-0100