RX610 Group
23. A/D Converter
R01UH0032EJ0120 Rev.1.20
Page 802 of 1006
Feb 20, 2013
23.6.5
Permissible Impedance of Signal Sources
To realize high-speed conversion 1.0
µ
s, the A/D conversion accuracy is guaranteed only when the impedance of the
signal sources for analog input signals of this LSI circuit is less than or equal to 1.0 kΩ. If a large external capacitance is
provided in the case of conversion in single mode, the input load becomes only the actual internal input resistance (6.5
kΩ), so the impedance of the signal source becomes insufficient. However, since the circuit has become a low-pass filter,
the rapid and large fluctuations in the analog signal produced by differentiation (for example, greater than 5 mV/μs)
become impossible to track (figure 23.15). Include a low-impedance buffer if high-rate analog signals are to be converted
or conversion is to be in scan mode.
This LSI
Equivalent circuit for the A/D converter
Cs
Rs
Figure 23.15 Equivalent Circuit for the Internal Circuit of Analog Input Pins
Table 23.9 Specifications of Analog Pins
Item
Min.
Max.
Unit
Permissible signal-source impedance
1.0
k
Ω
Values in the equivalent circuits for the internal
circuits of pins
Rs
6.5
k
Ω
Cs
6.0
pF
23.6.6
Factors Affecting Absolute Accuracy
Including a capacitor introduces ground coupling. A noisy ground can have a bad effect on absolute accuracy, so be sure
to connect the V
REFL
pins and so on to an electrically stable ground.
Furthermore, a mounted filter circuit can interfere with digital-signal lines on the board, so take care to ensure that the
design does not set up an antenna.