Datasheet

Section 19 A/D Converter
R01UH0166EJ0600 Rev. 6.00 Page 925 of 1434
Mar 02, 2011
H8S/2633 Group, H8S/2633 F-ZTAT
TM
,
H8S/2633R F-ZTAT
TM
, H8S/2695
Permissible Signal Source Impedance: H8S/2633 Group analog input is designed so that
conversion precision 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
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 AVSS.
Care is also required to insure that filter circuits do not communicate with digital signals on the
mounting board, so acting as antennas.
A/D converter
equivalent circuit
H8S/2633 Group
20 pF
C
in
=
15 pF
10 kΩ
Up to 5 kΩ
Low-pass
filter C
Up to 0.1 μF
Sensor output
impedance
Sensor input
Figure 19.11 Example of Analog Input Circuit