Permissible Signal Source Impedance: H8S/2148 Series and H8S/2144 Series 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.
But 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/µsec 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.
H8/3437 Series
chip
A/D converter
equivalent circuit
Sensor output
impedance,
up to 10 kΩ
10 kΩ
Sensor input
Cin
15 pF
=
20 pF
Low-pass
filter C,
up to 0.1 µF
Note: Figures are reference values.
Figure 15.10 Example of Analog Input Circuit
354
HITACHI [ HITACHI SEMICONDUCTOR ]
