HD6432621 Hitachi, HD6432621 Datasheet - Page 646

• Manufacturer Part Number: HD6432621
• Description: (HD64F262x Series) 16-Bit Microcomputer
• Manufacturer: Hitachi

Permissible Signal Source Impedance: H8S/2626 Series and H8S/2623 Series analog input is
designed so that conversion precision is guaranteed for an input signal for which the signal source
impedance is 10 k 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 k , 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 k , 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.
Sensor input
602
Sensor output
impedance
to 5 k
Low-pass
filter C
to 0.1 F
Figure 16-11 Example of Analog Input Circuit
H8S/2626 Series or
H8S/2623 Series
C
15 pF
in
=
A/D converter
equivalent circuit
10 k
20 pF