Datasheet

Section 19 A/D Converter
Rev. 4.00 Jun 06, 2006 page 572 of 1004
REJ09B0301-0400
Permissible Signal Source Impedance: H8S/2138 Group and H8S/2134 Group analog input is
designed so that conversion precision is guaranteed for an input signal for which the signal source
impedance is 10 kΩ (Vcc = 4.0 to 5.5 V, when φ ≤ 12 MHz or CKS = 0) 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 (Vcc = 4.0 to 5.5 V,
when φ ≤ 12 MHz or CKS = 0), 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 AV
SS
.
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/2138 Group or
H8S/2134 Group
chip
20 pF
C
in
=
15 pF
10 kΩ
Low-pass
filter
C to 0.1 µF
Sensor output
impedance,
up to 10 kΩ
Sensor input
Note: Values are reference values.
Figure 19.11 Example of Analog Input Circuit