SH7144 Datasheet, PDF (517/773 Pages) Renesas Technology Corp – Hitachi SuperH RISC engine

15.7 Usage Notes
15.7.1 Module Standby Mode Setting
Operation of the A/D converter can be disabled or enabled using the module standby control
register. The initial setting is for operation of the A/D converter to be halted. Register access is
enabled by clearing module standby mode. For details, refer to section 24, Power-Down Modes.
15.7.2 Permissible Signal Source Impedance
This LSI's analog input is designed such that conversion accuracy is guaranteed for an input signal
for which the signal source impedance is 1 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 1 kΩ, charging may be insufficient and it may not be
possible to guarantee A/D conversion accuracy. With a large capacitance provided externally for
A/D conversion in single mode, the input impedance will essentially comprise only the internal
input resistance of 10 kΩ, and the signal source impedance is ignored. However, as a low-pass
filter effect is obtained in this case, it may not be possible to follow a high speed switching analog
signal (e.g., 5 mV/µs or greater) (see figure 15.6). When converting a high-speed analog signal or
performing conversion in scan mode, a low-impedance buffer should be inserted.
15.7.3 Influences on Absolute Accuracy
Adding capacitance results in coupling with GND, and therefore noise in GND may adversely
affect absolute accuracy. 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 interfere in the accuracy by the digital
signals on the printed circuit board (i.e, acting as antennas).
Sensor output
impedance of
up to 1 k
Sensor input
Low-pass
filter
C = 0.1 F
This LSI
C_in =
20 pF
A/D converter
equivalent circuit
10 k
20 pF
Figure 15.6 Example of Analog Input Circuit
Rev. 2.0, 09/02, page 477 of 732