SH7108 Datasheet, PDF (381/560 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer

Section 11 A/D Converter
11.7 Usage Notes
11.7.1 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 18, Power-Down Modes.
11.7.2 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 than 3 kΩ. (For details, see table 11.6.) 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 is 1 kΩ or exceeds 3 kΩ,
charging may be insufficient and it may not be possible to guarantee A/D conversion accuracy.
However, for A/D conversion in single mode with a large capacitance provided externally, the
input load 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 an analog signal with a large differential coefficient (e.g., 5 mV/μs or
greater) (see figure 11.7). When converting a high-speed analog signal or converting in scan
mode, a low-impedance buffer should be inserted.
11.7.3 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 communicate with digital signals on the
mounting board (i.e, acting as antennas).
Sensor output
impedance of
up to 3 kΩ or to 3 kΩ
Sensor input
Low-pass
filter
C to 0.1 μF
This LSI
C_in =
15 pF
A/D converter
equivalent circuit
10 kΩ
20 pF
Figure 11.7 Example of Analog Input Circuit
Rev.1.00 Sep. 18, 2008 Page 347 of 522
REJ09B0069-0100