SH7047 Datasheet, PDF (434/764 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTMRISC engine Family/SH7000 Series

13.7 Usage Notes
13.7.1 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.
13.7.2 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 (20 MHz to 25 MHz) or 3 kΩ or less
(20MHz 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 or 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 13.6). When converting a high-speed analog signal or
converting in scan mode, a low-impedance buffer should be inserted.
13.7.3 Influences on Absolute Accuracy
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 interfere in the accuracy by the digital
signals on the printed circuit board (i.e, acting as antennas).
Sensor output
impedance of
up to 3 kΩ or
up to 1 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 13.6 Example of Analog Input Circuit
Rev. 2.00, 09/04, page 394 of 720