SH7730 Datasheet, PDF (923/1188 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTM RISC engine Family / SH7780 Series

Section 26 A/D Converter
26.7 Usage Notes
26.7.1 Allowable Signal-Source Impedance
For the analog input design of this LSI, conversion accuracy is guaranteed for an input signal with
signal-source impedance of 5 kΩ or less. The specification is for charging input capacitance of the
sample and hold circuit of the A/D converter within sampling time. When the output impedance of
the sensor exceeds 5 kΩ, conversion accuracy is not guaranteed due to insufficient charging. If
large external capacitance is set at conversion in single mode, signal-source impedance is ignored
since input load is only internal input resistance of 3 kΩ. However, an analog signal with large
differential coefficient (5 mV/µs or greater) cannot be followed up because of a low-pass filter
(figure 26.8). When converting high-speed analog signals or converting in scan mode, insert a
low-impedance buffer.
26.7.2 Influence to Absolute Accuracy
By adding capacitance, absolute accuracy may be degraded if noise is on GND because there is
coupling with GND. Therefore, connect electrically stable GND such as AVss to prevent absolute
accuracy from being degraded.
A filter circuit must not interfere with digital signals, or must not be an antenna on a mounting
board.
Output impedance of sensor
to 5 kΩ
Sensor input
Lowpass
filter
(C = 0.1µF)
This LSI
C_in =
15 pF
Equivalent circuit
of A/D converter
3 kΩ
20 pF
Figure 26.8 Analog Input Circuit Example
Rev. 1.00 Sep. 19, 2007 Page 875 of 1136
REJ09B0359-0100