Renesas SH7262 Series, Hardware Design Manual, Page: 24 / 38

SH7262/SH7264 Group Hardware Design Guide
REJ06B0999-0100 Rev. 1.00 Page 24 of 36
Jun. 30, 2010
6.4.2 Analog Pin Protection Circuit
As shown in Figure 15, connect the protection circuit between AVcc and AVss to prevent damage due to an abnormal
voltage, such as an excessive surge at analog input pins (AN0 to AN7) and analog reference voltage (AVREF). The
circuit also is used as the RC filter to minimize the noise. Note that the circuit shown in the figure below is an example.
Consider the circuit constants as appropriate before deciding.
Note: Only analog input pins 0 to 3 (AN0 to AN3) can be used on the SH7262.
SH7264
AVref
AVcc
AN0 to AN7 *
3
AVss
*
1
*
1
*
2
R
in
100
Ω
0.1
μ
F
Notes: Values are for reference.
2. R
in
: Input impedance
3. Only AN0 to AN3 can be used in the SH7262.
0.01
μ
F
10
μ
F
1.
Figure 15 Analog Input Pin Protection Circuit Example
6.4.3 Permissible Signal Source Impedance
The SH7264 analog input is designed to guarantee the conversion accuracy for the input signal when its source
impedance is equal to or less than 5 k
Ω
. This specification is to charge the input capacitance of the A/D Converter
(ADC) sample-and-hold circuit within the sampling time. When the sensor output impedance exceeds 5 k
Ω
, charging
may be insufficient and it may not be possible to guarantee A/D conversion accuracy. If a large capacitance is provided
externally when converting analog to digital in single mode, the input load essentially comprise only the 3k
Ω
internal
input resistance, and the signal source impedance is not required. However, the SH7264 ADC includes a low-pass filter,
it may not be able to handle the analog signal with a large differential coefficient (e.g., 5 mV/µs or greater). Install a
low-impedance buffer when converting a high-speed analog signal or converting signals in scan mode.
Figure 16 shows an example of the analog input circuit.
Sensor input
SH7264
Sensor output impedance
Low-pass filter
C to 0.1
μ
F
Up to 5 k
Ω
Cin =
15 pF
20 pF
3 k
Ω
Note: Values are for reference.
A/D Converter
equivalent circuit
Figure 16 Analog Input Circuit Example