Recommendation for Hardware Setup 32-Bit FR81S Family
Document No. 002-09375 Rev. *B
3
2.5
Analog Input Pins
Because the ADC works with an internal sample capacitor (Cadc) you must be aware of the time needed to fully charge
this capacitor to the corresponding analog signal source voltage level by the end of the sample time.
When the external impedance is too high, the sampling period for analog voltages may not be sufficient. In this case, it
is recommended to connect an external capacitor (Cext, approx. 0.1 μF) to the analog input pin.
An input impedance maximum (Rext) 15k Ohm is recommended. So, an appropriate sample time has to be selected
depending on the impedance Rext and the capacitance Cext.
Please refer to Datasheet “A/D Converter” chapter for further information.
Figure 2. Analog Input Circuit Model
AN1 pin
Rext
Analog
Signal
Source #1
Cext
AN2 pin
Rext
Analog
Signal
Source #2
Cext
ANn pin
Rext
Analog
Signal
Source #n
Cext
Cpad
Cpad
Cpad
Analog SW
(Closed during sample)
Analog SW
Analog SW
Radc
Cadc
ADC
MCU
Table 1. Reference Values
Component
Value
AVcc
Cadc
8.30 pF (Max)
4.5
V… 5.5 V
Cadc
8.30 pF (Max)
3
V… 3.6 V
Radc
1.9K (Max)
4.5
V… 5.5 V
Radc
4.3K (Max)
3
V… 3.6 V
Cpin
5pF..15pF
