7540 Group User
’
s Manual
2-134
APPLICATION
2.8 A-D converter
2.8.4 Notes on A-D converter
Notes on A-D converter are described below.
(1) Analog input pin
Figure 2.8.10 shows the internal equivalent circuit of an analog input. In order to execute the A-D
conversion correctly, to complete the charge to an internal capacitor within the specified time is
required. The maximum output impedance of the analog input source required to complete the
charge to a capacitor within the specified time is as follows;
About 35 k
(at f(X
IN
) = 8 MHz)
When the maximum output impedance exceeds the above value, equip an analog input pin with an
external capacitor of 0.01
μ
F to 1
μ
F between an analog input pin and V
SS
.
Further, be sure to verify the operation of application products on the user side.
G
Reason
An analog input pin includes the capacitor for analog voltage comparison. Accordingly, when
signals from signal source with high impedance are input to an analog input pin, charge and
discharge noise generates. This may cause the A-D conversion/comparison precision to be worse.
(2) Clock frequency during A-D conversion
The comparator consists of a capacity coupling, and a charge of the capacity will be lost if the clock
frequency is too low. Thus, make sure the following during an A-D conversion.
f(X
IN
) is 500 kHz or more
Do not execute the
STP
instruction
Fig. 2.8.10 Connection diagram
ANi (i=0 to 7: 36-pin version
i=0 to 5: 32-pin version)
C1
12 pF(Typical)
Notes 1:
This is a parasitic diode.
2:
Only the selected analog input pin is turned on.
C2
1.5 pF(Typical)
Chopper Amp.
A-D control circuit
Typical voltage
generation circuit
Switch tree,
ladder resistor
1.5 k
(Typical)
V
SS
V
SS
V
CC
V
SS
V
REF
R
(
Note 1
)
SW1
(
Note 2
)
(
Note 1
)