mpc5645s Freescale Semiconductor, Inc, mpc5645s Datasheet - Page 93

• Manufacturer Part Number: mpc5645s
• Description: Mpc5645s Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

A second aspect involving the capacitance network shall be considered. Assuming the three capacitances C
initially charged at the source voltage V
is installed when the sampling phase is started (A/D switch close).
In particular two different transient periods can be distinguished:
Freescale Semiconductor
•
A first and quick charge transfer from the internal capacitance C
is supposed initially completely discharged): considering a worst case (since the time constant in reality would be
faster) in which C
and the time constant is
V
A
Source
R
R
C
R
R
R
C
C
R
S
F
F
L
SW
AD
P
S
S
V
P2
V
V
V
CS
A
A2
A1
EXTERNAL CIRCUIT
Source Impedance
Filter Resistance
Filter Capacitance
Current Limiter Resistance
Channel Selection Switch Impedance (two contributions R
Sampling Switch Impedance
Pin Capacitance (three contributions, C
Sampling Capacitance
is reported in parallel to C
Figure 14. Input Equivalent Circuit (Extended Channels)
Figure 15. Transient Behavior during Sampling Phase
1
R
Filter
Voltage Transient on C
F
MPC5645S Microcontroller Data Sheet, Rev. 6
C
A
F
(refer to the equivalent circuit reported in
2
Current Limiter
R
L
P1
(call C
P1
S
, C
T
S
C
P2
P1
P
and C
V <0.5 LSB
t
= C
P3
P1
)

V
P1
INTERNAL CIRCUIT SCHEME
1
DD
+ C
SW1

< (R
2
and C
= R
P2
and R
Selection
SW
Channel
), the two capacitances C
L

R
(C
P2
SW1
SW2
+ R
S
C
Figure
to the sampling capacitance C
+ C
P3
)
AD
P1
) C
+ C
Extended
13): A charge sharing phenomenon
Switch
S
R
P2
<< T
SW2
)
C
P2
S
Electrical characteristics
Sampling
R
P
AD
and C
C
F
S
, C
P1
S
are in series,
S
and C
occurs (C
P2
are
93
S