mpc5602d Freescale Semiconductor, Inc, mpc5602d Datasheet - Page 55

• Manufacturer Part Number: mpc5602d
• Description: Mpc5602d 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
when the sampling phase is started (A/D switch close).
In particular two different transient periods can be distinguished:
Freescale Semiconductor
1.
2.
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
Equation 5
faster, but the A/D converter circuitry has been designed to be robust also in the very worst case: the sampling time T
is always much longer than the internal time constant:
The charge of C
according to
A second charge transfer involves also C
R
would be faster), the time constant is:
L
: again considering the worst case in which C
can again be simplified considering only C
Equation
P1
V
P2
V
V
V
CS
and C
A
A2
A1
is reported in parallel to C
7:
1
P2
Figure 14.
is redistributed also on C
Voltage transient on C
V A1
Preliminary—Subject to Change Without Notice
MPC5602D Microcontroller Data Sheet, Rev. 3.1
A
(refer to the equivalent circuit in

2

C S
 1
Transient behavior during sampling phase
 1
F
=
+

(that is typically bigger than the on-chip capacitance) through the resistance
C P1


R SW
R SW
P1
+
P2
(call C
C P2
+
+
S
and C
S
T
R AD
R AD
S
, determining a new value of the voltage V
S

P
as an additional worst condition. In reality, the transient is
=
S
V <0.5 LSB
t
= C


were in parallel to C
V A


C S T S
P1
-------------------- -
C P
C P C S

P1
1

Figure
+ C
«

+

< (R

2
and C
C P1
= R
C S
P2
SW
), the two capacitances C
L

13): A charge sharing phenomenon is installed
(C
+
P2
+ R
S
C P2
to the sampling capacitance C
+ C
AD
P1

P1
) C
+ C
(since the time constant in reality
S
P2
<< T
)
S
Electrical characteristics
P
A1
and C
on the capacitance
F
, C
P1
S
are in series,
S
and C
occurs (C
Eqn. 5
Eqn. 6
Eqn. 7
P2
are
55
S
S