
Electrical characteristics
MPC5604B/C Microcontroller Data Sheet, Rev. 4
Freescale Semiconductor
55
A second aspect involving the capacitance network shall be considered. Assuming the three capacitances CF, CP1 and CP2 are
initially charged at the source voltage VA (refer to the equivalent circuit in Figure 27): A charge sharing phenomenon is installed when the sampling phase is started (A/D switch close).
Figure 29. Transient behavior during sampling phase
In particular two different transient periods can be distinguished:
1.
A first and quick charge transfer from the internal capacitance CP1 and CP2 to the sampling capacitance CS occurs (CS
is supposed initially completely discharged): considering a worst case (since the time constant in reality would be
faster) in which CP2 is reported in parallel to CP1 (call CP = CP1 + CP2), the two capacitances CP and CS are in series,
and the time constant is
Eqn. 5
Equation 5 can again be simplified considering only CS as an additional worst condition. In reality, the transient is faster, but the A/D converter circuitry has been designed to be robust also in the very worst case: the sampling time TS
is always much longer than the internal time constant:
Eqn. 6
The charge of CP1 and CP2 is redistributed also on CS, determining a new value of the voltage VA1 on the capacitance
Eqn. 7
2.
A second charge transfer involves also CF (that is typically bigger than the on-chip capacitance) through the resistance
RL: again considering the worst case in which CP2 and CS were in parallel to CP1 (since the time constant in reality
would be faster), the time constant is:
VA
VA1
VA2
t
TS
VCS
Voltage transient on CS
ΔV < 0.5 LSB
1
2
τ
1 < (RSW + RAD) CS << TS
τ
2 = RL (CS + CP1 + CP2)
τ
1
R
SW
R
AD
+
()
=
C
P
C
S
C
P
C
S
+
---------------------
τ
1
R
SW
R
AD
+
()
<
C
S
T
S
V
A1
C
S
C
P1
C
P2
++
()
V
A
C
P1
C
P2
+
()
=
Because
of
an
order
from
the
United
States
International
Trade
Commission,
BGA-packaged
product
lines
and
part
numbers
indicated
here
currently
are
not
available
from
Freescale
for
import
or
sale
in
the
United
States
prior
to
September
2010:
MPC5604BxMG
products
in
208
MAPBGA
packages