mpc5604e Freescale Semiconductor, Inc, mpc5604e Datasheet - Page 40

• Manufacturer Part Number: mpc5604e
• Description: Mpc5604e Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

Electrical characteristics
The two transients above are not influenced by the voltage source that, due to the presence of the R
provide the extra charge to compensate the voltage drop on C
the filter is very high with respect to the sampling time (T
Calling f
according to the Nyquist theorem the conversion rate f
than or at least equal to twice the conversion period (T
which is just a portion of it, even when fixed channel continuous conversion mode is selected (fastest conversion rate at a
specific channel): in conclusion it is evident that the time constant of the filter R
sampling time T
sampling switch is closed.
The considerations above lead to impose new constraints on the external circuit, to reduce the accuracy error due to the voltage
drop on C
voltage on C
40
0
In this case, the time constant depends on the external circuit: in particular imposing that the transient is completed
well before the end of sampling time T
Of course, R
impedance) and R
(at the end of the charge transfer transient) will be much higher than V
balance assuming now C
S
the bandwidth of the source signal (and as a consequence the cut-off frequency of the anti-aliasing filter, f
; from the two charge balance equations above, it is simple to derive
S
:
S
, so the charge level on C
L
shall be sized also according to the current limitation constraints, in combination with R
Anti-Aliasing Filter (f
Analog Source Bandwidth (V
V A2
F
(filter resistance). Being C

S

Figure 12. Spectral representation of input signal
C S C P1 C P2 C F
already charged at V
f
f
0
F
Preliminary—Subject to Change Without Notice
+
MPC5604E Microcontroller Data Sheet, Rev. 2
10  2
F
= RC Filter pole)
S

Noise
cannot be modified by the analog signal source during the time in which the
+
S
=
f
f
A
, a constraints on R
)
10 R L
+

C
C
must be at least 2f
). Again the conversion period T
F
A1
definitively bigger than C

S

):
). The filter is typically designed to act as anti-aliasing.
=

C S
Sampled Signal Spectrum (f
S
V A C F
T
f
2 f
with respect to the ideal source V
F
C
+
 f
0

2 R
f
C P1
0
(Anti-aliasing Filtering Condition)
L
C
(Nyquist)
F
sizing is obtained:
C
f
+
0
F
+
0
V A1
(Conversion Rate vs. Filter Pole)
; it means that the constant time of the filter is greater
C P2

Equation 11

A1


F
C P1 C P2
C
.
T S
P1
Equation 10
C
F
= conversion Rate)
is definitively much higher than the
, C
+
f
C
C
P2
is longer than the sampling time T
and C
between the ideal and real sampled
+
C S
must be respected (charge
A
S
, then the final voltage V

; the time constant R
F
f
C
Freescale Semiconductor
F
filter, is not able to
S
(source
F
Eqn. 10
),
Eqn. 9
F
C
F
A2
of
S
,