ST10F272B_12 STMICROELECTRONICS [STMicroelectronics], ST10F272B_12 Datasheet - Page 147

• Manufacturer Part Number: ST10F272B_12
• Description: 16-bit MCU with 256 Kbyte Flash memory and 12/20 Kbyte RAM
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

ST10F272B/ST10F272E
The charge of C
voltage V
Equation 4
●
Equation 5
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
obtained:
Equation 6
Of course, R
combination with R
bigger than C
transient) will be much higher than V
balance assuming now C
Equation 7
The two transients above are not influenced by the voltage source that, due to the presence
of the R
C
respect to the sampling time (T
Figure
Calling f
the anti-aliasing filter, f
least 2f
the conversion period (T
T
selected (fastest conversion rate at a specific channel): in conclusion it is evident that the
time constant of the filter R
charge level on C
the sampling switch is closed.
S
S
, which is just a portion of it, even when fixed channel continuous conversion mode is
with respect to the ideal source V
A second charge transfer involves also C
capacitance) through the resistance R
and C
time constant is:
41).
0
F
0
; it means that the constant time of the filter is greater than or at least equal to twice
C
A1
the bandwidth of the source signal (and as a consequence the cut-off frequency of
F
S
on the capacitance according to the following equation:
filter, is not able to provide the extra charge to compensate the voltage drop on
were in parallel to C
L
P1
shall be sized also according to the current limitation constraints, in
, C
P1
V A2 C S C P1 C P2 C F
S
P2
and C
S
cannot be modified by the analog signal source during the time in which
(source impedance) and R
⋅
and C
(
F
), according to Nyquist theorem the conversion rate f
C
V A1
P2
S
). Again the conversion period T
+
10 τ 2
F
already charged at V
S
C
is redistributed also on C
⋅
, then the final voltage V
⋅
F
(
Doc ID 11917 Rev 3
C S
S
is definitively much higher than the sampling time T
+
=
). The filter is typically designed to act as anti-aliasing (see
P1
τ 2 R L
10 R
+
<
(since the time constant in reality would be faster), the
C P1
A1
A
⋅ L
+
; the time constant R
. The following equation must be respected (charge
⋅
+
⋅
(
C S
(
C P2
)
L
C S
=
: again considering the worst case in which C
V A C F
+
F
+
)
C P1
(that is typically bigger than the on-chip
F
A1
⋅
C P1
=
(filter resistance). Being C
):
V A
+
+
+
A2
S
V A1
C P2
⋅
, determining a new value of the
C P2
(
C P1
(at the end of the charge transfer
⋅
C
(
)
C P1 C P2
)
F
is longer than the sampling time
C
≤
+
S
F
, a constraints on R
C P2
T S
+
of the filter is very high with
Electrical characteristics
)
+
C S
)
F
C
definitively
must be at
L
S
sizing is
, so the
147/188
P2