st10f272z2 STMicroelectronics, st10f272z2 Datasheet - Page 151

• Manufacturer Part Number: st10f272z2
• Description: 16-bit Mcu With 256 Kbyte Flash Memory And 20 Kbyte Ram
• Manufacturer: STMicroelectronics

ST10F272Z2
Data about maximum input leakage current at each pin are provided in the Data Sheet
(Electrical Characteristics section). Input leakage is greatest at high operating temperatures,
and in general it decreases by one half for each 10° C decrease in temperature.
Considering that, for a 10-bit A/D converter one count is about 5mV (assuming
V
leads to an error of exactly one count (5mV); if the resistance were 100kΩ the error would
become two counts.
Eventual additional leakage due to external clamping diodes must also be taken into
account in computing the total leakage affecting the A/D converter measurements. Another
contribution to the total leakage is represented by the charge sharing effects with the
sampling capacitance: being C
equal to the conversion rate of a single channel (maximum when fixed channel continuous
conversion mode is selected), it can be seen as a resistive path to ground. For instance,
assuming a conversion rate of 250kHz, with C
obtained (R
channel). To minimize the error induced by the voltage partitioning between this resistance
(sampled voltage on C
must be designed to respect the following relation:
The formula above provides a constraints for external network design, in particular on
resistive path.
A second aspect involving the capacitance network shall be considered. Assuming the three
capacitances C
equivalent circuit reported in Figure 43), when the sampling phase is started (A/D switch
close), a charge sharing phenomena is installed.
Figure 44. Charge sharing timing diagram during sampling phase
In particular two different transient periods can be distinguished (see Figure 44):
●
AREF
A first and quick charge transfer from the internal capacitance C
sampling capacitance C
considering a worst case (since the time constant in reality would be faster) in which
C
are in series, and the time constant is:
P2
= 5 V), an input leakage of 100nA acting though an R
is reported in parallel to C
V
V
V
V
EQ
CS
A
A2
A1
= 1 / f
F
1
, C
P1
C
Voltage Transient on C
C
and C
S
S
) and the sum of R
, where f
V A
P2
2
S
⋅
R S
----------------------------------------------------------------------------- -
initially charged at the source voltage V
occurs (C
S
τ 1
substantially a switched capacitance, with a frequency
C
+
=
P1
represents the conversion rate at the considered
R F
(
R SW
(call C
+
S
R L
R EQ
S
is supposed initially completely discharged):
S
+
+
P
+ R
R AD
T
R SW
= C
S
S
equal to 4pF, a resistance of 1MΩ is
F
)
P1
+ R
∆V < 0.5 LSB
t
⋅
+
---------------------- -
C P
C P C S
+ C
R AD
L
+
+ R
⋅
P2
C S
<
), the two capacitance C
τ
τ
SW
1
2
1
-- - LSB
2
L
< (R
= R
= 50kΩ of external resistance
+ R
L
SW
Electrical characteristics
(C
AD
S
+ R
, the external circuit
P1
+ C
A
AD
and C
(refer to the
P1
) C
+ C
S
<< T
P2
P2
)
to the
S
P
and C
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S