
ST10F296E
Electrical characteristics
Input leakage and external circuit
The series resistor used to limit the current to a pin (see RL in Figure 99), in combination with a large source of impedance, can lead to a degradation of the ADC accuracy when
input leakage is present.
Data about maximum input leakage current at each pin is provided in
Section 24.5: DCcharacteristics. Input leakage is greatest at high operating temperatures and generally
decreases by one half a degree for each 10 °C decrease in temperature.
Considering that one count of a 10-bit ADC is about 5 mV (assuming VAREF = 5 V), an input
leakage of 100 nA acting though an RL = 50 k of external resistance, leads to an error of
exactly one count (5 mV). If the resistance is 100 k
, the error is two counts (10 mV).
Additional leakage due to external clamping diodes must also be taken into account in
computing the total leakage affecting the ADC measurements. Another contribution to the
total leakage is represented by the charge sharing effects with the sampling capacitance.
The sampling capacitance, CS, is essentially a switched capacitance with a frequency equal
to the conversion rate of a single channel (maximum when the fixed channel continuous
conversion mode is selected). It can be seen as a resistive path to ground. For instance,
assuming a conversion rate of 250 kHz and a CS of 4 pF, a resistance of 1 M is obtained
(REQ = 1/fCCS, where fC represents the conversion rate at the considered channel). To
minimize the error induced by the voltage partitioning between this resistance (sampled
voltage on CS) and the sum of RS + RF + RL + RSW + RAD, the external circuit must be
designed to respect the following relation:
Equation 25
Equation 25 places constraints on the external network design, in particular on the resistive
path.
A second aspect of the capacitance network must be considered. Assuming the three
capacitances, CF, CP1 and CP2, are initially charged at the source voltage VA (see
Figure 99), when the sampling phase is started (ADC switch closed), a charge-sharing
Figure 100. Charge sharing timing diagram during sampling phase
Two different transient periods can be distinguished in
Figure 100. They are described
below.
V
A
R
S
R
F
R
L
R
SW
R
AD
++
+
() R
EQ
×
12
()LSB
<
VA
VA1
VA2
t
TS
VCS
Voltage transient on CS
V < 0.5 LSB
1
2
τ 1 < (RSW + RAD) CS << TS
τ 2 = RL (CS + CP1 + CP2)