ST10F271 Datasheet, PDF (136/173 Pages) STMicroelectronics – 16-bit MCU with 128 Kbyte Flash memory and 12 Kbyte RAM

Electrical characteristics
ST10F271
An external resistance on V_AREF could introduce error under certain conditions: for this
reasons, series resistance are not advisable, and more in general any series devices in the
filter network should be designed to minimize the DC resistance.
Analog Input pins
To improve the accuracy of the A/D converter, it is definitively necessary that analog input
pins have low AC impedance. Placing a capacitor with good high frequency characteristics
at the input pin of the device, can be effective: the capacitor should be as large as possible,
ideally infinite. This capacitor contributes to attenuating the noise present on the input pin;
besides, it sources charge during the sampling phase, when the analog signal source is a
high-impedance source.
A real filter, can typically be obtained by using a series resistance with a capacitor on the
input pin (simple RC Filter). The RC filtering may be limited according to the value of source
impedance of the transducer or circuit supplying the analog signal to be measured. The filter
at the input pins must be designed taking into account the dynamic characteristics of the
input signal (bandwidth).
Figure 40. A/D converter input pins scheme
EXTERNAL CIRCUIT
Source
R_S
Filter
R_F
Current Limiter
R_L
INTERNAL CIRCUIT SCHEME
V_DD
Channel
Selection
Sampling
R_SW
R_AD
V_A
C_F
C_P1
C_P2
C_S
R_S Source Impedance
R_F Filter Resistance
C_F Filter Capacitance
R_L Current Limiter Resistance
R_SW Channel Selection Switch Impedance
R_ADSampling Switch Impedance
C_P Pin Capacitance (two contributions, C_P1 and C_P2)
C_S Sampling Capacitance
Input Leakage and external circuit
The series resistor utilized to limit the current to a pin (see R_L in Figure 40), in combination
with a large source impedance can lead to a degradation of A/D converter accuracy when
input leakage is present.
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_AREF = 5V),
an input leakage of 100nA acting though an R_L = 50kΩ of external resistance 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_S substantially a switched capacitance, with a frequency
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