
Analog / digital converter
UM0404
DocID13284 Rev 2
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 165. A/D converter input pins scheme
Input leakage and external circuit
The series resistor utilized to limit the current to a pin (see R
L
), 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 datasheet
(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
= 50k
Ω
of external resistance leads to an
error of exactly one count (5mV); if the resistance were 100k
Ω
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
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,
R
F
C
F
R
S
R
L
R
SW
C
P2
C
S
V
DD
Sampling
Source
Filter
Current Limiter
EXTERNAL CIRCUIT
INTERNAL CIRCUIT SCHEME
RS Source Impedance
RF Filter Resistance
CF Filter Capacitance
RL Current Limiter Resistance
RSW Channel Selection Switch
Impedance
C
P1
R
AD
Channel
Selection
V
A