st10f272z2 STMicroelectronics, st10f272z2 Datasheet - Page 150

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

Electrical characteristics
25.7.4
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Analog reference pins
The accuracy of the A/D converter depends on how accurate is its analog reference: a noise
in the reference results in at least that much error in a conversion. A low pass filter on the
A/D converter reference source (supplied through pins V
in order to clean the signal, minimizing the noise. A simple capacitive bypassing may be
sufficient in most of the cases; in presence of high RF noise energy, inductors or ferrite
beads may be necessary.
In this architecture, V
circuitry of the A/D converter: there is an effective DC current requirement from the
reference voltage by the internal resistor string in the R-C DAC array and by the rest of the
analog circuitry.
An external resistance on V
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 43. A/D converter input pins scheme
Input Leakage and external circuit
The series resistor utilized to limit the current to a pin (see R
with a large source impedance can lead to a degradation of A/D converter accuracy when
input leakage is present.
V
A
Source
R
S
AREF
EXTERNAL CIRCUIT
R
R
C
R
R
R
C
C
S
F
F
L
SW
AD
P
S
Current Limiter Resistance
Source Impedance
Filter Resistance
Filter Capacitance
Pin Capacitance (two contributions, C
Sampling Capacitance
and V
Sampling Switch Impedance
Channel Selection Switch Impedance
AREF
R
Filter
F
C
F
AGND
could introduce error under certain conditions: for this
Current Limiter
pins represents also the power supply of the analog
R
L
C
P1
P1
and C
INTERNAL CIRCUIT SCHEME
AREF
V
DD
P2
L
)
Selection
Channel
and V
in
R
SW
Figure
C
P2
AGND
Sampling
43), in combination
R
), is recommended
AD
C
S
ST10F272Z2