
ST72321xx-Auto Electrical characteristics
Doc ID 13829 Rev 1 217/243
Figure 99. Typical A/D converter application
19.12.1 Analog power supply and reference pins
Depending on the MCU pin count, the package may feature separate V
AREF
and V
SSA
analog power supply pins. These pins supply power to the A/D converter cell and function
as the high and low reference voltages for the conversion.
Separation of the digital and analog power pins allow board designers to improve A/D
performance. Conversion accuracy can be impacted by voltage drops and noise in the event
of heavily loaded or badly decoupled power supply lines (see Section 19.12.2: General PCB
design guidelines).
Figure 97. R
AIN
maximum versus f
ADC
with
C
AIN
=0pF
(1)
Figure 98. Recommended C
AIN
and R
AIN
values
(1)
1. C
PARASITIC
represents the capacitance of the PCB
(dependent on soldering and PCB layout quality) plus
the pad capacitance (3pF). A high C
PARASITIC
value will
downgrade conversion accuracy. To remedy this, f
ADC
should be reduced.
1. This graph shows that, depending on the input signal
variation (f
AIN
), C
AIN
can be increased for stabilization
time and decreased to allow the use of a larger serial
resistor (R
AIN)
.
0
5
10
15
20
25
30
35
40
45
0103070
C
PARASITIC
(pF)
Max. R
AIN
(Kohm)
2 MHz
1 MHz
0.1
1
10
100
1000
0.01 0.1 1 10
f
AIN
(KHz)
Max. R
AIN
(Kohm)
Cain 10 nF
Cain 22 nF
Cain 47 nF
AINx
ST72XXX
V
DD
I
lkg
V
T
0.6V
V
T
0.6V
C
ADC
12pF
V
AIN
R
AIN
10-bit A/D
conversion
2kmax
C
AIN
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