
DocID13284 Rev 2
385/564
UM0404
Analog / digital converter
Figure 164. A/D conversion characteristic
19.5.2
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
AREF
and V
AGND
), is recommended
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
AREF
and V
AGND
pins represents also the power supply of the analog
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
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.
19.5.3
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,
(2)
(1)
(3)
(4)
(5)
Offset Error OFS
Offset Error OFS
Gain Error GE
1 LSB (ideal)
VAIN (LSBIDEAL)
Digital
Out
3FF
3FE
3FD
3FC
3FB
3FA
005
004
003
002
001
000
007
006
(1) Example of an actual transfer
curve
(2) The ideal transfer curve
(3) Differential Non-Linearity Error
(DNL)
(4) Integral Non-Linearity Error (INL)
1
3
5
7
1024
1022
1020
1018
2
4
6
(6)
(7)
Bisector Characteristic
Ideal Characteristic