PXD20 Datasheet, PDF (91/130 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

4.16 ADC parameters
Electrical characteristics
The device provides a 10-bit Successive Approximation Register (SAR) Analog to Digital Converter.
1023
1022
1021
1020
1019
1018
Offset Error OSE
Gain Error GE
1 LSB ideal = V_DDA / 1024
(2)
code out
7
6
5
(5)
4
(4)
3
2
(3)
1
1 LSB (ideal)
0
1 23 45 67
Offset Error OSE
(1)
(1) Example of an actual transfer curve
(2) The ideal transfer curve
(3) Differential non-linearity error (DNL)
(4) Integral non-linearity error (INL)
(5) Center of a step of the actual transfer curve
1017 1018 1019 1020 1021 1022 1023
V_in(A) (LSB_ideal)
Figure 13. ADC characteristics and error definitions
4.16.1 Input impedance and ADC accuracy
In the following analysis, the input circuit corresponding to the precise channels is considered.
To preserve the accuracy of the A/D converter, it is 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; furthermore, it sources
charge during the sampling phase, when the analog signal source is a high-impedance source.
PXD20 Microcontroller Data Sheet, Rev. 2
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
91