mpc5645s Freescale Semiconductor, Inc, mpc5645s Datasheet - Page 91

• Manufacturer Part Number: mpc5645s
• Description: Mpc5645s Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

4.16.1
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.
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) and the equivalent input impedance of the ADC itself.
Freescale Semiconductor
code out
Input Impedance and ADC Accuracy
1023
1022
1021
1020
1019
1018
Offset Error OSE
7
6
5
4
3
2
1
0
1
2
Figure 12. ADC Characteristics and Error Definitions
3
4
1 LSB (ideal)
MPC5645S Microcontroller Data Sheet, Rev. 6
5
6
(4)
(3)
7
(2)
(5)
V
1017 1018 1019 1020 1021 1022 1023
in(A)
(LSB
ideal
(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
Offset Error OSE
)
1 LSB ideal = V
DDA
Gain Error GE
/ 1024
Electrical characteristics
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