MPC5646C datasheet(73/114 Pages) FREESCALE

73 / 114 page

MPC5646C Microcontroller Data Sheet, Rev. 3

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

Figure 15. ADC_0 characteristic and error definitions

4.17.1.1

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.

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.

(2)

(1)

(3)

(4)

(5)

Offset Error OSE

Gain Error GE

1 LSB (ideal)

Vin(A) (LSBideal)

(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

code out

1023

1022

1021

1020

1019

1018

1017 1018 1019 1020 1021 1022 1023

1 LSB ideal = VDD_ADC / 1024

MPC5646C Datasheet(PDF) 73 Page - Freescale Semiconductor, Inc