MPC5644BCAMMJ1 datasheet(72/116 Pages) NXP

72 / 116 page

MPC5646C Microcontroller DataSheet, Rev. 7

Electrical Characteristics

NXP Semiconductors

Figure 15. ADC_0 characteristic and error definitions

4.17.1.1

Input impedance and ADC accuracy

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.

In fact a current sink contributor is represented by the charge sharing effects with the sampling capacitance: being CS and Cp2

substantially two switched capacitances, with a frequency equal to the conversion rate of the ADC, it can be seen as a resistive

(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

MPC5644BCAMMJ1 Datenblatt(PDF) 72 Page - NXP Semiconductors