MPC5744P datasheet(80/114 Pages) FREESCALE

80 / 114 page

MPC5744P Data Sheet, Rev. 0.3

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

3.14

ADC electrical characteristics

The device provides a 12-bit Successive Approximation Register (SAR) Analog-to-Digital Converter.

Figure 8. ADC characteristics and error definitions

3.14.1

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; further, 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: CS being substantially a switched capacitance, with a frequency equal to the conversion rate

of the ADC, it can be seen as a resistive path to ground. For instance, assuming a conversion rate of 1 MHz,

(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

4095

4094

4093

4092

4091

4090

4089 4090 4091 4092 4093 4094 4095

1 LSB ideal =(VrefH-VrefL)/ 4096 =

3.3V/ 4096 = 0.806 mV

Total Unadjusted Error

TUE = +/- 6 LSB = +/- 4.84mV

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