MPC5643L datasheet(94/136 Pages) FREESCALE

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MPC5643L Microcontroller Data Sheet, Rev. 8.1

Electrical characteristics

Freescale Semiconductor

Figure 7. ADC characteristics and error definitions

3.15.1

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, with CS equal to 7.5 pF, a resistance of 133 k is obtained

(REQ =1 / (fC  CS), where fc represents the conversion rate at the considered channel). To minimize the error induced by the

voltage partitioning between this resistance (sampled voltage on CS) and the sum of RS +RF, the external circuit must be

designed to respect the Equation 4:

Eqn. 4

(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

---------------------



---LSB



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