MPC5604B datasheet(53/72 Pages) FREESCALE

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Electrical characteristics

MPC5604B/C Microcontroller Data Sheet, Rev. 4

Freescale Semiconductor

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 3 pF, a resistance of 330 kΩ 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 + RL + RSW + RAD, the external circuit

must be designed to respect the Equation 4:

Eqn. 4

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