SPC5604BF2MLL6 datasheet(72/109 Pages) FREESCALE

72 / 109 page

MPC5604B/C Microcontroller Data Sheet, Rev. 11

Package pinouts and signal descriptions

Freescale Semiconductor

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

path to ground. For instance, assuming a conversion rate of 1 MHz, with CS+Cp2 equal to 3 pF, a resistance of 330 k is

obtained (REQ = 1 / (fc ×(CS+Cp2)), 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+Cp2) and the sum of RS + RF, the external

circuit must be designed to respect the Equation 4:

Eqn. 4

Equation 4 generates a constraint for external network design, in particular on a resistive path.

Figure 20. Input equivalent circuit (precise channels)

RS RF

REQ

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



---LSB



RSW1

CP2

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

CP1

RAD

Channel

Selection

RS: Source impedance

RF: Filter resistance

CF: Filter capacitance

RL: Current limiter resistance

RSW1: Channel selection switch impedance

RAD: Sampling switch impedance

CP: Pin capacitance (two contributions, CP1 and CP2)

CS: Sampling capacitance

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