PPC5604BAMLL4R datasheet(73/109 Pages) NXP

73 / 109 page

Package pinouts and signal descriptions

MPC5604B/C Microcontroller Data Sheet, Rev. 15

NXP Semiconductors

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 path to ground. For instance, assuming a

conversion rate of 1 MHz, with CS+Cp2 equal to 3 pF, a resistance of 330 kΩ 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

PPC5604BAMLL4R Datasheet(PDF) 73 Page - NXP Semiconductors

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