MPC5644BCAMMJ1 datasheet(75/116 Pages) NXP

75 / 116 page

Electrical Characteristics

MPC5646C Microcontroller DataSheet, Rev. 7

NXP Semiconductors

Eqn. 6

The charge of CP1 and CP2 is redistributed on CS,determining a new value of the voltage VA1 on the capacitance according to

the following equation

Eqn. 7

•

A second charge transfer involves also CF (that is typically bigger than the on-chip capacitance) through the resistance

RL: again considering the worst case in which CP2 and CS were in parallel to CP1 (since the time constant in reality

would be faster), the time constant is:

Eqn. 8

In this case, the time constant depends on the external circuit: in particular imposing that the transient is completed well before

the end of sampling time TS, a constraints on RL sizing is obtained:

Eqn. 9

Of course, RL shall be sized also according to the current limitation constraints, in combination with RS (source impedance)

and RF (filter resistance). Being CF definitively bigger than CP1, CP2 and CS, then the final voltage VA2 (at the end of the charge

transfer transient) will be much higher than VA1. The following equation must be respected (charge balance assuming now CS

already charged at VA1):

Eqn. 10

The two transients above are not influenced by the voltage source that, due to the presence of the RFCF filter, is not able to

provide the extra charge to compensate the voltage drop on CS with respect to the ideal source VA; the time constant RFCF of

the filter is very high with respect to the sampling time (TS). The filter is typically designed to act as anti-aliasing

Figure 19. Spectral representation of input signal

τ1 RSW RAD

()

CS TS

•

VA1 CS CP1 CP2

()

•

VA CP1 CP2

()

•

τ2 RL

CS CP1 CP2

()

•

8.5

τ2

•

8.5

RL CS CP1 CP2

()

•

=TS

VA2 CS CP1 CP2 CF

()

•

VA CF

•

VA1

+CP1 CP2

+CS

()

•

Analog Source Bandwidth (VA)

Sampled Signal Spectrum (fC = conversion Rate)

Anti-Aliasing Filter (fF = RC Filter pole)

2 f0 ≤ fC (Nyquist)

fF = f0 (Anti-aliasing Filtering Condition)

TC ≤ 2 RFCF (Conversion Rate vs. Filter Pole)

Noise

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