MPC5645B datasheet(76/114 Pages) FREESCALE

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MPC5646C Microcontroller Data Sheet, Rev. 3

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

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. Equation 10 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

Calling f0 the bandwidth of the source signal (and as a consequence the cut-off frequency of the anti-aliasing filter, fF),

according to the Nyquist theorem the conversion rate fC must be at least 2f0; it means that the constant time of the filter is greater

than or at least equal to twice the conversion period (TC). Again the conversion period TC is longer than the sampling time TS,

which is just a portion of it, even when fixed channel continuous conversion mode is selected (fastest conversion rate at a

specific channel): in conclusion it is evident that the time constant of the filter RFCF is definitively much higher than the

sampling time TS, so the charge level on CS cannot be modified by the analog signal source during the time in which the

sampling switch is closed.

The considerations above lead to impose new constraints on the external circuit, to reduce the accuracy error due to the voltage

drop on CS; from the two charge balance equations above, it is simple to derive Equation 11 between the ideal and real sampled

voltage on CS:

Eqn. 11

2



10 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

VA2

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

CP1 CP2

+CF

CP1 CP2

+CF CS

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

MPC5645B Datasheet(PDF) 76 Page - Freescale Semiconductor, Inc

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