PPC5605BAMLL4R datasheet(80/114 Pages) NXP

80 / 114 page

MPC5607B Microcontroller Data Sheet, Rev. 10

Electrical characteristics

NXP Semiconductors

Eqn. 11

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 antialiasing.

Figure 20. Spectral representation of input signal

Calling f0 the bandwidth of the source signal (and as a consequence the cut-off frequency of the

antialiasing 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 12 between the ideal and real sampled voltage on CS:

Eqn. 12

From this formula, in the worst case (when VA is maximum, that is for instance 5 V), assuming to accept

a maximum error of half a count, a constraint is evident on CF value:

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

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

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