PXD10 datasheet(99/130 Pages) FREESCALE

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Electrical characteristics

PXD10 Microcontroller Data Sheet, Rev. 1

Freescale Semiconductor

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 21. 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 14 between the ideal and real sampled voltage on CS:

Eqn. 14

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

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

Eqn. 15

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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2048 C

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PXD10 Datasheet(PDF) 99 Page - Freescale Semiconductor, Inc