PXS30 Datasheet, PDF(84/139 Page) Freescale Semiconductor, Inc

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PXS30 Datasheet, PDF (84/139 Pages) Freescale Semiconductor, Inc – PXS30 Microcontroller

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Electrical characteristics

10 ï· ï´2 = 10 ï· RL ï· ï¨CS + CP1 + CP2ï© ï¼ TS

Eqn. 16

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 17 must be respected (charge balance assuming now CS already charged at

VA1):

VA2 ï· ï¨CS + CP1 + CP2 + CFï© = VAï· CF + VA1ï· ï¨CP1 + CP2 + CSï©

Eqn. 17

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.

Analog Source Bandwidth (VA)

Noise

Anti-Aliasing Filter (fF = RC Filter pole)

TC ï£ï 2 RFCF (Conversion Rate vs. Filter Pole)

fF ï½ f0 (Anti-aliasing Filtering Condition)

2 f0 ï£ï fC (Nyquist)

Sampled Signal Spectrum (fC = conversion Rate)

Figure 18. 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 19 between the ideal and real sampled voltage on CS:

--V-----A---- = --------C----P---1-----+-----C----P----2----+-----C----F---------

VA2 CP1 + CP2 + CF + CS

Eqn. 19

PXS30 Microcontroller Data Sheet, Rev. 1

PreliminaryâSubject to Change Without Notice

Freescale Semiconductor

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