SPC560P54X datasheet(79/105 Pages) STMICROELECTRONICS

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SPC56xP54x, SPC56xP60x

Electrical characteristics

104

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:

Equation 11

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:

Equation 12

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 CS

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

CF 2048 CS





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