SPC56AP54X datasheet(79/104 Pages) STMICROELECTRONICS

79 / 104 page

SPC56xP54x, SPC56xP60x

Electrical characteristics

Doc ID 18340 Rev 3

79/104

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

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2048

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