SPC56EC64L8 datasheet(86/118 Pages) STMICROELECTRONICS

86 / 118 page

Electrical Characteristics

SPC564Bxx - SPC56ECxx

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Doc ID 17478 Rev 4

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 ADC_0 (10-bit)

Equation 13 ADC_1 (12-bit)

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

CP1 CP2

+CF

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

CF 2048 CS

•

CF 8192 CS

•

SPC56EC64L8 Datasheet(PDF) 86 Page - STMicroelectronics