SPC56EL54X datasheet(123/165 Pages) STMICROELECTRONICS

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SPC56ELx, SPC564Lx

Electrical characteristics

164

and CS were in parallel to CP1 (since the time constant in reality would be faster), the

time constant is:

Equation 8

In this case, the time constant depends on the external circuit: in particular imposing

that the transient is completed well before the end of sampling time TS, a constraints on

RL sizing is obtained:

Equation 9

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 10 must be respected

(charge balance assuming now CS already charged at VA1):

Equation 10

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.

Figure 12. 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













10 R







+++









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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