RPC564L60L5BOSY datasheet(105/137 Pages) STMICROELECTRONICS

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RPC56EL60L5

Electrical characteristics

136

Figure 9. Transient Behavior during Sampling Phase

In particular two different transient periods can be distinguished:



A first and quick charge transfer from the internal capacitance CP1 and CP2 to the

sampling capacitance CS occurs (CS is supposed initially completely discharged):

considering a worst case (since the time constant in reality would be faster) in which

CP2 is reported in parallel to CP1 (call CP = CP1 + CP2), the two capacitances CP and

CS are in series, and the time constant is

Equation 5

Equation 5 can again be simplified considering only CS as an additional worst

condition. In reality, the transient is faster, but the A/D converter circuitry has been

designed to be robust also in the very worst case: the sampling time TS is always much

longer than the internal time constant:

Equation 6

The charge of CP1 and CP2 is redistributed also on CS, determining a new value of the

voltage VA1 on the capacitance according to Equation 7:

Equation 7



A second charge transfer involves also CF (that is typically bigger than the on-chip

capacitance) through the resistance RL: again considering the worst case in which CP2

VA1

VA2

VCS

Voltage Transient on CS

V <0.5 LSB





1 < (RSW + RAD) CS << TS



2 = RL (CS + CP1 + CP2)







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



















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