ST10F296 datasheet(308/346 Pages) STMICROELECTRONICS

308 / 346 page

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Electrical characteristics

ST10F296E

308/346

First transient period

This is a quick charge transfer from the internal capacitances, CP1 and CP2, to the sampling

capacitance, CS (initially CS is supposed to be completely discharged). Considering the

worst case scenario (since the time constant in reality is faster) in which CP2 is in parallel to

CP1 (call CP = CP1 + CP2), the two capacitances, CP and CS, are in series and the time

constant is:

Equation 26

Equation 26 can be simplified if only CS is considered as an additional worst condition. In

reality, the transient is faster, but the ADC circuitry has been designed to be robust in the

worst case situations. The sampling time, TS, is always much longer than the internal time

constant as in Equation 27.

Equation 27

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

VA1 voltage on the capacitance according to Equation 28.

Equation 28

Second transient period

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

capacitance) through the resistance RL. Consideringagain the worst case scenario in which

CP2 and CS are in parallel to CP1 (since the time constant in reality is faster), the time

constant is:

Equation 29

In Equation 29, the time constant depends on the external circuit. In particular, if the

transient is completed well before the end of the sampling time, TS, a constraint on RL sizing

is obtained, as shown in Equation 30.

Equation 30

RL must also be sized, according to the current limitation constraints, in combination with RS

(source impedance) and RF (filter resistance). As CF is greater than CP1, CP2 and CS, the

final voltage, VA2 (at the end of the charge transfer transient), is much higher than VA1.

Equation 31 (the charge balance) must be respected assuming that CS is already charged

at VA1.

Equation 31

()

⁄

()

() C

()

() T

≤

()

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