AN4092 datasheet(27/38 Pages) STMICROELECTRONICS

27 / 38 page

AN4092

ADC

Doc ID 023080 Rev 2

27/38

When the conversion switch closes, the charges between the capacitors are shared,

producing a new voltage that depends upon the initial voltages and the size of the

capacitors. This process takes places at the rate of the internal ADC time constant (RswCs).

At the same time the charge share voltage starts to recover towards the actual sample

voltage at the rate of the external RC time constant Rf(Cf+Cp). Even if the two processes are

interdependent, they can be treated as separated, as long as the internal time constant is

much smaller than the external one.

Being Vs the sample voltage and V1 the voltage after the charge sharing:

Equation 5

Therefore:

Equation 6

happening with an exponential law which time constant is:

Equation 7

Then the voltage V1 recovers exponentially towards Vs with the time constant:

Equation 8

Normally the latter is much larger than the first. This gives us the final constraint:

()V

()

--------------------------------Vs

swCs

f Cf

()

AN4092 Datasheet(PDF) 27 Page - STMicroelectronics