AN4092 datasheet(28/38 Pages) STMICROELECTRONICS

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ADC

AN4092

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Doc ID 023080 Rev 2

Equation 9

However the last constraint can’t be always fulfilled. For example when the time constant

Rf(Cf+Cp) has to be big in order to filter out very low frequencies. In this case it is preferable

to reduce the resistor Rf (as long as the current limitation constraint is kept) and increase Cf:

it results in a very small drop due to the charge sharing and eventually a small or negligible

error.

EXAMPLE 1

Let’s assume that:

●

Vamax = 12V

●

Rs = 100Ohm

●

f0 = 100KHz

Using the Equation 1:

Let’s choose Rf = 2.2KOhm.

Then, from the Equation 3:

Let’s choose Cf = 680pF.

Then, from the Equation 4:

Clocking the ADC at its maximum frequency (16MHz):

««

5.5

–

310

–

⋅

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

100

–

≅

≥

π100 10

2.2 10

⋅

()

⋅

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

723pF

200 10

⋅

()

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

μs

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