AN4092 datasheet(29/38 Pages) STMICROELECTRONICS

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AN4092

ADC

Doc ID 023080 Rev 2

29/38

Let’s choose Ts = 64 and continuos conversion.

The external RC time constant Rf(Cf+Cp) is 1.496us, therefore Ts is about 3 times that

constant, meaning that the second exponential transient is not really over. However, the

voltage drop due to the charge share is only (Equation 6: ) 4.41mV (worst case(n)), leading

to a negligible error (being Ts = 3 x time constant).

EXAMPLE 2

Let’s assume that:

●

Vamax = 24V

●

Rs = 100Ohm

●

f0 = 50Hz

Using the Equation 1:

Let’s choose Rf = 6.2KOhm.

Then, from the Equation 3:

Let’s choose Cf = 470nF.

In this case, being the signal slow, we can simply choose the maximum sampling time: Ts =

128. Then the ADC can be triggered by an internal timer to meet the Nyquist requirement fN

= 2f0.

The capacitance Cf is much bigger than the internal one, leading to a negligible voltage drop

and hence basically no conversion errors due to the charge sharing, hence allowing

potentially even smaller sampling times, reducing the overall conversion time.

The worst case is 3.75V, difference between the sample voltage and the residual voltage on the sample

capacitance.

66ADCclockcycles

–

5.5

–

310

–

⋅

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

100

–

≅

≥

π50 6.2 10

⋅

()

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

513nF

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