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AN2834

How to get the best ADC accuracy

3.2.7

Source frequency condition vs. source and parasitic capacitors

The external capacitance will not allow the analog input voltage to be exactly the same as

VAIN if the capacitor is not fully charged by the analog source.

If the analog input signal changes, then the analog signal frequency (FAIN) should be such

that the time period of this analog signal is at least: 10 × RAIN × (CAIN + Cp).

TAIN = analog signal time period = 1/FAIN.

We have:



Therefore:



For example:

For RAIN = 25 k, CAIN = 7 pF, Cp = 3 pF, this gives:



Thus, the maximum frequency of the source will be:

So for the above defined source characteristics (capacitance and resistance), the frequency

of the source must not exceed 400 kHz, otherwise the ADC conversion result will be not

accurate.

Figure 26. Recommended values for RAIN and CAIN vs. source frequency FAIN

TAIN 10 RAIN CAIN CP







FAIN

10 RAIN CAIN CP





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



FAINmax

10 25 10



 10

–



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

FAINmax 400 kHz

0.1

100

1000

0.01

0.1

FAIN(kHz)

CAIN 10 nF

CAIN 22 nF

CAIN 47 nF

ai15489

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