AN4173

Application code structure

Doc ID 023727 Rev 3

19/22

3.5

Conclusions

There are four possible ways to optimise input signal acquisition:

1.

Minimise the total source impedance. This means choosing sensors with low output

impedance (not always easy for some types of sensor) and minimising the serial

resistance of any protection devices between the analog source and the input pin

(while still providing a voltage protection level compatible with the circuit specification).

2.

Match the sample time to the analog source impedance. Using the formulas that relate

the sample time to the source internal resistance to match the source resistance to one

of the available sample times.

3.

Match the sample time to the analog filter cut-off frequency, to remove high

frequencies: the microcontroller sampling time (ADC silicon configuration) must be 5 to

10 times shorter than the period of the cut-off frequency of the low-pass filter on ADC

input signal.

4.

Reduce noise at the input pin. Add an external RC filter (with attention to the source

internal resistance).