AN4173

Considerations on the input impedance of the signal source

Doc ID 023727 Rev 3

5/22

1

Considerations on the input impedance of the signal

source

The impedance relative to the signal source can limit the ADC’s sample rate. Furthermore a

current limiter resistance and an RC filter are often necessary to minimize the current

request and to attenuate the noise present on the input pin. This external network can

generate accuracy problems for the ADC converter and for this reason it is important to

invest time in reaching the right adaptation.

the time needed to stabilize the incoming signal before the ADC can sample it, and if the

input signal has a high frequency, which means that it changes quickly over time, this low-

pass filter can distort the ADC’s acquisition.

Figure 1.

Input equivalent circuit

Moreover, considering a sinusoidal signal as input, if the source has a low impedance, the

RC time constant will not be relevant and the voltage on the capacitance will quickly match

the input voltage.

sampling, and the channel selection switch impedance, can limit the maximum frequency

applicable to the ADC input.

The internal resistance of the source must allow the capacitance to reach its final value

before the end of sample time.