SPC560P34L1 datasheet(69/103 Pages) STMICROELECTRONICS

69 / 103 page

SPC560P34L1, SPC560P34L3, SPC560P40L1, SPC560P40L3

Electrical characteristics

Doc ID 16100 Rev 7

69/103

3.14.1

Input impedance and ADC accuracy

To preserve the accuracy of the A/D converter, it is necessary that analog input pins have

low AC impedance. Placing a capacitor with good high frequency characteristics at the input

pin of the device can be effective: the capacitor should be as large as possible, ideally

infinite. This capacitor contributes to attenuating the noise present on the input pin; further, it

sources charge during the sampling phase, when the analog signal source is a high-

impedance source.

A real filter can typically be obtained by using a series resistance with a capacitor on the

input pin (simple RC filter). The RC filtering may be limited according to the source

impedance value of the transducer or circuit supplying the analog signal to be measured.

The filter at the input pins must be designed taking into account the dynamic characteristics

of the input signal (bandwidth) and the equivalent input impedance of the ADC itself.

In fact a current sink contributor is represented by the charge sharing effects with the

sampling capacitance: CS and CP2 being substantially two switched capacitances, with a

frequency equal to the ADC conversion rate, it can be seen as a resistive path to ground.

For instance, assuming a conversion rate of 1 MHz, with CS+CP2 equal to 3 pF, a resistance

of 330 k

Ω is obtained (REQ = 1 / (fc × (CS+CP2)), where fc represents the conversion rate at

the considered channel). To minimize the error induced by the voltage partitioning between

this resistance (sampled voltage on CS+CP2) and the sum of RS + RF, the external circuit

must be designed to respect the Equation 4:

Equation 4

Equation 4 generates a constraint for external network design, in particular on resistive

path.

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