SPC560D40L1 datasheet(62/90 Pages) STMICROELECTRONICS

62 / 90 page

Electrical characteristics

SPC560D30x, SPC56040Dx

62/90

Doc ID 16315 Rev 7

4.17.2

Input impedance and ADC accuracy

In the following analysis, the input circuit corresponding to the precise channels is

considered.

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;

furthermore, 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 value of source

impedance 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: being CS and Cp2 substantially two switched capacitances, with a

frequency equal to the conversion rate of the ADC, 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

 is obtained (R

EQ = 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 a resistive

path.

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