ST10F276 datasheet(189/229 Pages) STMICROELECTRONICS

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ST10F276

Electrical characteristics

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23.7.4

Analog reference pins

The accuracy of the A/D converter depends on the accuracy of its analog reference: A noise

in the reference results in proportionate error in a conversion. A low pass filter on the A/D

converter reference source (supplied through pins VAREF and VAGND), is recommended in

order to clean the signal, minimizing the noise. A simple capacitive bypassing may be suffi-

cient in most cases; in presence of high RF noise energy, inductors or ferrite beads may be

necessary.

In this architecture, VAREF and VAGND pins also represent the power supply of the analog cir-

cuitry of the A/D converter: There is an effective DC current requirement from the reference

voltage by the internal resistor string in the R-C DAC array and by the rest of the analog cir-

cuitry.

An external resistance on VAREF could introduce error under certain conditions: For this rea-

sons, series resistance is not advisable and more generally, any series devices in the filter

network should be designed to minimize the DC resistance.

23.7.5

Analog input pins

To improve the accuracy of the A/D converter, analog input pins must have low AC imped-

ance. 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; moreover, its source

charges during the sampling phase, when the analog signal source is a high-impedance

source.

A real filter is typically 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 imped-

ance 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 sig-

nal (bandwidth).

Figure 47.

A/D converter input pins scheme

RSW

CP2

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

Source impedance

Filter resistance

Filter capacitance

Current limiter resistance

RSW

Channel selection switch impedance

RAD

Sampling switch impedance

Pin capacitance (two contributions, CP1 and CP2)

Sampling capacitance

CP1

RAD

Channel

Selection

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