ST10F272Z2_08 datasheet(150/189 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F272Z2

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25.7.4

Analog reference pins

The accuracy of the A/D converter depends on how accurate is its analog reference: a noise

in the reference results in at least that much 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

sufficient in most of the cases; in presence of high RF noise energy, inductors or ferrite

beads may be necessary.

In this architecture, VAREF and VAGND pins represents also the power supply of the analog

circuitry 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 circuitry.

An external resistance on VAREF could introduce error under certain conditions: for this

reasons, series resistance are not advisable, and more in general any series devices in the

filter network should be designed to minimize the DC resistance.

Analog Input pins

To improve the accuracy of the A/D converter, it is definitively 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;

besides, 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).

Figure 43.

A/D converter input pins scheme

Input Leakage and external circuit

The series resistor utilized to limit the current to a pin (see RL in Figure 43), in combination

with a large source impedance can lead to a degradation of A/D converter accuracy when

input leakage is present.

RSW

CP2

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

RS Source Impedance

RF Filter Resistance

CF Filter Capacitance

RL Current Limiter Resistance

RSW Channel Selection Switch Impedance

RADSampling Switch Impedance

CP Pin Capacitance (two contributions, CP1 and CP2)

CS Sampling Capacitance

CP1

RAD

Channel

Selection

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