ST10F273 datasheet(138/179 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F273

138/179

Figure 37.

A/D conversion characteristic

24.7.4

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.

(2)

(1)

(3)

(4)

(5)

Offset error OFS

Gain error GE

1 LSB (ideal)

VAIN (LSBIDEAL)

[LSBIDEAL = VAREF / 1024]

Digital

out

(HEX)

3FF

3FE

3FD

3FC

3FB

3FA

005

004

003

002

001

000

007

006

(1) Example of an actual transfer curve

(2) The ideal transfer curve

(3) Differential Non-Linearity Error (DNL)

(4) Integral Non-Linearity Error (INL)

(5) Center of a step of the actual transfer curve

(6) Quantization Error (1/2 LSB)

(7) Total Unadjusted Error (TUE)

1024

1022

1020

1018

(6)

(7)

Bisector characteristic

Ideal characteristic

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