ST10F296 datasheet(306/346 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F296E

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24.7.3

Analog reference pins

The accuracy of the ADC converter depends on the accuracy of its analog reference. A

noise in the reference results in the same proportion of error in a conversion. A low pass

filter on the ADC converter reference source (supplied through the VAREF and VAGND pins),

is recommended to clean the signal thereby minimizing the noise. A simple capacitive

bypassing may be sufficient in most cases. In the presence of high RF noise energy,

inductors or ferrite beads may be necessary.

In the ST10F296E architecture, the VAREF and VAGND pins also represent the power supply

of the analog circuitry of the ADC. An effective DC current is required from the reference

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

circuitry.

An external resistance on VAREF could introduce error under certain conditions. For this

reason, series resistance is not advisable. Any series devices in the filter network should be

designed to minimize the DC resistance.

24.7.4

Analog input pins

To improve the accuracy of the ADC, analog input pins must 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. Moreover, the source of the

capacitor 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). RC filtering may be limited according to the value of the impedance

source of the transducer or circuit supplying the analog signal to be measured. The filter at

the input pins must be designed to account for the dynamic characteristics of the input

signal (bandwidth).

Figure 99.

ADC input pins scheme

Legend:

RS: Source impedance

RF: Filter resistance

CF: Filter capacitance

RL: Current limiter resistance

RSW: Channel selection switch impedance

RAD: Sampling switch impedance

cp: Pin capacitance (two contributions, CP1 and CP2)

CS: Sampling capacitance

VA: Source voltage

RSW

CP2

VDD

Sampling

Source

Filter

Current limiter

External circuit

internal circuit

CP1

RAD

Channel

selection

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