ST10F271 datasheet(136/173 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F271

136/173

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 40.

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 40), in combination

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

input leakage is present.

Data about maximum input leakage current at each pin are provided in the Data Sheet

(Electrical Characteristics section). Input leakage is greatest at high operating temperatures,

and in general it decreases by one half for each 10°C decrease in temperature.

Considering that, for a 10-bit A/D converter one count is about 5mV (assuming VAREF = 5V),

an input leakage of 100nA acting though an RL = 50kΩ of external resistance leads to an

error of exactly one count (5mV); if the resistance were 100k

Ω the error would become two

counts.

Eventual additional leakage due to external clamping diodes must also be taken into

account in computing the total leakage affecting the A/D converter measurements. Another

contribution to the total leakage is represented by the charge sharing effects with the

sampling capacitance: being CS substantially a switched capacitance, with a frequency

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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