PXS30 Datasheet, PDF(82/139 Page) Freescale Semiconductor, Inc

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PXS30 Datasheet, PDF (82/139 Pages) Freescale Semiconductor, Inc – PXS30 Microcontroller

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Electrical characteristics

3.14.1 Input impedance and ADC accuracy

To preserve the accuracy of the A/D converter, it is 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; further, 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) and the equivalent input

impedance of the ADC itself.

In fact a current sink contributor is represented by the charge sharing effects with the sampling

capacitance: CS being substantially a switched capacitance, with a frequency equal to the conversion rate

of the ADC, it can be seen as a resistive path to ground. For instance, assuming a conversion rate of 1 MHz,

with CS equal to 3 pF, a resistance of 330 kï is obtained (REQ = 1 / (fC ï´ CS), where fC represents the

conversion rate at the considered channel). To minimize the error induced by the voltage partitioning

between this resistance (sampled voltage on CS) and the sum of RS + RF + RL + RSW + RAD, the external

circuit must be designed to respect the Equation 9:

ï·

-R----S-----+----R-----F-----+----R-----L-----+-----R----S----W-------+-----R----A-----D--

REQ

ï¼

1--

LSB

Eqn. 9

Equation 9 generates a constraint for external network design, in particular on resistive path. Internal

switch resistances (RSW and RAD) can be neglected with respect to external resistances.

EXTERNAL CIRCUIT

Source

Filter

Current Limiter

CP1

INTERNAL CIRCUIT SCHEME

VDD

Channel

Selection

Sampling

RSW1

RAD

CP2

RS Source Impedance

RF Filter Resistance

CF Filter Capacitance

RL Current Limiter Resistance

RSW1 Channel Selection Switch Impedance

RAD Sampling Switch Impedance

CP Pin Capacitance (two contributions, CP1 and CP2)

CS Sampling Capacitance

Figure 10. Input equivalent circuit

PXS30 Microcontroller Data Sheet, Rev. 1

PreliminaryâSubject to Change Without Notice

Freescale Semiconductor

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