MPC5673K datasheet(82/140 Pages) FREESCALE

82 / 140 page

MPC5675K Microcontroller Data Sheet, Rev. 4

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

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 k is obtained (REQ =1 / (fC  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:

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.

Figure 10. Input equivalent circuit

---------------------------------------------------------------------------



---LSB



RSW1

CP2

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

Source Impedance

Filter Resistance

Filter Capacitance

Current Limiter Resistance

RSW1 Channel Selection Switch Impedance

RAD Sampling Switch Impedance

Pin Capacitance (two contributions, CP1 and CP2)

Sampling Capacitance

CP1

RAD

Channel

Selection

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