PXS20 datasheet(89/119 Pages) FREESCALE

89 / 119 page

Electrical characteristics

PXS20 Microcontroller Data Sheet, Rev. 1

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

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 4:

Eqn. 4

Equation 4 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 9. Input Equivalent Circuit

A second aspect involving the capacitance network shall be considered. Assuming the three capacitances CF, CP1 and CP2 are

initially charged at the source voltage VA (refer to the equivalent circuit reported in Figure 9): A charge sharing phenomenon is

installed when the sampling phase is started (A/D switch close).

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



---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

PXS20 Datasheet(PDF) 89 Page - Freescale Semiconductor, Inc

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