PXD10 datasheet(96/130 Pages) FREESCALE

96 / 130 page

Electrical characteristics

PXD10 Microcontroller Data Sheet, Rev. 1

Freescale Semiconductor

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 7:

Eqn. 7

Equation 7 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 18. Input equivalent circuit (precise channels)

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



---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

PXD10 Datasheet(PDF) 96 Page - Freescale Semiconductor, Inc

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