MPC5646C datasheet(74/114 Pages) FREESCALE

74 / 114 page

MPC5646C Microcontroller Data Sheet, Rev. 3

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

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 16. Input equivalent circuit (precise channels)

Figure 17. Input equivalent circuit (extended channels)

RS RF RL RSW RAD

REQ

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



---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

RSW1

CP3

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

Source Impedance

Filter Resistance

Filter Capacitance

Current Limiter Resistance

RSW Channel Selection Switch Impedance (two contributions RSW1 and RSW2)

RAD Sampling Switch Impedance

Pin Capacitance (three contributions, CP1, CP2 and CP3)

Sampling Capacitance

CP1

RAD

Channel

Selection

CP2

Extended

RSW2

Switch

MPC5646C Datasheet(PDF) 74 Page - Freescale Semiconductor, Inc

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