MPC5643L datasheet(95/136 Pages) FREESCALE

95 / 136 page

Electrical characteristics

MPC5643L Microcontroller Data Sheet, Rev. 8.1

Freescale Semiconductor

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 8. 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 8): A charge sharing phenomenon is

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

Figure 9. Transient Behavior during Sampling Phase

In particular two different transient periods can be distinguished:

•

A first and quick charge transfer from the internal capacitance CP1 and CP2 to the sampling capacitance CS occurs (CS

is supposed initially completely discharged): considering a worst case (since the time constant in reality would be

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

VA1

VA2

VCS

Voltage Transient on CS

V <0.5 LSB





1 < (RSW + RAD) CS << TS



2 = RL (CS + CP1 + CP2)

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