PXD10 datasheet(97/130 Pages) FREESCALE

97 / 130 page

Electrical characteristics

PXD10 Microcontroller Data Sheet, Rev. 1

Freescale Semiconductor

Figure 19. Input equivalent circuit (extended channels)

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 18): A charge sharing phenomenon is installed when the sampling phase is started (A/D switch

close).

Figure 20. 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

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

VA1

VA2

VCS

Voltage transient on CS

V <0.5 LSB





1 < (RSW + RAD) CS << TS



2 = RL (CS + CP1 + CP2)

PXD10 Datasheet(PDF) 97 Page - Freescale Semiconductor, Inc

PXD10 Datasheet(HTML) 97 Page - Freescale Semiconductor, Inc

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