MPC5644BCAMMJ1 datasheet(74/116 Pages) NXP

74 / 116 page

MPC5646C Microcontroller DataSheet, Rev. 7

Electrical Characteristics

NXP Semiconductors

Figure 17. Input equivalent circuit (extended channels)

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

initially charged at the source voltage VA (refer to the equivalent circuit reported in Figure 16): when the sampling phase is

started (A/D switch close), a charge sharing phenomena is installed.

Figure 18. 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

faster) in which CP2 is reported in parallel to CP1 (call CP = CP1 + CP2), the two capacitances CP and CS are in series,

and the time constant is

Eqn. 5

This relation can again be simplified considering CS as an additional worst condition. In reality, transient is faster, but the A/D

converter circuitry has been designed to be robust also in very worst case : the sampling time Ts is always much longer than the

internal time constant.

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

ΔV < 0.5 LSB

τ1 < (RSW + RAD) CS << TS

τ2 = RL (CS + CP1 + CP2)

τ1 RSW RAD

()

CP CS

•

CP CS

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

•

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