MPC5601PEF0MLL4R datasheet(58/95 Pages) FREESCALE

58 / 95 page

MPC5602P Microcontroller Data Sheet, Rev. 4.1

Freescale Semiconductor

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 14. 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 14): A charge sharing phenomenon

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

RS RF RL RSW RAD

REQ

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



---LSB



RSW1

CP2

VDD

Sampling

Source

Filter

Current Limiter

EXTERNAL CIRCUIT

INTERNAL CIRCUIT SCHEME

CP1

RAD

Channel

Selection

RS: Source impedance

RF: Filter resistance

CF: Filter capacitance

RL: Current limiter resistance

RSW1: Channel selection switch impedance

RAD: Sampling switch impedance

CP: Pin capacitance (two contributions, CP1 and CP2)

CS: Sampling capacitance

MPC5601PEF0MLL4R датащи(PDF) 58 Page - Freescale Semiconductor, Inc