MPC5744P datasheet(81/114 Pages) FREESCALE

81 / 114 page

Electrical characteristics

MPC5744P Data Sheet, Rev. 0.3

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

with CS equal to 3 pF, a resistance of 330 kΩ 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 9:

Eqn. 9

Equation 9 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 29. 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 29): A charge sharing phenomenon is installed when the sampling phase is started (A/D switch

close).

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

•

---LSB

RSW1

CP2

VDD_HV_IOx

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

MPC5744P bảng dữ liệu(PDF) 81 Page - Freescale Semiconductor, Inc