SPC5702BECLU datasheet(67/98 Pages) FREESCALE

67 / 98 page

Electrical Characteristics

MPC560xS Microcontroller Data Sheet Data Sheet, Rev. 1

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

In fact a current sink contributor is represented by the charge sharing effects with the sampling capacitance: CS being

substantially a switched capacitance, with a frequency equal to the conversion rate of the ADC, it can be seen as a resistive path

to ground. For instance, assuming a conversion rate of 1 MHz, with CS equal to 3 pF, a resistance of 330kΩ 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 5:

Eqn. 5

Equation 5 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 (Precise Channels)

RS RF RL RSW RAD

REQ

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

•

---LSB

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

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