MPC5675KFF0VMS2R datasheet(81/147 Pages) NXP

81 / 147 page

Electrical characteristics

MPC5675K Microcontroller Data Sheet, Rev. 9

NXP Semiconductors

In fact a current sink contributor is represented by the charge sharing effects with the sampling capacitance: CS and CP2 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 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 , the external circuit must be

designed to respect 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 9. 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 (please see the equivalent circuit in Figure 9): A charge sharing phenomenon is

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

Figure 10. Transient behavior during sampling phase

In particular two different transient periods can be distinguished:

RS RF

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

VA1

VA2

VCS

Voltage Transient on CS

ΔV < 0.5 LSB

τ1 < (RSW + RAD) CS << TS

τ2 = RL (CS + CP1 + CP2)

MPC5675KFF0VMS2R Datasheet(PDF) 81 Page - NXP Semiconductors

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