SPC5643LFF2MLL6 datasheet(95/136 Pages) NXP

95 / 136 page

Electrical characteristics

MPC5643L Microcontroller Data Sheet, Rev. 10

NXP Semiconductors

A real filter can typically be obtained by using a series resistance with a capacitor on the input pin (simple RC filter). The RC

filtering may be limited according to the value of source impedance of the transducer or circuit supplying the analog signal to

be measured. The filter at the input pins must be designed taking into account the dynamic characteristics of the input signal

(bandwidth) and the equivalent input impedance of the ADC itself.

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 Cp2 + CS equal to 7.5 pF, a resistance of 133 kΩ is obtained

(REQ = 1 / (fS*(Cp2+CS)), where fS 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 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 8. 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 8): A charge sharing phenomenon is

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

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

SPC5643LFF2MLL6 Datasheet(PDF) 95 Page - NXP Semiconductors

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