PPC5605BAMLL4R datasheet(77/114 Pages) NXP

77 / 114 page

Electrical characteristics

MPC5607B Microcontroller Data Sheet, Rev. 10

NXP Semiconductors

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: being CS and Cp2 substantially two switched capacitances, 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+Cp2 equal to 3 pF, a resistance of 330 kΩ is obtained (REQ = 1 /

(fc ×(CS+Cp2)), 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+Cp2) 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 a resistive path.

Figure 17. Input equivalent circuit (precise channels)

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

PPC5605BAMLL4R 데이터시트(PDF) 77 Page - NXP Semiconductors

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