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53 page 
Electrical characteristics MPC5607B Microcontroller Data Sheet, Rev. 3 Preliminary—Subject to Change Without Notice Freescale Semiconductor 53 Figure 17. Input equivalent circuit (extended channels) 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 16): A charge sharing phenomenon is installed when the sampling phase is started (A/D switch close). Figure 18. Transient behavior during sampling phase In particular two different transient periods can be distinguished: 1. A first and quick charge transfer from the internal capacitance CP1 and CP2 to the sampling capacitance CS occurs (CS is supposed initially completely discharged): considering a worst case (since the time constant in reality would be faster) in which CP2 is reported in parallel to CP1 (call CP = CP1 + CP2), the two capacitances CP and CS are in series, and the time constant is RF CF RS RL RSW1 CP3 CS VDD Sampling Source Filter Current Limiter EXTERNAL CIRCUIT INTERNAL CIRCUIT SCHEME RS Source Impedance RF Filter Resistance CF Filter Capacitance RL Current Limiter Resistance RSW Channel Selection Switch Impedance (two contributions RSW1 and RSW2) RAD Sampling Switch Impedance CP Pin Capacitance (three contributions, CP1, CP2 and CP3) CS Sampling Capacitance CP1 RAD Channel Selection VA CP2 Extended RSW2 Switch VA VA1 VA2 t TS VCS Voltage Transient on CS ΔV < 0.5 LSB 1 2 τ 1 < (RSW + RAD) CS << TS τ 2 = RL (CS + CP1 + CP2) |
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