MPC5644B View Datasheet(PDF) - Freescale Semiconductor
Part Name
Description
MFG CO.
'MPC5644B' PDF : 114 Pages View PDF
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 330 k 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 4:
VA -R----S-----+-----R----F-----+----R----R-L---E--+--Q---R----S----W-------+-----R----A-----D-- 12-- LSB
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.
EXTERNAL CIRCUIT
Source
Filter
Current Limiter
INTERNAL CIRCUIT SCHEME
VDD
Channel
Selection
Sampling
RS
RF
RL
RSW1
RAD
VA
CF
CP1
CP2
CS
RS Source Impedance
RF Filter Resistance
CF Filter Capacitance
RL Current Limiter Resistance
RSW1 Channel Selection Switch Impedance
RAD Sampling Switch Impedance
CP Pin Capacitance (two contributions, CP1 and CP2)
CS Sampling Capacitance
Figure 16. Input equivalent circuit (precise channels)
EXTERNAL CIRCUIT
Source
RS
VA
Filter
RF
Current Limiter
RL
CF
CP1
INTERNAL CIRCUIT SCHEME
VDD
Channel Extended
Selection
Switch
Sampling
RSW1
RSW2
RAD
CP3
CP2
CS
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
Figure 17. Input equivalent circuit (extended channels)
MPC5646C Microcontroller Data Sheet, Rev. 3
74
Preliminary—Subject to Change Without Notice
Freescale Semiconductor
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