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| SPC5607BF1MLU6 Datasheet, PDF (75/110 Pages) Freescale Semiconductor, Inc – Microcontroller Data Sheet | |||
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Electrical characteristics
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 kï 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 ï· -R----S-----+----R-----F-----+----R----R-L---E--+--Q---R----S----W-------+-----R----A-----D-- ï¼ 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
RS
VA
Filter
RF
Current Limiter
RL
CF
CP1
INTERNAL CIRCUIT SCHEME
VDD
Channel
Selection
Sampling
RSW1
RAD
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 17. Input equivalent circuit (precise channels)
MPC5607B Microcontroller Data Sheet, Rev. 6
Freescale Semiconductor
75
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