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| PXD10 Datasheet, PDF (96/130 Pages) Lumins Inc. – 4” Dia.X 6” extruded aluminum step poles | |||
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Electrical characteristics
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 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 ï´ 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 7:
VA
ï·
-R----S-----+----R-----F-----+----R-----L-----+-----R----S----W-------+-----R----A-----D--
REQ
ï¼
1--
2
LSB
Eqn. 7
Equation 7 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 18. Input equivalent circuit (precise channels)
PXD10 Microcontroller Data Sheet, Rev. 1
96
Freescale Semiconductor
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