MMC2107 - Freescale Semiconductor

Freescale Semiconductor, Inc.
Queued Analog-to-Digital Converter (QADC)
In Figure 18-54, R F , R SRC and C F comprise the external filter circuit. C P
is the internal parasitic capacitor. C Samp is the capacitor array used to
sample and hold the input voltage. V I is an internal voltage source used
to provide charge to C samp during sample phase.
The following paragraphs provide a simplified description of the
interaction between the QADC and the user’s external circuitry. This
circuitry is assumed to be a simple RC low-pass filter passing a signal
from a source to the QADC input pin. These simplifying assumptions are
made:
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Freescale Semiconductor, I
• The external capacitor is perfect (no leakage, no significant
dielectric absorption characteristics, etc.)
• All parasitic capacitance associated with the input pin is included
in the value of the external capacitor.
• Inductance is ignored.
18.11.7.1 Settling Time for the External Circuit
• The "on" resistance of the internal switches is 0 ohms and the "off"
resistance is infinite.
The values for R SRC , R F , and C F in the user's external circuitry
determine the length of time required to charge C F to the source voltage
level (V SRC ). At time t = 0, V src changes in Figure 18-54 while S1 is
open, disconnecting the internal circuitry from the external circuitry.
Assume that the initial voltage across C F is 0. As C F charges, the voltage
across it is determined by the equation, where t is the total charge time:
V CF = V SRC (1 –e –t/(RF + RSRC) CF )
As t approaches infinity, V CF will equal V SRC . (This assumes no internal
leakage.) With 10-bit resolution, 1/2 of a count is equal to 1/2048
full-scale value. Assuming worst case (V SRC = full scale), Table 18-16
shows the required time for C F to charge to within 1/2 of a count of the
actual source voltage during 10-bit conversions. Table 18-16 is based
on the RC network in Figure 18-54.
NOTE:
The following times are completely independent of the A/D converter
architecture (assuming the QADC is not affecting the charging).
Technical Data MMC2107 – Rev. 2.0
498 Queued Analog-to-Digital Converter (QADC) MOTOROLA
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