mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 231

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

10.8.6.1 Settling Time for the External Circuit
MC68HC16Y3/916Y3
USER’S MANUAL
In Figure 10-10, R
sample capacitor. Each channel has its own capacitor. C
retains the value of the last sample. V
the DAC capacitor array (C
2, or 2.5 volts for 5-volt operation.
The following paragraphs provide a simplified description of the interaction between
the ADC 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 ADC input pin. The following sim-
plifying assumptions are made:
The values for R
required to charge C
At time t = 0, S1 in Figure 10-10 closes. S2 is open, disconnecting the internal circuitry
from the external circuitry. Assume that the initial voltage across CF is 0. As CF
charges, the voltage across it is determined by the following equation, where t is the
total charge time:
When t = 0, the voltage across C
(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
the required time for C
during 10-bit conversions. Table 10-10 is based on the RC network in Figure 10-10.
• The source impedance is included with the series resistor of the RC filter.
• The external capacitor is perfect (no leakage, no significant dielectric absorption
• All parasitic capacitance associated with the input pin is included in the value of
• Inductance is ignored.
• The “on” resistance of the internal switches is zero ohms and the “off” resistance
characteristics, etc.)
the external capacitor.
is infinite.
The following times are completely independent of the A/D converter
architecture (assuming the ADC is not affecting the charging).
F
F
and C
Freescale Semiconductor, Inc.
and C
F
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to the source voltage level (V
F
to charge to within 1/2 of a count of the actual source voltage
F
ANALOG-TO-DIGITAL CONVERTER
F
in the user's external circuitry determine the length of time
DAC
comprise the user's external filter circuit. C
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) before each sample. The value of this supply is V
V
CF
F
= 0. As t approaches infinity, V
=
I
is an internal voltage source used to precharge
V
SRC
NOTE
1 e
–
SRC
t – R
SRC
= full scale), Table 10-10 shows
F
C
).
F
S
is never precharged; it
CF
will equal V
S
is the internal
MOTOROLA
10-23
DDA
SRC
/
.