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Sample Timing Considerations
When SAMPCON = 0 all Ax inputs are high impedance. When SAMPCON =
1, the selected Ax input can be modeled as an RC low-pass filter during the
sampling time t
input resistance R
by the source. The capacitor C
of the source voltage V
Figure 17−5. Analog Input Equivalent Circuit
R S
V I
V S
The resistance of the source R
can be used to calculate the minimum sampling time t
conversion:
Substituting the values for R
For example, if R
, as shown below in Figure 17−5. An internal MUX-on
sample
(max. 2 kΩ) in series with capacitor C
I
for an accurate 12-bit conversion.
S
MSP430
V I = Input voltage at pin Ax
V S = External source voltage
R I
R S = External source resistance
V C
R I = Internal MUX-on input resistance
C I = Input capacitance
C I
V C = Capacitance-charging voltage
ln(2 13 )
u (R
) R
t
)
sample
S
I
I
u (R
) 2kW)
t
sample
S
is 10 kΩ, t
S
I
must be charged to within 1/2 LSB
voltage V
I
C
and R
affect t
. The following equation
S
I
sample
) 800ns
C
I
and C
given above, the equation becomes:
I
40pF ) 800ns
9.011
must be greater than 5.13
sample
ADC12 Operation
(max. 40 pF) is seen
for a 12-bit
sample
µ
s.
ADC12
17-9
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