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ADC10 Operation
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 18−4. 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.
When ADC10SR = 0:
When ADC10SR = 1:
Substituting the values for R
For example, if R
ADC10SR = 0, or 4.33
18-8
ADC10
, as shown below in Figure 18−4. An internal MUX-on
sample
(max. 2 kΩ) in series with capacitor C
I
for an accurate 10-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 11 )
u (R
) R
t
)
sample
S
I
ln(2 11 )
u (R
) R
t
)
sample
S
I
I
u (R
) 2k)
t
7.625
sample
S
u (R
) 2k)
t
7.625
sample
S
Ω
is 10 k
S
µ
s when ADC10SR = 1.
I
must be charged to within ½ LSB
voltage V
I
C
and R
affect t
.The following equations
S
I
sample
) 800ns
C
I
) 2.5ms
C
I
and C
given above, the equation becomes:
I
20pF ) 800ns
20pF ) 2.5ms
, t
must be greater than 2.63
sample
(max. 20 pF) is seen
for a 10-bit
sample
(ADC10SR = 0)
(ADC10SR = 1)
µ
s when
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