#%325 =
:
P
O+D
® B
#&%%.-
;
F 1
G = ln l
2
J
OAPPHEJC ANNKN
p
#%325 =
:
322.5JO ® 30/*V
;
F 1 = 8.7 \ 9
P
O+D
= 2.5 ® 129JO = 322.5JO
G = ln F
2
12
.5$
0.25 .5$
GFln l
2200L( + 5L(
1.6L(
p = 9.7 F7.2 = 2.5
ì =
:
563 + 34003
;:
1.6L(
;
+
:
563
;:
2200L( + 5L(
;
= 5.5JO + 123.5JO = 129JO
ADC Circuit
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Analog-to-Digital Converter (ADC)
(5)
And the number of required time constants would be:
So the S+H time should be set to at least:
Finally, the minimum ACQPS value is calculated and rounded up to the nearest supported value:
While this gives a rough estimate of the required acquisition window, a better method would be to setup a
circuit with the ADC input model, a model of the source impedance/capacitance, and any board parasitics
in SPICE (or similar software) and simulate to verify that the sampling capacitor settles to the desired
accuracy.
7.2.2.2 ACQPS Approximation for Low Bandwidth Signals
Signals that are sampled infrequently and are tolerant of low-pass filtering (such as ambient temperature
sensors) can be treated as low bandwidth signals. A large C
s
that is sized to be much larger than C
h
will
allow the ADC to sample the R
s
C
s
filtered signal quickly at the expense of increased phase delay. C
h
will
receive the bulk of its charge from C
s
during the S+H window, and C
s
will recover its charge through R
s
between ADC samples.
7.2.2.2.1 ACQPS Approximation Equations for Low Bandwidth Signals
The desired settling accuracy will determine the value of C
s
:
The desired recovery time and acceptable charge error will determine the value of R
s
:
With this configuration, C
s
acts as the effective source signal, which simplifies the equations for calculating
(k) and (τ) as follows:
So the total S+H time (t
S+H
)should be set to at least:
(6)
Finally, t
S+H
is used to determine the minimum value to program into the ACQPS field of the
ADCSOCxCTL registers:
(7)
Where the following parameters are provided by the ADC input model:
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