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0000
1100
1101
1110
1111
0000
0001
0010
0011
1100
1101
1110
1111
CxSHORT
Sampling capacitor, C
S
Analog Inputs
Comparator_D Operation
Figure 17-2. Comparator_D Sample-And-Hold
The required sampling time is proportional to the size of the sampling capacitor (C
S
), the resistance of the
input switches in series with the short switch (R
i
), and the resistance of the external source (R
S
). The total
internal resistance (R
I
) is typically in the range of TBD k
Ω
. The sampling capacitor C
S
should be greater
than 100 pF. The time constant, Tau, to charge the sampling capacitor C
S
can be calculated with the
following equation:
Tau = (R
I
+ R
S
) × C
S
Depending on the required accuracy, 3 to 10 Tau should be used as a sampling time. With 3 Tau the
sampling capacitor is charged to approximately 95% of the input signals voltage level, with 5 Tau it is
charged to more than 99%, and with 10 Tau the sampled voltage is sufficient for 12-bit accuracy.
17.2.5 Output Filter
The output of the comparator can be used with or without internal filtering. When control bit CDF is set,
the output is filtered with an on-chip RC filter. The delay of the filter can be adjusted in four different steps.
All comparator outputs are oscillating if the voltage difference across the input terminals is small. Internal
and external parasitic effects and cross coupling on and between signal lines, power supply lines, and
other parts of the system are responsible for this behavior as shown in
. The comparator
output oscillation reduces the accuracy and resolution of the comparison result. Selecting the output filter
can reduce errors associated with comparator oscillation.
465
SLAU272C – May 2011 – Revised November 2013
Comparator_D
Copyright © 2011–2013, Texas Instruments Incorporated