Datasheet

Sampling Capacitor, C

CASHORT

Analog Inputs

Comparator_A+ Operation

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21.2.3 Input Short Switch

The CASHORT bit shorts the comparator_A+ inputs. This can be used to build a simple sample-and-hold

for the comparator as shown in Figure 21-2.

Figure 21-2. Comparator_A+ Sample-And-Hold

The required sampling time is proportional to the size of the sampling capacitor (C

), the resistance of the

input switches in series with the short switch (R

), and the resistance of the external source (R

). The total

internal resistance (R

) is typically in the range of 2 to 10 kΩ. The sampling capacitor C

should be greater

than 100 pF. The time constant, Tau, to charge the sampling capacitor C

can be calculated with the

following equation:

Tau = (R

+ R

) x C

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

charge to more than 99% and with 10 Tau the sampled voltage is sufficient for 12-bit accuracy.

21.2.4 Output Filter

The output of the comparator can be used with or without internal filtering. When control bit CAF is set, the

output is filtered with an on-chip RC-filter.

Any comparator output oscillates 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 Figure 21-3. The comparator output

oscillation reduces accuracy and resolution of the comparison result. Selecting the output filter can reduce

errors associated with comparator oscillation.

526

Comparator_A+ SLAU144J–December 2004–Revised July 2013

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