National Instruments DAQ X Series User Manual page 173

Again the measurement time for the one counter measurement is lowest but
the accuracy is lower. Note that the accuracy and measurement time of the
sample clocked and two counter large range are almost the same. The
advantage of the sample clocked method is that even when the frequency to
measure changes, the measurement time does not and error percentage
varies little. For example, if you configured a large range two counter
measurement to use a divide down of 50 for a 50 k signal, then you would
get the accuracy measurement time and accuracy listed in Table 7-3. But if
your signal ramped up to 5 M, then with a divide down of 50, your
measurement time is 0.01 ms, but your error is now 0.1%. The error with a
sample clocked frequency measurement is not as dependent on the
measured frequency so at 50 k and 5 M with a measurement time of 1 ms
the error percentage is still close to 0.001%. One of the disadvantages of a
sample clocked frequency measurement is that the frequency to be
measured must be at least twice the sample clock rate to ensure that a full
period of the frequency to be measured occurs between sample clocks.
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Low frequency measurements with one counter is a good method for
many applications. However, the accuracy of the measurement
decreases as the frequency increases.
High frequency measurements with two counters is accurate for high
frequency signals. However, the accuracy decreases as the frequency
of the signal to measure decreases. At very low frequencies, this
method may be too inaccurate for your application. Another
disadvantage of this method is that it requires two counters (if you
cannot provide an external signal of known width). An advantage of
high frequency measurements with two counters is that the
measurement completes in a known amount of time.
Measuring a large range of frequencies with two counters measures
high and low frequency signals accurately. However, it requires
two counters, and it has a variable sample time and variable error %
dependent on the input signal.
Again, the measurement time for the one counter measurement is
lowest, but the accuracy is lower. Note that the accuracy and
measurement time of the sample clocked and two counter large range
are the same. The advantage of the sample clocked method is that even
when the frequency to measure changes, the measurement time and
error % does not. For example, if you configured a large range
two counter measurement to use a divide down of 50 for a 50 kHz
signal, then you would get the accuracy measurement time and
accuracy listed in table 7-3. But if your signal ramped up to 5 MHz,
then with a divide down of 50, your measurement time would be
0.01 ms, but your error would now be 0.001%. The error with a sample
7-21
Chapter 7 Counters
X Series User Manual