Measurement Computing DaqBoard 3000USB Series - Manual (Page 72)

5-10 Counter Input Modes 887794 DaqBoard/3000USB Series User’s Manual
Upper 16-bits of the 32-bit counter Lower 16-bits of the 32-bit counter
Range (Hz) Ticksize (nS) Averaging
Option
Range (Hz) Ticksize (nS) Averaging
Option
15u – 1500u 20833.333 1 1 – 100 20833.333 1
150u – 15m 2083.333 1 10 – 1k 2083.333 1
1500u – 150m 208.333 1 100 – 10k 208.333 1
15m – 1500m 20.833 1 1k – 100k 20.833 1
150m – 15 20.833 10 10k – 1M 20.833 10
1500m – 150 20.833 100 100k – 5M 20.833 100
15 – 1500 20.833 1000 1M – 5M 20.833 1000
Frequency Ranges for a 16-bit value, sampling error is less than 0.21%
Each frequency range given in the previous table-set can be exceded. If the input waveform goes under-
range by too much, the counter value will top out at 65535 indicating you have reached the lowest possible
frequency that can be measured on that range. If the input waveform goes over range by too much, the
counter will return values that are very course and have a lot of sampling error. The values returned will
have a small number of counts for the period duration. If an input waveform cannot fit within one of the
16-bit ranges shown above or requires much higher accuracy, then a 32-bit range should be considered.
Full 32-bit Counter
Range (Hz)
Ticksize (nS) Averaging Option
15u – 100 20833.333 1
150u – 1k 2083.333 1
1.5m – 10k 208.333 1
15m – 100k 20.833 1
150m – 1M 20.833 10
1.5 – 5M 20.833 100
15 – 5M 20.833 1000
Frequency Ranges for a 32-bit Value,
Sampling Error is Less than 0.21%
The 32-bit ranges shown above are much wider than the 16-bit ranges, but also require the full 32-bit value
to be returned. Since digital or counter channels do not take up any time in the scan period there is no
disadvantage in reading a 32-bit counter versus a 16-bit counter. The 32-bit frequency ranges can also be
exceeded with a loss of accuracy or topping out at 4,294,967,295 counts.
Some measurements will require the accuracy of an input waveform to be free of sampling error, having only
the absolute accuracy of the internal timebase as the source of error. Sampling error can be averaged out to
give the required result. In most cases, the 3000USB Series board can perform the required averaging on
the values before they are returned to the PC. The frequency ranges shown below will give a sampling error
that is less than 10ppm or 1ppm.
Full 32-bit Counter <10 ppm Full 32-bit Counter <1 ppm
Range (Hz) Ticksize (nS) Averaging
Option
Range (Hz) Ticksize (nS) Averaging
Option
15u – 500m 20833.333 1 15u – 50m 20833.333 1
150u – 5 2083.333 1 150u – 500m 2083.333 1
1.5m – 50 208.333 1 1.5m – 5 208.333 1
15m – 500 20.833 1 15m – 50 20.833 1
150m – 5k 20.833 10 150m – 500 20.833 10
1.5 – 50k 20.833 100 1.5 – 5k 20.833 100
15 – 500k 20.833 1000 15 – 50k 20.833 1000
High Accuracy Frequency Ranges for a 32-bit Value
that has little sampling error (<10ppm, <1ppm)
If the input frequency is required to have less than 1 ppm sampling error and is greater than 50kHz, use the
15– 50kHz, 1ppm range. The values returned will have sampling error that is greater than 1ppm but they
can be averaged by the PC software to further reduce the sampling error.