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USB-1616HS-2 User's Guide Functional Details
21
The counter channels may return only the lower 16-bits of count value if that is sufficient for the application.
They could also return the full 32-bit result if necessary. Similarly, the digital input channel could be the full 24
bits if desired or only eight bits if that is sufficient. If the three counter channels are all returning 32-bit values
and the digital input channel is returning a 16-bit value, then 13 samples are being returned to the PC every scan
period, with each sample being 16-bits. The 32-bit counter channels are divided into two 16-bit samples—one
for the low word, and the other for the high word. If the maximum scan frequency is 166,666 Hz, then the data
bandwidth streaming into the PC is 2.167 MS/s. Some slower PCs may have a problem with data bandwidths
greater than 6 MS/s.
The USB-1616HS-2 has an onboard 1 MS buffer for acquired data.
Example: Sampling digital inputs for every analog sample in a scan group
The scan is programmed pre-acquisition and is made up of six analog channels (Ch0, Ch2, Ch5, Ch11, Ch13,
Ch15) and four digital channels (16-bits of digital input, three counter inputs.) Each of the analog channels can
have a different gain.
The acquisition is triggered and the samples stream to the PC via the USB cable. Each analog channel requires
one microsecond of scan time therefore the scan period can be no shorter than 6 µs for this example. All of the
digital channels are sampled at the start of scan and do not require additional scanning bandwidth as long as
there is at least one analog channel in the scan group. The 16-bits of digital input are sampled for every analog
sample in the scan group. This allows up to 1 MHz digital input sampling while the 1 MHz analog sampling
bandwidth is aggregated across many analog input channels.
The scan period can be made much longer than 6 µs—up to 1 second. The maximum scan frequency is one
divided by 6 µs, or 166,666 Hz. Note that digital input channel sampling is not done during the "dead time" of
the scan period where no analog sampling is being done either.
Figure 10. Analog and digital scanning, once per scan mode example
If the three counter channels are all returning 32-bit values and the digital input channel is returning a 1-bit
value, then 18 samples are returned to the PC every scan period, with each sample being 16-bits. Each 32-bit
counter channel is divided into two 16-bit samples—one for the low word and the other for the high word. If the
maximum scan frequency is 166,666 Hz, then the data bandwidth streaming into the PC is 3 MS/s. Some slower
PCs may have a problem with data bandwidths greater than 6 MS/s.
The USB-1616HS-2 has an onboard 1 MS buffer for acquired data.