Measurement Computing USB-2533, User Manual

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USB-2533 User's Guide Functional Details
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Synchronous I/O – mixing analog, digital, and counter scanning
The USB-2533 can read analog, digital, and counter inputs, while generating digital pattern outputs at the same
time. Digital and counter inputs do not affect the overall A/D rate because these inputs use no time slot in the
scanning sequencer.
For example, one analog input channel can be scanned at the full 1 MHz A/D rate along with digital and counter
input channels. Each analog channel can have a different gain, and counter and digital channels do not need
additional scanning bandwidth as long as there is at least one analog channel in the scan group.
Digital input channel sampling is not done during the "dead time" of the scan period where no analog sampling
is being done either.
Analog input
The USB-2533 has a 16-bit, 1-MHz A/D coupled with 64 single-ended, or 32 differential analog inputs. Seven
software programmable ranges provide inputs from ±10 V to ±100 mV full scale.
Analog input scanning
The USB-2533 has several scanning modes to address various applications. You can load the 512-location scan
buffer with any combination of analog input channels. All analog input channels in the scan buffer are measured
sequentially at 1 µs per channel.
For example, in the fastest mode, with a 0 delay between the end of scan and the start of scan, a single analog
channel can be scanned continuously at 1 MS/s; two analog channels can be scanned at 500 kS/s each; 16
analog input channels can be scanned at 62.5 kS/s.
Example: Analog channel scanning of voltage inputs
Figure 8 shows a simple acquisition. The scan is programmed pre-acquisition and is made up of six analog
channels (Ch0, Ch1, Ch3, Ch4, Ch6, and Ch7). Each of these analog channels can have a different gain. The
acquisition is triggered and the samples stream to the PC via DMA. Each analog channel requires one
microsecond of scan time—therefore the scan period can be no shorter than 6 µs for this example. The scan
period can be made much longer than 6 µs—up to 1 s. The maximum scan frequency is one divided by 6 µs or
166,666 Hz.
Figure 8. Analog channel scan of voltage inputs example
Example: Analog channel scanning of voltage and temperature inputs
Figure 9 shows a programmed pre-acquisition scan made up of six analog channels (Ch0, Ch1, Ch5, Ch11,
Ch12, Ch13). Each of these analog channels can have a different gain. You can program channels 0 and 1 to
directly measure TCs.