Measurement Computing USB-2627 - Manual (Page 14)

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Chapter 4
Functional Details
Analog input modes
The USB-2627 can acquire analog input data in two modes – software paced and hardware paced.
Software paced
You can acquire one analog sample at a time in software paced mode. You initiate the A/D conversion with a
software command. The analog value is converted to digital data and returned to the computer. Repeat this
procedure until you have the total number of samples that you want.
The sample rate in software paced mode is system-dependent and can range from 33 S/s to 4000 S/s.
Hardware paced
You can acquire data from up to 16 channels in hardware paced mode. The analog data is continuously
acquired, converted to digital values, and written into the 4k FIFO buffer on the device until you stop the scan.
The FIFO buffer is serviced in blocks as the data is transferred from the FIFO buffer to the computer memory
buffer. You start a continuous scan with either a software command or with an external hardware trigger event.
The maximum sampling rate in hardware paced mode from one to 16 channels is 1,000 kS/s, max.
Burst mode
Burst mode is an optional scan mode used with the onboard pacer to obtain more precise timing between
samples. When burst mode is enabled, each successive channel in a scan is sampled at the maximum A/D rate.
This ensures that samples from each channel are taken as close as possible to the same absolute point in time.
When burst mode is disabled, data is sampled at evenly spaced intervals, allowing you to increase the sample
period time; doing so can improve settling time and overall measurement accuracy.
Multi-channel scanning with burst mode enabled and disabled is shown in Figure 7.
Figure 7. Multi-channel scan with burst mode enabled and disabled
The burst mode sample period is 1 µs.
You can trigger the acquisition with the external trigger, and control the clock period with the internal A/D
pacer clock.