Measurement Computing USB-2527 - Manual (Page 25)

USB-2527 User's Guide Functional Details
25
Synchronous I/O
– mixing analog, digital, and counter scanning
The USB-2527 can read analog, digital, and counter inputs, while generating up to four analog outputs and
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-2527 has a 16-bit, 1-MHz A/D coupled with 16 single-ended, or eight differential analog inputs.
Seven software programmable ranges provide inputs from ±10 V to ±100 mV full scale.
Analog input scanning
The USB-2527 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 by default.
For example, in the fastest mode, with a 1 µs settling time for the acquisition of each channel, 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.
Settling time
For most applications, leave the settling time at its default of 1 µs.
However, if you are scanning multiple channels, and one or more channels are connected to a high-impedance
source, you may get better results by increasing the settling time. Remember that increasing the settling reduces
the maximum acquisition rate.
You can set the settling time to 1 µs, 5 µs, 10 µs, or 1 ms.
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. Using the default settling time, 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 1 divided by
6 µs, or 166,666 Hz.