Measurement Computing Personal Daq rev.6.0 - Manual (Page 59)

Personal Daq User’s Manual 989394 Personal DaqView 4-17
Acquisition Parameters
The lower section of the Configure Acquisition box is used for adjusting the scan rate, as well as enabling
or disabling Continuous Calibration and Overrange Protection modes. Each of these categories is
discussed below.
Scan Rate, Period, and Frequency
The lower region of the dialog box displays the maximum scan rate and the actual scan rate. This section
of the dialog box allows you to set the scan timing, i.e., the rate at which Personal DaqView takes readings.
It can be set to a frequency (number of scans per second) or period (the time length of one scan) value.
The maximum scan rate depends on the channel configuration and is displayed in the configure acquisition
window. It is determined by the number of enabled channels and their measurement duration.
To get the highest scan rate possible:
• turn off all unused channels
• set the measurement duration to its lowest setting on all analog channels
These steps can be performed from the Channel Configuration Window (when selected for Analog Input
Spreadsheet). Setting the highest possible scan rate inversly affects the resolution of your analog channel
readings. When setting the scan rate, it is important to be aware of various issues that arise with a low scan
rate. Among these are under sampling and aliasing.
Note:
The scan period can only change by increments of 1 msec. If you enter an invalid value,
Personal DaqView will automatically convert your entry to the nearest acceptable value.
Note: Aliasing errors can result from having a scan rate that is too low (under sampling).
Continuous Calibration
The continuous calibration option is useful in applications that make use of very long acquisitions periods.
When the continuous calibration box is checked (9), Personal DaqView continuously calibrates the unit
while the acquisition is in progress. Although this provides greater reading accuracy, it can result in a
lowering of the maximum scan rate. For this reason the continuous calibration option has a default setting
of disabled (unchecked).
Overrange Protection
By default, the overrange protection option is enabled. This is because certain ranges are slow to recover
from overload conditions. It should be noted that the slow recovery does not indicate any problem with the
unit; however, if there is such an overload on a channel, erroneous readings on other channels may occur.
If your application presents the possibility of a channel overload, then you should keep the overrange
protection function enabled (9). If using a custom application, use the
DcotpDaqOverrangeProtect
option-type setting with the daqSetOption API (see Appendix B for additional information regarding
daqSetOption ).