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National Instruments Corporation
5
NI USB-443x Calibration Procedure
Changing Coupling on NI USB-443x Devices
NI USB-443
x
devices support both AC and DC coupling. After changing the coupling on a device from
DC to AC, wait 30 seconds for the inputs to settle. If samples are acquired before the 30 seconds has
elapsed, the acquired data may be distorted. This section illustrates a deterministic method for setting
the delay.
Figure 1.
Implicitly Setting Configuration Settings with the DAQmx Start Task VI
When channel configuration settings are set in software, NI-DAQmx keeps track of the desired settings
and downloads them to hardware simultaneously when the DAQmx Start Task VI is called. This means
that in Figure 1, the settings on the hardware change when the DAQmx Start Task VI is called, and not
when they are set earlier. Therefore, there is no 30 seconds wait between setting the coupling on the
device and starting the acquisition.
For flexibility, NI-DAQmx gives you the ability to download channel configuration settings to hardware
at any point by calling the DAQmx Control Task VI with the
commit
option as shown in Figure 2. When
programmed this way, the DAQmx Control Task VI ensures that the wait occurs after the coupling
setting has changed on the device.
Figure 2.
Explicitly Setting Configuration Settings with the DAQmx Control Task VI
Be sure to perform an explicit commit and wait 30 seconds whenever you change the coupling on an
NI USB-443
x
from DC to AC.
Note
NI-DAQmx may initialize the coupling on an NI USB-443
x
to DC when the device first
powers on. Similarly, NI-DAQmx may set the coupling to DC at the end of the execution of one of
the calibration adjustment VIs. Perform the 30 seconds wait procedure described in this section when
performing an AC-coupled acquisition after the device is powered on, and after calling any of the
calibration adjustment VIs.
