National Instruments Data Acquisition Device NI USB-621x - Manual (Page 38)

Chapter 4 Analog Input
© National Instruments Corporation 4-5 NI USB-621x User Manual
Configuring AI Ground-Reference Settings in Software
You can program channels on an USB-621x device to acquire with
different ground references.
To enable multimode scanning in LabVIEW, use NI-DAQmx Create
Virtual Channel.vi of the NI-DAQmx API. You must use a new VI for
each channel or group of channels configured in a different input mode. In
Figure 4-3, channel 0 is configured in differential mode, and channel 1 is
configured in RSE mode.
Figure 4-3. Enabling Multimode Scanning in LabVIEW
To configure the input mode of your voltage measurement using the DAQ
Assistant, use the Terminal Configuration drop-down list. Refer to the
DAQ Assistant Help for more information about the DAQ Assistant.
To configure the input mode of your voltage measurement using the
NI-DAQmx C API, set the terminalConfig property. Refer to the
NI-DAQmx C Reference Help for more information.
Multichannel Scanning Considerations
USB-621x devices can scan multiple channels at high rates and digitize the
signals accurately. However, you should consider several issues when
designing your measurement system to ensure the high accuracy of your
measurements.
In multichannel scanning applications, accuracy is affected by settling
time. When your USB-621x device switches from one AI channel to
another AI channel, the device configures the NI-PGIA with the input range
of the new channel. The NI-PGIA then amplifies the input signal with the
gain for the new input range. Settling time refers to the time it takes the
NI-PGIA to amplify the input signal to the desired accuracy before it is
sampled by the ADC. The NI USB-621x Specifications lists settling time.
USB-621x devices are designed to have fast settling times. However,
several factors can increase the settling time which decreases the accuracy