Configuring Ai Ground-Reference Settings In Software; Multichannel Scanning Considerations - National Instruments 622 Series User Manual

Chapter 4 Analog Input
AI ground-reference setting is sometimes referred to as AI terminal configuration.

Configuring AI Ground-Reference Settings in Software

You can program channels on an M Series device to acquire with different ground references.
To enable multimode scanning in LabVIEW, use the 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 referenced single-ended 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

M Series 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
M Series 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
specifications document for your DAQ device lists its settling time.
M Series devices are designed to have fast settling times. However, several factors can increase
the settling time which decreases the accuracy of your measurements. To ensure fast settling
times, you should do the following (in order of importance):
Use Low Impedance Sources—To ensure fast settling times, your signal sources should
1.
have an impedance of <1 kΩ. Large source impedances increase the settling time of the
NI-PGIA, and so decrease the accuracy at fast scanning rates.
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