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Analog Input

Module by: National Instruments. E-mail the author

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Summary: In this lesson, you will learn how to perform analog input.

Use analog input to perform analog-to-digital (A/D) conversions.

The available analog input measurement types for a task are voltage, temperature, strain, current, resistance, or frequency.

Figure 1
Figure 1 (aitasks.png)

Each measurement type has its own characteristics, such as resistor values for current measurements or strain gauge parameters for strain measurements.

Task Timing

When performing analog input, the task can be timed to Acquire 1 Sample, Acquire n Samples, or Acquire Continuously.

Acquire 1 Sample

Acquiring a single sample is an on-demand operation. In other words, NI-DAQmx acquires one value from an input channel and immediately returns the value. This operation does not require any buffering or hardware timing. For example, if you periodically monitor the fluid level in a tank, you would acquire single data points. You can connect the transducer that produces a voltage representing the fluid level to a single channel on the measurement device and initiate a single-channel, single-point acquisition when you want to know the fluid level.

Acquire n Samples

One way to acquire multiple samples for one or more channels is to acquire single samples in a repetitive manner. However, acquiring a single data sample on one or more channels over and over is inefficient and time consuming. Moreover, you do not have accurate control over the time between each sample or channel. Instead you can use hardware timing, which uses a buffer in computer memory, to acquire data more efficiently. Programmatically, you need to include the timing function and specify the sample rate and the sample mode (finite). As with other functions, you can acquire multiple samples for a single channel or multiple channels.

With NI-DAQmx, you also can gather data from multiple channels. For instance, you might want to monitor both the fluid level in the tank and the temperature. In such a case, you need two transducers connected to two channels on the device.

Acquire Continuously

If you want to view, process, or log a subset of the samples as they are acquired, you need to continually acquire samples. For these types of applications, set the sample mode to continuous.

Task Triggering

When a device controlled by NI-DAQmx does something, it performs an action. Two very common actions are producing a sample and starting a waveform acquisition. Every NI-DAQmx action needs a stimulus or cause. When the stimulus occurs, the action is performed. Causes for actions are called triggers. The start trigger starts the acquisition. The reference trigger establishes the reference point in a set of input samples. Data acquired up to the reference point is pretrigger data. Data acquired after the reference point is posttrigger data.

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