PHY 406 - Microprocessor Interfacing Techniques

Module 1 - Simple Analog Input and Output

Objective

There are three parts to this module: Analog Output, Analog Input and Advanced. The Advanced section is optional and carries no marks.

Preparation

We will be using functions from the Functions>>Data Acquisition>>Analog Input and Functions>>Data Acquisition>>Analog Output pallettes.

You will need the following Vis

• Functions>>Data Acquisition>>Analog Input>>AI Sample Channel
• Functions>>Data Acquisition>>Analog Input>>AI Acquire Waveform
• Functions>>Data Acquisition>>Analog Output>>AO Update Channel
• Functions>>Data Acquisition>>Analog Output>>AO Generate Waveform

If you stick with channel 0 as the input and output channels, you can use the default values for the device and channel. You can also ignore the "limits" inputs.

Remember that Ctrl-h brings up help on any VI

Equipment

• Interface cable to LabVIEW system with "box" to break out AD0 and DA0
• Oscilloscope
• Waveform generator
• Connecting cables

Analog Output

1. What is the largest size of sine wave you can produce?
2. How does this relate to the fact that this is a 12-bit digital-to-analog converter?
3. What is the highest frequency of sine wave which you can produce which looks "reasonable"?
4. If an output is to be produced at fixed sample rate such that every state of the converter is to be used, what is the maximum output frequency as a function of the sample output rate?

Using the example VI E:\l_view\examples\anlg_ot2:

1. Replace the constant update rate with a variable number (ie a control).
2. Repeat the above exercise with a multi-point output variable output rate.
3. How do your conclusions for 1, 2, 3 and 4 above change or not with this new configuration?
4. Why doesn't the output sine wave look like a sine wave at high frequencies even if there are enough points per cycle?

Analog Input

Functions>>Data Acquisition>>Analog Input>>AI Sample Channel

and display the result as a chart on the front panel. You should include a control to vary the rate at which the samples are taken. You can use the signal generator as an input.

1. Your chart is in volts, but the A/D converter is described as a 12-bit converter. If the maximum range of the converter is 10V, what is the smallest theoretical change in input which can be detected? - Can you devise any practical test to see if this is in fact the case?
2. What is the fastest rate of input that you can get?
3. What is the highest frequency of sine wave that you can realistically examine at any given sampling rate? - (ie how many samples/cycle of the sine wave are required)
4. If it is required that the input changes less than the smallest measurable change (ie changing a single bit), what is the relationship between the sampling frequency, bit resolution, sine wave amplitude and sine wave frequency?

Change your VI to get an array of 100 points using the multi-point input VI and show it on a graph and then repeat the process. (You will need to adjust the sampling rate input of the multi-point VI).

1. How do your conclusions for 1, 2, 3 and 4 above change or not with this new configuration?

Advanced

• The waveform in the first example "jitters" because there is no trigger point - a point at which the waveform always starts. Devise a means to produce a trigger and then display a steady waveform.
• The problem of the sine wave not looking like a sine wave in the second part can be addressed by using a more sophisticated buffering scheme. It is possible to use a better buffering scheme with the VIs AO Config, AO Write, AO Start and AO Clear. Make a better sine wave generator.
• Build a VI which "listens" to a waveform, locates a single cycle, and then repeatedly outputs the waveform. This is the basis of an "intelligent" arbitrary waveform generator.

Notes