Servo - June 2009

approximation of “011” or 3V
is compared with 3.25V and
again this is below the incom-
ing voltage. Therefore, the
1 remains and is placed in the
holding register in the LSB’s
place. So, when an analog
voltage of 3.25V is converted
in this three bit SAR, the
binary conversion is “011” or 3.

Enough Theory; Let’s Measure!

FIGURE 3

FIGURE 4

Now let’s look at converting analog voltages with the USB- 6008 unit. You probably noticed that two values were given for the number of bits used in the A-to-D converters in the USB- 6008( 9) units. When these converters are used in the differential mode — that is, two sides of an analog voltage are fed into the converter — they use the higher number of bits for the conversions, with a

FIGURE 5

total of four analog inputs possible. When they are used in the single-ended mode or RSE (respect to ground single ended) mode, the conversion bits are one less with a total of eight analog inputs possible.

In this article, we are going to use a 10K potentiometer, an LM34 temperature transducer, and an LDR (light dependent resistor) which can all be used in the single-ended or RSE mode,

FIGURE 7

and we are going to use a voltage range of 0V up to +5V. Therefore, for the USB-6008 unit the resolution will be: Resolution = (5V-0V) / (2¹¹- 1) or 5V /

2047 = 2.44 mV. Since the LM34 changes its output by 10 mV/°F, it will fit within the USB-6008’s A-to-D capabilities quite well.

Let’s first connect a 10K potentiometer to AI0 as shown in Figure 2. Now let’s make a VI that will display the analog voltage conversion result. Open a blank VI and place a numeric indicator and a stop button on the front panel by selecting from the controls palette: Express>Numeric Indicators>Num Ind and Express>Buttons>Stop. In the block diagram, open a DAQ Assistant by selecting from the functions palette: Express>Input>DAQ Assistant. From the DAQ Assistant pop-ups, select: Acquire Signals>Analog>Voltage; see Figure 3. Next select AI0 and finish; see Figure 4. From the final pop-up window, set the Max to 5 and the Min to 0 and the terminal configuration to RSE as shown in Figure 5, then select OK. Place the DAQ Assistant, the numeric

FIGURE 6

FIGURE 8

indicator, and the stop button inside of a while loop on the block diagram by selecting from the functions palette: Programming>Structures>While Loop and wire as in Figure 6.

When you run the VI, you will see the analog voltage change as you turn the potentiometer wiper. You can also connect a multimeter to the potentiometer and compare its readings against the VI display. Analog inputs can be displayed in more graphical ways than just through a numeric indicator. Let’s add a meter to the front panel by selecting: Express>Numeric Indicators> Meter. Open the meter’s properties window and under the scale tab set the maximum to five; see Figure 7. In the block diagram, place the meter’s icon inside of the while loop and wire it to the output of the DAQ Assistant as shown in Figure 8. Now you have a graphical display of the analog voltage along with the numeric indicator as in Figure 9.