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Unformatted text preview: horizontal line makes with the pattern and dividing by the number of intersection points a vertical line makes with the pattern, you can find the ratio of the two frequencies. In the following example, there are four intersection points on a horizontal line and two on a vertical line, giving a ratio of 2:1. In addition, if the two signals are perfect harmonics, the Lissajous pattern can also give the phase between the two signals. Load Lissajous1.vi and investigate the phase relationship of two harmonically related signals. Figure 7-8. LabVIEW Simulation for a Lissajous Plot 2=2*1 National Instruments Corporation 7-7 Fundamentals of Digital Electronics Lab 7 Digital-to-Analog Converter Lab 7 Library VIs (Listed in the Order Presented) DAC.vi (8-bit DAC simulation) ADD R1,R2.vi (8-bit binary adder) MC1408.vi (simulation of a Motorola 1408 DAC IC) DAC Resolution.vi (4-bit, 6-bit and 8-bit DAC simulation) DAC+/-.vi (unsigned and signed DACs) Lissajous.vi (simulation of a Lissajous plot) DAC8/12.vi (resolution of an 8-bit and a 12-bit DAC) DAC12.vi (subVI used in DAC8/12.vi) BIN_RST.vi (8-bit binary counter with reset) Half Adder.vi (subVI used in ADD R1,R2.vi) Full Adder.vi (subVI used in ADD R1,R2.vi) FlipFlop.vi (subVI used in ADD R1,R2.vi) Fundamentals of Digital Electronics 7-8 National Instruments Corporation Lab 8 Analog-to-Digital Converters, Part I
The analog-to-digital converter, known as the A/D converter (read as A-to-D converter) or the ADC, is the second key component to bridging the analog and digital worlds. The ADC is the basis of digital voltmeters, digital multimeters, multichannel analyzers, oscilloscopes, and many other instruments. There are many different ADC designs, of which the ramp, tracking, and successive approximation converters are common. This lab looks at the ramp and tracking A/D converters. Purpose of the Analog-to-Digital Converter
The purpose of an ADC converter is to produce a digital binary number that is proportional to an analog input signal. The fundamental conversion process is shown in the following diagram. Input Voltage + C Test Voltage DAC b7 Counter b0 Reset Figure 8-1. Symbolic Design for an 8-Bit Analog-to-Digital Converter A counter creates a test binary sequence, and its digital output is converted into an analog voltage using a digital-to-analog converter. The DAC is a basic element of many ADC circuits and was discussed in Lab 7. (This is a good time to review its operation if you are not familiar with the DAC.) The test voltage is then compared with the input signal. If the input signal is larger than the test signal, the counter is increased to bring the test signal closer to the input level. If the input signal is smaller than the test signal, the counter is decreased to bring the test signal closer to the input level. The National Instruments Corporation 8-1 Fundamentals of Digital Electronics Lab 8 Analog-to-Digital Converters, Part I process continues until the comparator changes sign, at which time the test level will be within one count of the input level. Increasing the number of bits of the counter and DAC increases the conversion resolution. The Ramp ADC
The ramp ADC uses a binary counter and digital-to-analog converter to generate a ramp test waveform. In this demonstration, an 8-bit binary up counter, Binary Counter.vi, together with the 8-bit DAC, DAC.vi (introduced in the last lab), generate the test waveform. The test level will rise from 0 to 255 and repeat if left in the free running mode. However, when the test level becomes greater than--or in this case, equal to--the input level, the comparator will change sign and stop. Figure 8-2. LabVIEW VI to Simulate an 8-Bit Ramp ADC The last value on the binary bits (b7-b0) is the digitized value of the input voltage level. In the LabVIEW simulation, a wait time of 60 ms is chosen so that the eye can follow the action. The comparator function is simulated with the LabVIEW Equal function. Load and run the simulation VI Ramp.vi and follow the action on the front panel. Try other values of the input level and note that the conversion time depends on the input voltage level. Fundamentals of Digital Electronics 8-2 National Instruments Corporation Lab 8 Analog-to-Digital Converters, Part I Figure 8-3. LabVIEW Front Panel of 8-Bit ADC Converter. The Comparator LED Indicator Changes State When the Test Waveform Numeric Value Exceeds the Voltage Input In the next simulation, Ramp4.vi, the binary counter is allowed to free run. Whenever the test signal is greater than the input level, the comparator changes sign. This intersection of the ramp waveform with the input level can be seen on a chart display. The binary value of the counter at the intersection point is the digitized signal. The transition of the comparator indicates this event. If the changing state of the comparator resets the binary counter, a true ramp ADC is simulated. In this case, the binary counter is replaced with the binary counter with reset, featured in Lab 6. Load the VI Ramp2....
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This note was uploaded on 12/01/2009 for the course S ss taught by Professor S during the Spring '09 term at Universidad Autonoma de Nuevo Leon - School of Business.
- Spring '09