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lab2_Fall10

# lab2_Fall10 - ME 365 EXPERIMENT 2 INTRODUCTION TO DIGITAL...

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1 ME 365 EXPERIMENT 2 INTRODUCTION TO DIGITAL DATA ACQUISITION Objectives After completing this experiment, you should be able to Understand and explain how an analog signal is converted to a digital signal Explain why clipping occurs and how to prevent it Explain why aliasing occurs and how to prevent it Determine the quantization interval for an analog to digital converter and how it is related to the quantization error Background For this experiment a simple "Digital Data Acquisition System" (DDAS) has been constructed. A simplified block diagram of the system is shown below. A more detailed diagram is shown in Figure 2 and a circuit diagram is shown in Figure 5. A single input is digitized with an analog to digital converter (ADC) and then converted back to an analog signal with a digital to analog converter (D/A), with the option of passing this output through a low pass filter. Figure 1: Block Diagram of analog to digital back to analog system. Note that there is no "front end" containing signal conditioning elements (filters and amplifiers). There are two golden rules for putting signals into analog to digital converters: 1. make sure that the signal amplitude is as big as possible but still within the input range of the analog to digital converter (amplify or attenuate the signal to achieve this). 2. make sure that the highest frequency in the signal is less than half the sample rate of the analog to digital converter (low pass filter the signal to be sure that this is true and make sure that the filter cut off frequency is much less than half the sample rate). 5 V D/A Zero order Hold Analog Input ADC 8 bits integers 0 2 8 - 1 Timing Control 5 V Analog Output Optional Low pass filter

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Digital Data Acquisition ME 365 2 There is only a single input channel in this system. Multichannel systems like the one used in the PC in the ME365 Lab, have a multiplexer in front of the analog to digital converter. A multiplexer enables analog to digital conversion to take place on each channel in sequence. The total sample rate for a digital system is equal to the number of channels times the sample rate on each channel. So, if you are quoted a maximum sample rate of 44KHz. for a two channel system, that often means you can sample at 44,000 samples per second if you use 1 channel but you can only sample at 22,000 samples per second if you wish to use 2 channels. Most systems would also have a sample and hold in front of the multiplexer. This piece of hardware holds the signal constant while the analog to digital conversion is taking place. If the analog to digital conversion is fast, relative to the speed at which the signals are changing, the system will work acceptably without a sample and hold. If you are doing multichannel data acquisition, then the sample and hold device becomes very important. This is because the sample and hold device holds all signals constant while it converts each signal in turn, and thus eliminates any time delays between samples taken on different channels. Figure 2: System Components
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lab2_Fall10 - ME 365 EXPERIMENT 2 INTRODUCTION TO DIGITAL...

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