barcode reader in asp.net codeproject Performance Analysis of an Integrated-Circuit ADC in Software

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EXAMPLE 1511 Performance Analysis of an Integrated-Circuit ADC
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Using the data sheets for the AD574 (supplied in the enclosed CD-ROM), answer the following questions: 1 2 What is the accuracy (in volts) of the AD574 What is the highest frequency signal that can be converted by this ADC without violating the Nyquist criterion
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is a form of signal distortion that occurs when an analog signal is sampled at an insuf cient
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Known Quantities: ADC supply voltage; input voltage range Find: ADC accuracy; maximum signal frequency for undistorted A/D conversion Schematics, Diagrams, Circuits, and Given Data: VCC = 15 V; 0 Vin 15 V ADC
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speci cations found in device data sheet
Analysis:
From the data sheet we determine that the AD574 is a 12-bit converter The accuracy is limited by the least-signi cant bit (LSB) For a 0 15-volt range, we can calculate the magnitude of the LSB to be: Vin, max Vin, min 15 = 12 ( 1 bit) = 366 mV 2n 1 2 1
The data sheet states that the maximum guaranteed conversion time of the ADC is 35 s; therefore the highest conversion frequency for this ADC is: fmax = 1 = 2857 kHz 35 10 6
Since the Nyquist criterion states that the maximum signal frequency that can be sampled wthout aliasing distortion is one half of the sampling frequency, we conclude that the maximum signal frequency that can be acquired by this ADC is approximatly 14 kHz
Comments: In practice, it is a good idea to oversample by a certain amount A
reasonable rule of thumb is to oversample by a factor of 2 to 5 Suppose we chose to oversample by a factor of 2; then we would not expect to have signal content above 7 kHz One way to ensure that the signal being sampled is limited to a 7-kHz bandwidth is to pre lter the signal with a low-pass lter having a cutoff frequency at or below 7 kHz The active lters discussed in an earlier section of this chapter are often used for this purpose
Data Acquisition Systems The structure of a data acquisition system, shown in Figure 1532, can now be analyzed, at least qualitatively, since we have explored most of the basic building blocks A typical data acquisition system often employs an analog multiplexer, to process several different input signals A bank of bilateral analog MOSFET switches, such as the one we described together with the sample-and-hold ampli er, provides a simple and effective means of selecting which of the input signals should be sampled and converted to digital form Control logic, employing standard gates and counters, is used to select the desired channel (input signal), and to trigger the sampling circuit and the ADC When the A/D conversion is completed, the ADC sends an appropriate end of conversion signal to the control logic, thereby enabling the next channel to be sampled In the block diagram of Figure 1532, four analog inputs are shown; if these were to be sampled at regular intervals, the sequence of events would appear as depicted in Figure 1533 We notice, from a qualitative analysis of the gure, that the effective sampling rate for each channel is one fourth the actual external clock rate; thus, it is important to ensure that the sampling rate for each individual
15
Electronic Instrumentation and Measurements
External clock
Control logic
V1 End of Amplifier Trigger Trigger conversion Sample Digital A/D and output hold
Analog input signals
V4 Analog multiplexer
Internal clock
Figure 1532 Data acquisition system
channel satis es the Nyquist criterion Further, although each sample is held for four consecutive cycles of the external clock, we must notice that the ADC can use only one cycle of the external clock to complete the conversion, since its services will be required by the next channel during the next clock cycle Thus, the internal clock that times the ADC must be suf ciently fast to allow for a complete conversion of any sample within the design range These and several other issues are discussed in the next Focus on Measurements box
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