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Digitization
• When data acquisition hardware receives an analog signal it converts it to a voltage. An A/D (analog-to-digital) converter then digitizes the signal and makes it ready for transfer to a computer or to a display.
• Digitization of an analog signal requires two separate operations.– Define the number of points and the rate at which data
are acquired.– Quantization--conversion of data into numerical form.
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Signal Inputs into an A/D converter
• Single-Ended– All inputs are referenced to a common ground
• Adequate for high level signals• Less expensive but problematic if grounding
problems exist. (Ground Loop Problems)• Differential
– Differences between Hi input and Lo input are measured directly without the influence of ground loop interference.
• About 2 times the expense of single ended inputs
• Needs 2 times as many wires• Always use for thermocouples and low voltage
applications
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Conversion Scheme
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A/D (analog-to-digital) converters
• Many specifications are quoted by hardware manufacturers. Here, we’ll try to explain what some of them mean in practice. For example:– Resolution– Linearity– Throughput– Gain
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Resolution
• Resolution of an A/D converter is the number of steps into which the input range is divided. Resolution is usually expressed as bits (N) and the number of steps is 2 to the power of N.
• Example: A converter with a 12-bit resolution divides the range into 212 , or 4096 steps. – A 0-10 Volt range will be resolved to 10V/4096 or 0.25
mV.– A 0-100 mV range will be resolved to 0.0025 mV.– A -10 to 10 V range is resolved by 20V/4096.
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Throughput
• Throughput is the maximum rate at which the A/D converter can output data values.
• A converter that takes 10 microseconds to acquire and convert will generate about 100,000 samples per second.
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Accuracy
• % Reading + Count– Look at the scale being used. It will display the value,
as given by that scale.– Multiply the readying by the % reading value.– Add the “Counts” x the value of the least digit
presented.
The example in the manual:
V=134.2 mV
Accuracy = 134.2 x 0.008 + 2 x 0.1 = 1.3 mV
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Gain
• On board amplifiers may permit you to reduce the range and thus increase resolution.
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GPIB(General Purpose Interface Bus)
• Also known as IEEE-488• Started by HP (HPIB)
– 16 line parallel connection
• Advantages– Fast data transfer rates
• Up to 1 MB/s
– Multiple devices (15) on each GPIB
• Disadvantages– Limited transmission lengths (2
m to 4 m) between devices– Need GPIB adapter in PC