Download Interfacing and A/D Conversion

A/D Conversion and
Interfacing
Physics 270
Voltmeters
Analog Voltmeter
Digital Voltmeter
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•
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Signal Scaler
(attenuator amplifier)
Digital conditioner
(reduces scaled input
signal to a DC
voltage within range
of A/D converter)
A/D Converter
Control Logic
Analog versus Digital
• Analog is continuous.
• Digital is discrete.
• A/D = analog to digital
• D/A = digital to analog
• Digital… base 2 system
• Analog… base 10 system
Digital Representation
Digital Representation
Digital Representation
How many bits would you need to divide 10 V into 0.01 V
intervals?
• If you divide 10 V into 0.01 V intervals you need 1000
intervals.
• If you need 1000 intervals you need to think about a
power of 2 that is larger than 1000.
• The smallest power of 2 that is larger than 1000 is
210 which is equal to 1024.
• That means that you need 10 bits in the converter, and
then the count in the counter/register will run from 0 to
1023.
• And that leads us to observe that real converters often
go to 10.23 V, not 10V, because that gives perfect 0.01 V
increments between resolvable voltages.
• And another converter might run from -5.12v to
+5.11v for the same reason.
Digital Representation
Digital Representation
Quantization Error
Quantization Error
Offset Error
Gain Error
Other sources of error
Come from non-linearity,
Missing bits, etc.
Noise
Voltage-Measurement Error
Current-Measurement Error
To minimize the percentage error, an ammeter's input resistance should be less
than the Thevenin resistance of the original circuit by 20 times or more.
Conversely, a voltmeter should have an input resistance that is larger than the
Thevenin resistance of the original circuit by more than 20 times. The same
goes for the ohmmeter; it should have an input resistance that is at least 20
times the Thevenin resistance of the original circuit. By following these simple
rules, it is possible to reduce the error to below 5 percent.
LabView
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Environment – used everywhere
• Learning Curve
• Labview Introduction Video
– http://zone.ni.com/wv/app/doc/p/id/wv-411/nextonly/y