Lab ADA: Analog to Digital to Analog Exponential Decay Demo
Version: Nov 2002
analog signal sent into a computer must pass through an A-D converter.
You can think of an A-D converter as a "universal sensor" of analog voltages...
This lab explores the amplitude resolution aspects of A-D conversion, using external chips
from Analog Devices.
Go to the Analog Devices website and access the datasheets for
the 7576 8-bit A-D converter, and the 7524 DAC. Hook up the 7576 to read a
voltage from a VI: A suggested VI is shown below, where the Analog-OUT provides the
7576 ADC input.
Clock the 7576 at 1MHz, from
your Agilent waveform generator.
How many clock cycles are needed for one conversion? See datasheet.
Read in the 7576 datasheet
about mode control, pin 3 (page 5).
Let the mode be synchronous, i.e., the 7576 waits for a start-conversion
signal. See Fig 5 in the 7576 datasheet.
Send a digital signal from your VI to start a conversion once per second.
See waveform generator and Digital Out icons in the VI above.
DAC. Let the digital
outputs of the ADC chip be the inputs to the
7524 DAC, which in turn will project to an Analog-In channel on the
green connector card. On the next cycle of the FOR loop,
A fraction (1/10) of the Analog-In will be subtracted from the previous
Analog-Out to become the next Analog-Out, which will become the next
Analog-In, etc. A closed loop is created. See figure below and VI above.
What you need to show.
When your VI For loop runs, show that the Analog
signal decreases "exponentially" over the 10 steps suggested by the VI.
In LabVIEW or on your scope show the Analog-In and Analog-Out signals
on the same graph, and compare what 8-bit resolution does.
Can LabVIEW display the difference between the two waveforms? Can the scope?
Disable the 4 least significant bits from the ADC to the DAC
and repeat the comparison.
DAQ card: When
the Analog-In is read by LabVIEW it is converted to digital
by an internal A-D chip, and when an Analog-Out is generated by the DAQ card, it is
converted from digital by an internal DAC chip. Thus there are really 4 converters in the
overall loop you have created.