Digital Electronics and Computer
Interfacing
Tim Mewes
5. Computer Interfacing – DAQ cards
5.5 Analog to Digital conversion
5.5.1 Comparator
• Device that compares two Voltages
and switches its output to indicate
which one is larger
V1
• An OP-Amp can be used as a
comparator:
VSupl. +
Vout
V2
V out
  V Supl for V 1  V 2
 
  V Supl for V 1  V 2
Digital Electronics and Computer Interfacing
VSupl. -
2
5.5.2 Direct conversion (flash) ADC
Vref=3 V
Comparator output HIGH for Vin>Vi
V 3  V ref (1 
R
VSupl. -
V3
2R
3
VSupl.+
VSupl. -
V2
2R
VSupl. -
R
2
1
6R
)
5
6
V ref
3R
)
R tot
1
2
V ref
V 1  V ref (1 
5R
)
R tot
1
6
V ref
 V 1  0 . 5 Volts
0
5V
R tot
R
 V 2  1 . 5 Volts
VSupl.+
Vin
)  V ref (1 
 V 3  2 . 5 Volts
V 2  V ref (1 
VSupl.+
V1
R
Advantage:
Speed - conversion typically takes about 10 ns!
Disadvantage:
Large number of comparators!
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5.5.2 Successive approximation
• Input signal Vin is compared (using a comparator) with a
signal VDAC generated by a DAC
• Approximate Vin by successively setting the bits of the DAC:
• Turn off all bits
• Turn on most significant bit
if Vin > VDAC leave the bit on otherwise turn it off again
• Turn on the next significant bit
if Vin > VDAC leave the bit on otherwise turn it off again
…
• For an n-bit ADC it takes n-steps to converge to the final result
• Time for conversion: of the order of s
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5.5.3 Single slope integration
• Start ramp generator (constant current source & capacitor) together
with a counter that counts clock pulses
• When the ramp voltage equals the input Voltage a comparator stops
the counter
• Number of clock pulses counted is proportional to the input Voltage
• Resolution depends on the clock-frequency:
the higher the clock-frequency the better the resolution
• More bits for the counter needed for higher resolution
• Stable clock needed
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Digital Electronics and Computer
Interfacing
Tim Mewes
6. Computer Interfacing – GPIB bus
6.1 GPIB bus
• Digital communication standard for test and measurement
devices
• Initially developed by Hewlett-Packard (HP), also known as
 HP-IB (Hewlett-Packard Instrument Bus)
 GPIB (General Purpose Instrumentation Bus)
 IEEE-488.x (IEEE Standard Digital Interface for
Programmable Instrumentation x:1 or 2)
• 8-bit parallel communication
• data transfer rates up to 1 Mbyte/s
• One System Controller (PC)
• up to 15 additional instruments
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6.2 GPIB commands
• Over the years three levels of standardization
developed
• IEEE 488.1
• IEEE 488.2
• SCPI: Standard Commands
for Programmable Instruments
highest level – devices using SCPI
commands are easily to exchange
For example: all Voltmeters using
SCPI will respond to the
same command
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6.3 GPIB and LabVIEW
• GPIB write sends the
specified data string to
the device referenced by
the address string
• The address string consists of the primary
and secondary address of the device in the format
primary+secondary (use MAX to determine those)
• Both primary and secondary address can range from 0-30
• Example:
for a primary address “0” and secondary address “10”
use 0+10 as the address string
• The data string depends on the device – use its manual to
determine the string for a particular command
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6.3 GPIB and LabVIEW
• GPIB read reads byte count
number of bytes from the device
referenced by the address string
• The command terminates when the specified number of
bytes is read or when a Carriage Return/Line Feed
character is received
• The data string holds the string received from the
instrument – typically this string needs to be processed
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6.3 GPIB and LabVIEW
• GPIB query
first sends a command and then reads the response of
the instrumentquery
• Query commands typically end with a question mark: ?
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6.4 GPIB examples
• HP 54200 digitizing Oscilloscope
Display message:
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6.4 GPIB examples
• HP 54200 digitizing Oscilloscope
Set the timebase:
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6.4 GPIB examples
• HP 54200 digitizing Oscilloscope
Set the timebase:
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6.4 GPIB examples
• HP 54200 digitizing Oscilloscope
Query voltage of a specified point:
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6.5 GPIB using MAX
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6.5 GPIB using MAX
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6.5 GPIB using MAX
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