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Digital Electronics and Computer Interfacing
Tim Mewes
5. Computer Interfacing – DAQ cards
Digital Electronics and Computer Interfacing 2
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
• An OP-Amp can be used as a comparator:
VSupl. +
VSupl. -
V1
V2
Vout
21
21
VVforV
VVforVV
Supl
Suplout
Digital Electronics and Computer Interfacing 3
5.5.2 Direct conversion (flash) ADC
VSupl.+
VSupl. -
VSupl.+
VSupl. -
VSupl.+
VSupl. -
R
2R
2R
R
Vref=3 V
Vin0
1
2
3
Comparator output HIGH for Vin>Vi
5 V
V3
V2
V1
refreftot
ref VR
RV
R
RVV
6
5)
61()1(3
VoltsV 5.23
reftot
ref VR
RVV
2
1)
31(2
VoltsV 5.12
reftot
ref VR
RVV
6
1)
51(1
VoltsV 5.01
Advantage: Speed - conversion typically takes about 10 ns!
Disadvantage: Large number of comparators!
Digital Electronics and Computer Interfacing 4
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
Digital Electronics and Computer Interfacing 5
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
Digital Electronics and Computer Interfacing
Tim Mewes
6. Computer Interfacing – GPIB bus
Digital Electronics and Computer Interfacing 7
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
Digital Electronics and Computer Interfacing 8
6.2 GPIB commands
• Over the years three levels of standardizationdeveloped
• IEEE 488.1• IEEE 488.2• SCPI: Standard Commands
for Programmable Instrumentshighest level – devices using SCPI commands are easily to exchangeFor example: all Voltmeters usingSCPI will respond to thesame command
Digital Electronics and Computer Interfacing 9
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 primaryand secondary address of the device in the formatprimary+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
Digital Electronics and Computer Interfacing 10
6.3 GPIB and LabVIEW
• GPIB read reads byte countnumber of bytes from the devicereferenced by the address string
• The command terminates when the specified number ofbytes 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
Digital Electronics and Computer Interfacing 11
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: ?
Digital Electronics and Computer Interfacing 12
6.4 GPIB examples• HP 54200 digitizing Oscilloscope
Display message:
Digital Electronics and Computer Interfacing 13
6.4 GPIB examples• HP 54200 digitizing Oscilloscope
Set the timebase:
Digital Electronics and Computer Interfacing 14
6.4 GPIB examples• HP 54200 digitizing Oscilloscope
Set the timebase:
Digital Electronics and Computer Interfacing 15
6.4 GPIB examples• HP 54200 digitizing Oscilloscope
Query voltage of a specified point:
Digital Electronics and Computer Interfacing 16
6.5 GPIB using MAX
Digital Electronics and Computer Interfacing 17
6.5 GPIB using MAX
Digital Electronics and Computer Interfacing 18
6.5 GPIB using MAX