Digital Electronics and Computer Interfacing Tim Mewes 5. Computer Interfacing – DAQ cards

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


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!


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


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


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


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


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


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


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: ?


6.4 GPIB examples• HP 54200 digitizing Oscilloscope

Display message:



Set the timebase:



Set the timebase:



Query voltage of a specified point:


6.5 GPIB using MAX


6.5 GPIB using MAX


6.5 GPIB using MAX

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Digital Electronics and Computer Interfacing Tim Mewes 5. Computer Interfacing – DAQ cards