Digital Electronics and Computer Interfacing

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

VSupl. + V1 Vout V2 VSupl. - 5.5 Analog to Digital conversion 5.5.1 Comparator • Device that compares two Voltagesand switches its output to indicate which one is larger • An OP-Amp can be used as a comparator: Digital Electronics and Computer Interfacing

5.5.2 Direct conversion (flash) ADC Vref=3 V Comparator output HIGH for Vin>Vi R VSupl. - V3 3 2R VSupl.+ VSupl. - V2 2 2R VSupl.+ VSupl. - V1 1 R VSupl.+ Advantage: Speed - conversion typically takes about 10 ns! 0 Vin 5 V Disadvantage: Large number of comparators! Digital Electronics and Computer Interfacing

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

5.5.3 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

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 Digital Electronics and Computer Interfacing

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

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

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

6.3 GPIB and LabVIEW • GPIB queryfirst sends a command and then reads the response of the instrumentquery • Query commands typically end with a question mark: ? Digital Electronics and Computer Interfacing

6.4 GPIB examples • HP 54200 digitizing Oscilloscope Display message: Digital Electronics and Computer Interfacing

6.4 GPIB examples • HP 54200 digitizing Oscilloscope Set the timebase: Digital Electronics and Computer Interfacing

6.4 GPIB examples • HP 54200 digitizing Oscilloscope Query voltage of a specified point: Digital Electronics and Computer Interfacing

6.5 GPIB using MAX Digital Electronics and Computer Interfacing

Digital Electronics and Computer Interfacing