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 An OP-Amp can be used as a comparator: VSupl. + VSupl. - V1 V2 Vout Digital Electronics and Computer Interfacing

5.5.2 Direct conversion (flash) ADC Vref=3 V Comparator output HIGH for Vin>Vi R V3 VSupl. - 3 2R VSupl.+ V2 VSupl. - 2 2R VSupl.+ V1 VSupl. - 1 R VSupl.+ Advantage: Speed - conversion typically takes about 10 ns! 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

Digital Electronics and Computer Interfacing 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

Digital Electronics and Computer Interfacing 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 Digital Electronics and Computer Interfacing

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 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 Digital Electronics and Computer Interfacing

Digital Electronics and Computer Interfacing 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 Digital Electronics and Computer Interfacing

Digital Electronics and Computer Interfacing 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

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

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

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

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

Digital Electronics and Computer Interfacing 6.5 GPIB using MAX Digital Electronics and Computer Interfacing

Digital Electronics and Computer Interfacing 6.5 GPIB using MAX Digital Electronics and Computer Interfacing

Digital Electronics and Computer Interfacing 6.5 GPIB using MAX Digital Electronics and Computer Interfacing