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

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
Digital Electronics and Computer Interfacing

3. 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

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

6. Digital Electronics and Computer Interfacing
Tim Mewes
6. Computer Interfacing – GPIB bus

7. 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

8. 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

9. 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

10. 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

11. 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

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

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

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

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

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

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

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