Laboratory

1
Laboratory 4
Corrected Version
Digital-to-Analog and Analog-to-Digital Converters
In this lab you will build and test several basic
DAC and ADC circuits.

• Equipment
• Knight Electronics Minilab with connecting wires
  
• Digital Multimeter
• PC with Digital Storage Oscilloscope attachment
• 1 of 15kW, 2 of 30kW, 3 of 60kW , and 1 of
  120kW resistors
• Operational amplifier

• Preparation
• Review operational amplifiers in Nilsson and
  Riedel. Op Amps are also covered in Chapter 14 of
  our textbook.

Description of Experiment A The circuit diagram
of a simple DAC is shown below.
SPDT Switch

• Connect the circuit for N3, using R 30kW. You
  will need a total of four resistors. Use a Vref
  of 5V.
• We dont have a sufficient number of SPDT
  switches but you can simulate throwing the
  switches by plugging jumper wires into the Knight
  Minilab board.
• There are eight combinations of switch settings.
  Measure the output voltage for each of these
  combinations.
• Calculate the output voltage you would expect
  using the analyses from Nilsson and Riedel.
  Remember that, for a high gain Op Amp, the
  voltage difference between the and
  terminals on the op amp will approach zero when a
  resistor is connected between the output terminal
  and the terminal. Therefore, it is
  straightforward to calculate i0 and then v0.

2
Corrected Version

• Compare the calculated values of v0 to those you
  measured.
• You will very likely see some error. Precision
  resistors would be used in a real DAC.
• Use the digital multimeter to measure each
  resistor and recalculate v0. You should now have
  very good agreement between the measurements and
  the calculations.
• Make a plot showing v0 versus the digital input
  (x-axis) for the three cases
• Original calculations
• Measured values
• Calculations using measurements of each
  resistor.
• Explain the differences

• Description of Experiment B
• The DAC can be used as the basis for an Analog to
  Digital Converter (ADC) as shown to the right.
• Use the DAC you built in Experiment A. Keep
  vref 5V.
• Use the digital multimeter as a voltage
  comparator. Attach the positive terminal of the
  multimeter to the vin terminal. When vin gt vo,
  the multimeter shows a positive voltage and vice
  versus.
• You can provide the functions performed by the
  control circuits by observing the polarity of the
  feedback voltage from the comparator and then
  throwing the appropriate switches.

• The final position of the switches indicates the
  digital output.
• The control sequence uses a binary search and
  you can proceed as follows
• First calibrate the system by setting all the
  switches at the 1 position. The output voltage
  should be zero.
• Next set all the switches at the 2 position.
  The output voltage should be at the maximum
  level, vmax which you measured in Experiment A.

3
Corrected Version

• Reset all the switches to the 1 position.
  Throw the switch for the most significant digit
  from the 1 to the 2 position. This will swing
  the output voltage from zero to about ½ the
  maximum voltage you measured above with all
  switches at the 2 position.
• If the voltage comparator reads a positive
  value, we know that vin is greater than this
  voltage. This means that the most significant
  digit is 1.
• Conversely, if the voltage comparator reads a
  negative value, we know that vin is less than
  approximately vmax/2. If this is the case, throw
  the switch back to the 1 position. This tells
  you that the most significant digit is 0.
• Next proceed to the next significant bit and
  repeat the sequence.
• Then proceed to the third significant bit.
• The settings of the switches now tell you the
  digital value of vin.

• The Lab Instructor will provide you an unknown
  voltage to measure.
• Please use the ADC to measure the unknown
  voltage first.
• After you have measured the digital value of
  the unknown voltage, calculate the decimal value.
  
• Then use the digital multimeter to measure the
  unknown.
• Compare the results.
• How much error do you have?
• Explain the sources of the error? What is the
  effect of having a three bit DAC rather than a
  12 bit DAC.

Description of Experiment C
The DAC in experiment A requires a wide range of
resistor values. For a 12 bit DAC, the resistor
range would be 213/1 8192/1. This is difficult
to manufacture to high accuracy. Therefore, the
more practical implementation is the ladder
network shown below.
4
Corrected Version

• Connect up this circuit with R 30kW. Use R/2
  for the feedback resistor of the Op Amp.
• Repeat the measurements made in Experiment A.
• Compare the results to those of Experiment A.

• Lab Report
• Follow the guidelines given for the earlier labs
• Be sure to clearly document all of your
  measurements, observations and conclusions.