CP/PC364 Data Communications & Networks Laboratory
D/A and A/D Conversion
Overview
Digital transmission of analog data, (such as sound and video), requires analog input signals to be converted to digital signals for processing. Once received, the signals then have to be converted to back analog. These conversions are done by analog to digital (A/D) and digital to analog (D/A) converters, respectively. Although you could construct these converters from discrete components, integrated circuits specifically designed for these purposes are usually used.
Objectives
The objectives for this lab are:
- to test the D/A and A/D ICs for proper operation
- to investigate the effects of the D/A and A/D conversion process
At completion of the lab, you should have both an A/D circuit and a D/A circuit on one breadboard. One of the configurations you will be using in this lab is shown below. Use this as a guideline for positioning the circuits on the breadboard.
Do not dismantle the A/D and D/A circuits. Store these circuits in your lockup area for use later.
Preparation
Great care should be taken to avoid static discharge into CMOS based chips. |
Before starting the lab, review the CMOS handling procedures.
- Always use a ground strap. If your grounding mat doesn't have two grounding straps, one for each of the partners, see the lab instructor.
- CMOS devices should be stored pin down in conductive foam when they are not in a circuit.
- Never leave unused inputs floating; connect to ground or +5V to prevent excessive current consumption and erratic behaviour.
- Never connect an input signal to a CMOS device when the power is off.
Equipment
- bench supply, oscilloscope, function generator
- breadboard, debugger board, resistors, capacitors
- DAC0800 [PDF: 11 pages], a digital-to-analog converter from National Semiconductor
- ADC0804 [PDF: 41 pages], an analog-to-digital converter from National Semiconductor
- LM 336-2.5 2.5 V zener diode [PDF: 12 pages]
- LM 336-5.0 5.0 V zener diode [PDF: 11 pages]
Task
NOTE: For both the DAC and the ADC, use a zener diode circuit with a diode that is appropriate for the reference voltage. Do NOT use a supply voltage by itself or a resistor based voltage divider for the reference voltage.- Using a DAC0800, design a circuit to take in an
8-bit digital value and produce an appropriate analog output
between 0 and 5 volts.
The sample circuits shown in FIGURE 3. Basic Positive
Reference Operation and
FIGURE 9. Positive Low Impedance Output Operation
should be of use.
Demonstrate your circuit to the lab supervisor. Keep this circuit for future labs.
Analog Ground
versusDigital Ground
Take careful note of the different ground symbols. For any circuit handling analog inputs that has a digital component, separate grounds must be used for the analog and digital portions of the circuit to avoid digital noise from getting into the analog input circuit.
* Use separate buses for the analog ground and the digital ground.
* Bring the grounds together at a single point as close to the power supply as possible.[Explanation of second sentence Note 2 on page 4 of ADC0804 data sheet.]
- Using the ADC0804, construct a circuit to convert
a 0-5 volt input signal to a digital value from 0 to 255 and
run the ADC in continuous or free running mode.
Construct the circuit on the same breadboard that was used for
the DAC circuit.
- For the circuit you built, what is the speed of the ADC internal clock? See, §2.6 Clocking Option and do the calculations.
- What relationship does this speed have to the number of clock cycles required to complete a conversion?
- Determine the smallest voltage change which can be detected by the circuit. Measure the change in voltage by slowly varying the input voltage and watching for the change in state of the LEDs.
- Is this change in voltage the same for input voltages over the entire 0-5 volt range?
- Are these results consistent with the
expected resolution of the ADC? Show the calculations.
- Combine the DAC and ADC circuits by using the
ADC output lines to drive the DAC0800 as shown in the
figure below.
Warning: carefully check for the most and least significant
bits when connecting the digital lines;
the numbering of the bits is not consistent.
Drive the ADC input with a sine wave of the appropriate amplitude. Compare the analog output of the DAC with the original sine wave and sketch the signals.- How high a frequency can this system accept before the output becomes too distorted?
- What part of your system dominates this frequency limit?
A square wave input might make seeing where the output
deviates from the input easier.
Demonstrate your circuit to the lab supervisor.