CP/PC364 Data Communications & Networks Laboratory

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:

Do not dismantle the A/D circuit. Store this circuit in your lockup area for use later.

Preparation

image Great care should be taken to avoid static discharge into CMOS based chips.

Before starting the lab, review the CMOS handling procedures.

  1. 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.
  2. CMOS devices should be stored pin down in conductive foam when they are not in a circuit.
  3. Never leave unused inputs floating; connect to ground or +5V to prevent excessive current consumption and erratic behaviour.
  4. Never connect an input signal to a CMOS device when the power is off.

Equipment

Task

To wire the ADC, there are 3 different sub-circuits to wire. They are: You will wire each of these sections in turn.
  1. Wire up the zener diode with an appropriate resistor to produce a 2.5 V reference voltage.
    Demonstrate your circuit to the lab supervisor.

  2. Look in the data sheet for the ADC0804 at the figure in dicated under Typical Applications.
    Note: All of the connections shown should be part of any circuit, except where something different is shown. The "8080 Interface" figure, for instance, don't show any power (or input!) connections so they assume wiring as in the previous figure.
    Wire up the ADC clock and power supply connections as shown, and wire the outputs to a debugger board.
  3. Look in the data sheet for the ADC0804 and find the diagram showing it operatiing in "self-clocking" or "free running" mode. This is the configuration you want to use.
    Wire the connections shown.

  4. Here is the circuit symbol for Analog Ground.
    image
    Here is the circuit symbol for Digital Ground.
    image

    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.]


  5. Connect the voltage reference you wired above, if you have not already done so. If you have done everything correctly, you should have a circuit which can take in a 0-5 volt input signal and produce a digital value from 0 to 255.
    Demonstrate your circuit to the lab supervisor.

    • 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.

Resources

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