Mastering Electronics Via Labs

Introduction

In this experiment you will become familiar with an analog-to- digital converter (ADC) integrated circuit. The ADC is used to convert an analog signal to a digital representation. Once the signal is represented digitally, it can be processed by a computer in any number of interesting ways. The analog signal might be an audio signal or it may be a video signal or it may be the output of a thermometer or some other sensor-based system.

The Experiment Procedure

1 )  Connect the 0804 as shown in Figure 4. Define one set of connections as analog ground and another set as digital ground. Run separate wires from the power supply ground connection to each of the ground connections on the breadboard. Note the circuit notations for analog and digital ground and be sure that components are connected to the appropriate ground point in the circuit. The 0.1 mF capacitors from pins 6 and 9 to analog ground are noise suppressors, as is the 10 mF electrolytic capacitor from pin 20 to digital ground. The 150 pF capacitor from pin 4 to digital ground establishes the internal clock frequency along with the 10 k resistor from pin 19 to pin 4. The 1.0 k resistors in series with the LEDs (Light Emitting Diodes) are current limiting resistors. The switch from pin 3 to ground can be a wire which is momentarily touched to the circuit digital ground connection. Be careful with the bare leads on the 8 sets of resistor/LEDs from the digital outputs to Vcc so they do not short circuit.  It is best to run them perpendicular to the chip. Make sure the LED polarity is correct.

2 ) Adjust the voltage from pin 20 to ground to as close to 5.120 volts as you can get. Measure this voltage with the Keithley meter. Record the value you end up with. Note that because of the type of control in the power supply, you may need to settle for a slightly different voltage.

3 ) Measure the voltage from pin 9 to analog ground. It should be exactly half the voltage on pin 20, and hopefully, 2.560 volts. Record your exact value.

4) Now measure the internal clock frequency and waveform to be sure the clock is working. Use your CRO and your frequency counter. The frequency should be 640 kHz, but it usually ends up closer to 300 kHz. The exact frequency should be f = 1 / (1.1RC).

First measure the frequency and note the waveform at pin 19. Then observe the frequency and the waveform at pin 4. Record the results.

Next, observe the frequency and waveform at pin 5, noting that the voltage at pin 5 goes low after each conversion (64 clock cycles per conversion). Thus, the frequency at pin 5 should be 1/64th the frequency at pin 19 or pin 4. The waveform will be narrow, negative-going pulses with an amplitude of approximately 5 volts.

Can you explain the waveforms observed and any discrepancies between frequencies measured at the three pins? Note particularly the input capacitance of the CRO and compare with the 150 pF timing capacitor. The connecting cable also has capacitance. Do you expect the clock frequency remains constant as it is measured with the CRO?

5)   The jumper from pin 3 to pin 5 is used to produce free-running conversion. This means that the ADC does a conversion and then tells itself to do another after completing the first and continues to do conversion after conversion at the rate of about 10,000 conversions per second. To get the conversion process started properly, however, lots of internal parts need to be initialized. This is done by momentarily touching the wire from pin 3 to the digital ground. Once this is done, the ADC enters its free-running mode. Initialize your ADC by momentarily grounding pin 3.

6 ) Vary the input voltage over the range 0 - 5 V,  while measuring it with the Keithley. Your output indicator LEDs should begin counting in binary. Since +5V at a digital output represents binary "1", the LEDs will light when a "0" is present at the output and will extinguish when a "1" is present. Verify with a table that the LSB is 20 mV and that each additional 20 mV of d-c input voltage causes the output to increase by one binary count. (You may decide not to include all 256 entries in the table. Enter enough to be convincing.) Do the outputs change at exactly 20 mV input increments?

7) Now observe the effect of changing V_ref/2. Set the input voltage to 2.000 volts, using d-c output from the function generator. Connect the output of a power supply (0~5 V) between pin 9 (V_ref/2) and analog ground. Set this voltage at the value of V_ref/2 which will result in 10 mV for the LSB. Then check to see whether the ADC digital output is an accurate representation of the input voltage.

8) Noting that the input voltage of the ADC needs to remain between 0 and 5 for the output to register properly (and, more importantly, to avoid damaging the ADC), disconnect your function generator from the ADC input and adju