Lab Exercise 6: Digital/Analog conversion logic

Transcription

1 Lab Exercise 6: Digital/Analog conversion logic Introduction In this lab exercise, you will build and investigate circuits for analog-to-digital and digital-to-analog conversion Preparation Before arriving at the lab, you should do the following: Review the lecture notes and handouts. Read through the lab, and plan the circuits for each section. Lab setup For this lab, you will need the following equipment An experiment breadboard A pulse-generator An ADC0809 analog-to-digital converter IC A DAC08 digital-to-analog converter Exercises 1. Analog-to-digital converter (ADC) Read and consult the data sheet for the ADC0809 IC for this portion of the lab. Mount the ADC on your breadboard, and connect the power and ground pins to +5V and GND, respectively. You will need an analog voltage signal to test. Connect a potentiometer between +5V and GND, and use the sliding contact as an adjustable input to the ADC. The ADC has eight selectable inputs (IN0 to IN7), but for this lab only one is needed. Connect your analog input to IN0, and select this input by setting all three input address pins (ADD A, B, C) to zero (GND). To latch this address into the ADC, connect the address latch enable pin ALE to one (+5V). The ADC needs two reference voltages (Vref+ and Vref-) to set the absolute measurement scale. Since we want to make measurements in the range 0 to 5V, connect the two Vref pins to power and ground. The ADC requires a clock signal as input to perform the conversion. Generate a symmetric square wave between 0 and 5V with frequency around 600 khz, and connect it to the clock input pin.

2 To begin a new conversion, the start conversion pin (SC) must be set high. At the end of the conversion, the ADC sets the end of conversion (EOC) pin to high. For continual conversion of your input signal, you can connect the EOC output to the SC input, so that the end of each conversion automatically starts a new one. The digital output pins D0 to D7 must be enabled by setting the output-enable (OE) pin high. Otherwise the output pins will be tri-stated (set to high impedance). Finally, to read the output, connect eight LEDs between the digital output pins and GND. The internal resistors of the TTL outputs are enough to drive a single LED without requiring an additional current-limiting resistor. By reading a lit LED as logic 1 and unlit ones as 0, you can read off and decode the binary-encoded result. Draw and label the schematic of your ADC design below (this would be very good to do before arriving to the lab!) Power up and test your design. Measure the binary output for several different analog input voltages, and record your results here:

3 Plot your measurements and fit a line to your results. What can you conclude about the ADC scale? ADC output (counts) Input voltage (V)

4 2. Digital-to-analog conversion Now you will connect and test a high-speed multiplying D/A converter IC. Read and consult the data sheet for the DAC08 for this portion of the lab. Connect 5V power and ground to V+ and V-, respectively. The ADC requires eight parallel digital signals as input. Use the output pins of your ADC from section 1, connecting output 0 to input 0, 1 to 1, and so on. The logic threshold control pin (VLC) should be left unconnected. As for the ADC, a DAC also requires two reference voltages to set the output scale. As before, you can use +5V and GND. Instead of direct connection, you should make these connections through 1k resistors (see Figure 4 in the data sheet, for example) to provide a reference current of 0 to 5 ma. The compensation input (COMP) may be left unconnected. The inverted and non-inverted output pins drive current, instead of voltage. The maximum value of IO is 2 ma. To produce an output voltage, connect a 2 kω between IO and ground. What output voltage range do you expect? Draw a schematic of your DAC circuit below. Again, it is recommended to plan this before class. Finally, test your circuit with a number of ADC input voltages, and record the resulting output voltage from your DAC circuit:

5 Plot your measurements and fit a line to your results. What can you conclude about the ADC scale? DAC output (V) Input voltage (V) When you are done, put all components away and show your results to the lab instructor.