ELEC 225, Fall 2011
Prof. Rich Kozick

Laboratory 2:
D/A Conversion, Superposition, and Thevenin Models

This lab will provide practice with circuit analysis and Thevenin equivalent circuits as we analyze and design a digital-to-analog (D/A) conversion circuit.

A1
 
A0
 
vo (t)
 
 
 
 
 
 
 
0V
 
0V
 
 
 
0V
 
5V
 
 
 
5V
 
0V
 
 
 
5V
 
5V
 
 

a)      Find vo(t) using source superposition as a function of A1, A0, and R.

b)      Let A1 and A0 be as shown in the table and fill in the vo column.

c)      Simulate the circuit in PSpice to validate your answers. What value of R did you use? How does the value of R affect vo and the current drawn from each source?

d)     Imagine yourself to be v0(t).  What is the Thevenin equivalent of the circuit that you see? Please express the Thevenin equivalent circuit model as a function of A1, A0, and R.

e)      Let R = 5 Kohms and suppose the circuit drives a load resistor, RL. What is the value of vo with the load attached (as a function of RL)? What value of RL would provide at most a 5% drop in vo(t) compared with no load (open circuit)?

f)       This circuit is a two bit digital-to-analog converter (DAC). Design a 4-bit DAC by adding two more voltage sources, A3 and A2, to the given circuit. Analyze your 4-bit DAC to relate the output voltage vo (with open circuit) to the inputs A3, A2, A1, and A0.

g)      Design an up counter using the 74LS193 chip and drive your DAC (see notes below about the counter chip).  Drive your counter with a square wave from the function generator as a clock. Record your output waveform, vo(t), and explain the measured waveform in terms of your analysis. (Measure the voltages at the counter output pins when they are High and Low. Are the values 0 V and 5 V?)

h)      Vary the frequency of the function generator (clock) and record vo(t).

i)        Set up the 74LS193 so that the output bits are all High. Devise an experiment to determine the Thevenin equivalent circuit at one of the output pins, and analyze your measured data to determine an approximate Thevenin model. You may find it helpful to use PSpice (with the 74LS library) to help you design the experiment. Does your measured data fit the theoretical Thevenin model? Does knowing the Thevenin model allow you to understand design tradeoffs in choosing the value of R in the DAC? What happens to the High voltage value at the counter output pins if R is chosen too small?

 

Lab Report:  Each student is asked to individually write a lab report that presents your analysis and results. Please submit your report at the beginning of your lab session on Thursday, October 6.

 

Review:

A binary count that changes uniformly in time can be generated with the 74193 chip. The counter chip is described in this link (search for SN74LS193). The 74193 is an UP-DOWN counter, so it can count up or down depending on a control input.

A. The outputs are pins 2, 3, 6, and 7.
B. To count UP, hold pin 4 high (1), and put the pulse input into pin 5.
C. To count DOWN, hold pin 5 high (1), and put the pulse input into pin 4.
D. To clear the counter, pulse pin 14 high.
E. Be careful about the other pins.

Thank you and have fun!