ELEC 225, Fall 2009
Prof. Rich Kozick

Laboratory 3:
Nodal Analysis and D/A Conversion

This lab will provide practice with nodal 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 nodal analysis 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?

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)        As you have probably figured out by now, that 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 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. Record your output waveform, vo(t).

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

 

Lab Report:  Each student is asked to individually write a lab report following the IEEE Region 6 paper contest guidelines . Please submit your report by 4 PM on Friday, October 9 at Prof. Kozick’s office or in the EE department office. Additional materials to help with the report are posted on Blackboard under “Course Materials/Laboratory Materials/Lab 3.”

 

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!