ELEC 225, Fall 2009
Prof. Rich Kozick
Laboratory 2:
Thevenin Equivalent Circuit Models for Digital Logic Gates
Digital logic gates are the basic building blocks of digital systems
such as computers, calculators, cell phones, and MP3 players.
In this lab we will examine the 7400 series of logic chips.
Recall that in ELEC 120 we used FPGAs to design and test our digital designs.
We will experiment with the 7400 chip which contains four NAND gates.
Your first task is to verify the truth table for a NAND gate using PSpice
and experimental measurements.
- Create a PSpice project and load the "74LS"
library.
Copy a 74LS00 NAND
gate into the schematic and perform all of the
lab activities in PSpice as well as experimental measurements.
- Set up a 7400 chip on your breadboard and
measure the output voltage Yout for the input conditions shown below.
Please note that you must power up the chip before taking your output voltage
measurements.
You can find the pin diagram for the 7400 chip at
http://www.ti.com/
(search for SN74LS00).
INPUTS OUTPUT
Va Vb Yout
0V 0V ___
0V 5V ___
5V 0V ___
5V 5V ___
You should develop a strategy for determining the Thevenin equivalent circuit
model.
You may find it useful to use PSpice to help you determine a plan before
you take experimental measurements.
- Be sure to check all four of the NAND gates on the chip.
- Determine the range of input voltages which are always accepted as
"0"and "1" for your NAND gates using PSpice and experimental measurements.
- Check the input of a NAND gate in PSpice and experimentally.
What is the voltage when nothing is connected to the input?
(This is the open circuit voltage looking into the input.)
Determine a Thevenin equivalent circuit for the input to the gate.
- Set the inputs to a NAND gate to 0 volts and
determine the Thevenin equivalent model looking into the gate output.
Compare the models you obtain using PSpice and experimental measurements.
- Set the inputs to a NAND gate to 4 volts and repeat
the previous step.
Lab Report
Each lab group should determine a Thevenin equivalent circuit model
for a NAND gate with inputs set to 0 V and 5 V.
Show how you obtained the Thevenin model using PSpice and
experimental measurements.
Please submit a brief summary describing your group's approach and
results before you leave the lab session.
Thank you and have fun!