EET231/EGR244 (4/3/2007) Digital-to-Analog Converter from “Scratch” Lab
The purpose of this lab is to wire and explore the functioning of a binary-weighted DAC using a summing inverting amplifier. You will be wiring this chip on a protoboard and using a the basic digital logic trainer.
Equipment Required
1 – Digital Logic Trainer (the basic board that we usually use)
1 - Power supply (for trainer)
1 – Dual voltage power supply (for the Op Amp)
1 – LM741 Operational Amp
1 – Digital multimeter (DMM)
1 - Oscilloscope
5 – Resistors (2x1K, 1.8K, 3.9K, 8.2K)
Assorted length wires and jumpers
Procedure
1. Read pages 728-730 in your textbook so that you have a solid understanding of what this circuit is supposed to do.
2. Figure 11-5 is repeated below.
3. Place the LM741 IC on the protoboard (oriented toward the left). The pinout for the chip follows:
4. Wire +12v to pin 7, -12v to pin 4. Note that there is no ground and that pin 8 is not used.
5. Connect the 1k feedback resistor (Rf) between the output pin 6, and the inverting input pin 2.
6. The 4 resistors that are approximately binary-weighted all connect one end to pin 2. There aren’t enough holes below pin 2 to accomplish this directly, so you can run a short jumper away from the chip to use for the other connections.
7. Connect power to the digital trainer using the standard trainer supply (+5v, ground).
8. Use 4 of the logic switches to provide the logic inputs (D, C, B, A) to the other end of the 4 binary-weighted resistors.
9. Connect the DMM to read the output voltage on pin 6.
10. Turn on all the power and note the output voltage for each 4 bit binary input. Complete the following table.
0 / 0 / 0 / 0
0 / 0 / 0 / 1
0 / 0 / 1 / 0
0 / 0 / 1 / 1
0 / 1 / 0 / 0
0 / 1 / 0 / 1
0 / 1 / 1 / 0
0 / 1 / 1 / 1
1 / 0 / 0 / 0
1 / 0 / 0 / 1
1 / 0 / 1 / 0
1 / 0 / 1 / 1
1 / 1 / 0 / 0
1 / 1 / 0 / 1
1 / 1 / 1 / 0
1 / 1 / 1 / 1
11.
On the average, what is the resolution (stepsize)?
q = ______v
12. Wire the logic trainer as a 4 bit ripple-up counter using 4 JK flipflops. Recall that the not Q drives the clock for the flipflops following the first one, and that the flipflop that changes the most quickly is the least significant bit (A). This should be the flipflop that is connected to the clock on the trainer.
13. Set the clock to a relative slow speed. Check the correct operation of the counter by wiring 4 LEDs to the Q outputs and observing the counter. If it counts down instead of up, check that not Q is connected to all clock inputs after the first one.
14. If the counter is working properly, connect the LED’s to the binary-weighed resistors in place of the switches (disconnect the switches as we won’t need them any longer).
15. You should now see the voltage output reduce step-by-step to zero, then start again.
16. Connect the oscilloscope to the output and turn the clock frequency to HIGH and the control full clockwise. Adjust the display so that the down stairstep (also called a sawtooth) is shown. Show the display to your instructor.
Instructor checkoff______
17. From the display, determine the resolution, q. q = ______v