Second Exam

CHEMISTRY 595

March 1997

You may use as reference your copy of Malmstadt, Enke, and Crouch, the lab handouts, your problem set answers, your lab reports, and the class notes. Any other published material is acceptable as well. Unpublished work by others (such as anyone else's homework or old exam solutions) is not. If you make use of any material from a published source, please use normal referencing practices and give credit where it is due.

SHOW ALL YOUR WORK so partial credit can be assigned if necessary. All eight (8) problems are weighted equally.

Turn in your exam in Gilbert 8; if the door is locked, slide the exam gently under the door. Questions during the exam: Schuyler's office 737-6735; home 758-3395 (if not home, leave message with your phone number and time of call).

1. You used the LM311 analog comparator as a Schmitt trigger with hysteresis in Exp. 13, Fig. 13-13.

a. Explain clearly how this circuit resists the effect of noise when converting a noisy 0-3 V waveform such as the one shown to the right into the corresponding TTL waveform shown below it. Specifically, discuss the magnitude of the noise immunity (how much noise can it tolerate?) when the 10 k pot is set at its mid-point.

b. What would the LM311's output look like if the 10k pot were adjusted all the way to the bottom? Explain your point.

2. The following circuits have been suggested to add V₁ to V₂. Discuss briefly the reasons why each circuit would, or would not, be a good solution.

3. Instrumentation Amplifiers are often used with bridge input circuits such as the one shown to the right. Assume the bridge is nearly balanced, and all four resistance values are similar. Discuss which situation is wiser to use, if it makes any difference: One in which the point on the bridge labeled V⁺ is +15 V and the point labeled V^- is -15 V, versus one in which V⁺ is +30 V and V^- is 0 V (these voltages are with respect to the ground or common of the amplifier). Assume the Instrumentation Amplifier is operating with ±15 V power supplies. Explain your reasoning. Incidentally, this bridge configuration is commonly used with thermistors and gas chromatography thermal conductivity detectors.

4. Discuss briefly why CMOS digital circuits draw much less current from the power supply than TTL circuits do. A discussion along the lines of the lecture is expected.

5. Why is the simple Op Amp differentiator circuit useless for differentiating noisy waveforms? How can this difficulty be overcome? Explain why the solution works, and try to give a specific example which proves your point.

6. Design a circuit that would be able to route ten different analog signals to an oscilloscope one at a time, automatically. The rate at which the different signals are switched should be 1000 Hz (that is, each signal is sent through to the CRO for only 1.0 ms before the circuit switches to the next one, and so forth). Once each of the ten signals has been sent to the CRO, the whole process repeats, over and over. Assume the analog signals are on the order of 0.1 to 10 volts in magnitude and of either + or - polarity. Try to avoid wasting time during the switch between signals (that is, little time should be spent with no analog signal being passed through). Your circuit should also produce a TTL trigger pulse with a rising edge each time the switch circuit switches to the next analog signal (this could be used to trigger the CRO to begin a new sweep).

7. Derive the equation which describes the output voltage of the Op Amp in the circuit shown to the right in terms of the input voltages and resistor values. It is not sufficient to just state the result: you must show the derivation and state any assumptions.

8. a. The Op Amp circuit shown to the right produces a constant output voltage when the switch is in the IC position. What is the voltage and why does the circuit behave this way?

b. When the switch is moved to the RUN position, the output voltage follows a well-known pattern. What is the pattern (you will probably need to derive this result) and explain why it results.

c. Suggest a very simple circuit, without an Op Amp, that would produce a similar pattern.