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midterm 2006

# midterm 2006 - Chemical Instrumentation CHEM*3440 Mid-Term...

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Chemical Instrumentation CHEM*3440 Mid-Term Examination Fall 2006 Tuesday, October 24 Duration: 2 hours. You may use a calculator. No additional aids will be necessary as a series of data and equation sheets are included as part of the examination. 1. (10 points) Analyze the following operational amplifier circuit. The op amps are driven with a ±3.00 V power supply. + + A B C Ω µ V input V output The input signal is a sine wave whose behaviour is described by the following equation: V in (t) = 1.00 V + 1.50 V * cos(2 π t) (t measured in seconds) (a) What name describes the function of the first operational amplifier? (b) What name describes the function of the second operational amplifier? (c) On the accompanying graph (next page), carefully sketch and label the behaviour of the voltage at the three positions labelled A, B, and C. Overlay them on the graph and clearly label them. (Suggestion: Make B a dashed line and the three curves will be more clearly distinguishable.) (d) A common assumption is that after 5 time constants, a circuit has essentially reached its final value. How long does it take for this circuit to settle to its final value fol- lowing a change at the input?

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-4 -3 -2 -1 0 1 2 3 4 0 1 2 3 4 Time (s) Signal (V) 2. (6 points) (a) Compare a Flash and a Staircase ADC (analog-to-digital converter). Discuss the relative complexity of construction, resolution, conversion times, and any- thing else you feel is pertinent. (b) You have a 10-bit DAC operating over the range of 0 to +5V. (i) In millivolts, what is the magnitude of an incremental step in the output volt- age? (ii) In volts, what is the output voltage corresponding to the number 291? 3. (12 points) Describe each of the following terms in a couple of sentences each. (a) Conductivity (b) Hyperchromic (c) Drift (d) White noise (e) Ensemble Averaging (f) % Transmittance
4. (4 points) A resistance thermometer is a device whose resistance changes with tem- perature. Below is one used in a Wheatstone Bridge configuration with an error signal measurement. R 2 = R 4 = 1.000 k Ω . R 3 is the temperature sensor. R 1 is an adjustable precision resistor which can be adjusted to one of three positions 100 Ω , 200 Ω , or 500 Ω and thereby adjust the range setting for the instrument. (a) At a particular temperature, the sensor has a resistance of 120 Ω . To which resistor should R 1 be set for in order to give the best sensitivity to the instrument and why? (b) If the error signal produces a current of +10 μA, what should the value of R feedback be in order to provide an output signal of -100 mV from the current-to-voltage amplifier? + R 4 R 2 R 1 R 3 R feedback 5. (4 points) A substance with a molar absorptivity of 850 M -1 cm -1 at 500 nm is found in a solution of unknown concentration. A cell with a path length of 1.00 cm is used in a spectrometer and the %transmittance found for the solution at 500 nm is 30%. What is the concentration of this compound in the solution?

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6. (15 points) The following data were obtained in the calibration of an electrochemical sensor. Each data point in the calibration data is the average of three measurements.
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midterm 2006 - Chemical Instrumentation CHEM*3440 Mid-Term...

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