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Chemistry 402-- Exam 1, February 12, 2001 Answers are provided in italic type name ___________________________________

Answer all questions unless otherwise indicated.

100 points (14 questions @ 7 points + 2 points if you get your name right)

1. Describe briefly what the terms mean (select any 4.) Your explanation should go beyond the name

ROM-- read only memory (stored data that can be retrieved but is permanently locked in memory)
EPROM-- erasable, programmable read only memory (ROM that can be created and relatively easily changed when desired)
CPU-- central processing unit (the logical brains of a computer; that which processes and performs instructions)
Bus-- a set of wires treating a single purpose (such as a data us or an address bus)
byte-- a collection of binary digits (bits) that becomes a word; traditionally 8 bits
Analog (signal)-- a signal that is capable of gradually or infinitesimally changing in value.

2. Look at the signal on the right; estimate the S/N. Briefly justify your approach.

A pair of horizontal lines drawn through the upper and lower bounds of a signal. This typically brackets the noise to +/- 3 sigma (sigma being the standard deviation.)
Here that's roughly between 0.7-0.5 ; so noise (as 1 sigma) is about 0.2/6 = 0.03.
The signal is (average) 0.6
ratio is 0.6/.03 = 20 = S/N

a more conservative estimate of noise is 2 sigma (S/N=10 then)

3. What's the resolution of a 10 bit ADC that covers the range -5 to +5 volts ?

10 bits can count 0-1023 or 1024 volts.
the voltage range is 10 volts
each bit then represents 10.00 V / 1024 = nearly 0.010 Volts (resolution)

4. which radiation has the higher energy per photon?

a) blue or yellow light blue
b). 200 nm or 400 nm radiation _200__
c). X-rays or microwaves __Xrays_____
d). 25,000 cm^-1 or 20,000 cm^-1 light __25,000____

5. Give an example of a transducer. Tell what property is measured and what property (or domain) the transducer produces.

example of thermocouple

information = Temperature, Analog

TC produces voltage , Analog

6. Convert binary to decimal and decimal to binary (or hexadecimal)

start with values for each binary digit 128-64-32-16-8-4-2-1

value of each binary bit is that value (if 1) or zero (if 0)

a. 1011 (binary) = ______ (decimal)
- 8+0+2+1=11
b. 1101 1100 (binary) = ________ (decimal)
- 128+64+0+16 + 8+4+0+0 =120
c. 25 (decimal) = _______ (binary)
- 16+8+0+0+1 = 11001
d. 11 (decimal) = __ (binary) = __ (hexadecimal)
- 8+0+2+1 = 1011
in hexadecimal we count 0-1-...9-A-B-C-D-E so
11(dec)=B(hex)

7. Consider the op amp circuit on the right

what is the voltage at point 1 __-1.0 V__

R1, Rf set inverting gain of -5.0

what is the voltage at point 2 ______

combination of R2 and Rf, second amplifier
gain is -0.5
so the -1.0 V (part a) becomes +0.5V
but we also sum second voltage
gain here is -5.0
signal is +0.12 V
amplifier produces -0.60
the two signals are summed:
+0.5 + (-0.60) = -0.10 volts

what is the voltage at point 3 _zero_

noninverting terminal grounded (zero)
rule: with feedback both terminals at same voltage

8. The circuit on the left provides what voltage?

32000 ohms overall
8000 ohms (two lower resistors)
voltage divider, producing 8/32 or 1/4 of the signal
1/4 or 3.0 volts is 0.75Volts

alternative: 3V and 32000 ohms total
current is 3V/32000ohm = 9.38x10-5 amps
voltage drop across last two resistors (8000 ohm)

Vx = 9.38x10-5 x 8000 = 0.75 ohms

9. What is ensemble averaging? Why is it useful in laser fluorescence but not in gas chromatography?

ensemble average involves collecting a series of data points (e.g., in time.) The event is repeated and the results are combined, point by point. Random noise, on average, will begin to cancel out.

to be useful, the signal must be repetitive. GC certainly does not meet that requirement (long runs, not likely to rerun 10-1000 times.) Fluorescence can be repeated very often for a single sample.

10. What kind of a filter (name) would be useful

in improving the signal pictured in problem 2

use the language of a Fourier Series within your explanation

signal appear to be a relatively constant or very slowly varying signal (about 0.6V)

superimposed is a rapidly changing noise component.

Fourier Series-- think frequency instead of time

signal is of very low frequency with a lot of high frequency noise

a LOW PASS filter would be useful.

(passes low frequency--the signal,-- but blocks high frequency -- the noise.)

11. Evaluate the standard deviation, s, of the numbers in this list

11.21, 12.08, 11.99, 11.54, 11.82, 12.14

sum is 70.78 (6 terms)
mean or average is 11.80
deviations are .61, .28, .19, .26, .02, .34>
sum of these squared is .6702
divide by 5 (n-1), square root -->0.37 (sigma)

12. We often represent a normal distribution of errors

with a Gaussian or bell shaped graph

what is the label on the horizontal axis? property or deviation from mean

what is the label for the vertical axis? probability or frequency of measurement

which arrow (a,b,c,d) is approximately equal to one standard deviation must be d

13. A 5.0 volt square wave is sent into a string of four JK Flip Flops.

(These are negative edge triggered devices) thus, state of device changes once per pulse, when signal drops.

All FF are cleared before the pulse train arrives
What is present at point a after the pulse train is complete?
What is present at point b after the pulse train is complete?
What is present at point c after the pulse train is complete?
What is present at point d after the pulse train is complete?

Briefly explain:

suggest a small table, listing status at points a,b,c,d after each pulse (after the falling portion of the pulse)

pulse# at a at b at c at d
before 0 0 0 0
1 1 0 0 0
2 0 1 0 0
3 1 1 0 0
4 0 0 1 0
5 1 0 1 0

pulse#	at a	at b	at c
before	0	0	0
1	1	0	0
2	0	1	0
3	1	1	0
4	0	0	1
5	1	0	1

4. Precision and Accuracy

Illustrate a case where a set of measurements might exhibit high precision but be of low accuracy.

weighing with a good balance that is calibrated incorrectly

missing the bull's eye but having the arrows all tightly clustered (archery)

end of exam