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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles, most of which are invited contributions by guest authors, are intended to serve the readers of THIS JOURNALby calling attention to new developments i n the theory, design, or availability of chemical laboratmy instrumentation, or by presenting useful insights and explanations o j topics that are of practical importance to Wose who use, or teach the use of, modern instrumenfatwn and instrumental techniques.
XXXIX. Signal to Noise Optimization in Chemistry-P art Two: SIN Considerations in Designing a Measuring System THOMAS COOR, Princeton Applied Research Corporation, Princeton, New Jersey 08540 INTRODUCTION
I n the last section we reviewed the important specifications to be considered in designing chemical experiments or measuring systems. I f the system or experiment is to achieve the maximum signal-to-noise ratio (S/N), i t must be designed and specified in such a. way as to minimize all the sources of noise that were discussed. Also, special noise reduction techniqnes may be advisable if i t is important to achieve the best possible S/N. System design will now be discussed from the standpoint of optimum S/N.
measurements where ground-loop voltages are a problem are best handled with differential measuring equipment: amplifiers, oscilloscopes, metem, etc. having two inputs, responding to only the difference between the two inputs and rejecting the common mode signals. A ground-loop avoiding system for a hypothetical chemical set-np is shown in Figure 8.
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DC versus A C Systems
From the standpoint of S/N the most important decision to be made in chemiealinstrument or experiment design is that of determining whether the electrical signals to he measured will be ac or dc. Returning to Figure 7, the signal Q(t) from the system under observation will in general not be ac, but will have some steady or slowly varying value. A necessary part of the apparatus is the provision far some means of "zeroing" the system: This shoidd be done in such a way as to leave 6s much of the system ,mehanged as possible, inch~dingsources of noise. The ideal way would be to reduce only the phenomena of interest to zero, leaving the transducer or probe and all followi~rg equipment ntrchanged. To avoid noise, drifts, etc. the zero should be tsken frequently. If the quantity of interest and the output of the transducer are dc, the simplest way to make the measurement is to amplify and record the signals as de information. If the signals are small, the system is likely to be subject to drift and to all the other unpleasant properties of l/j' flicker and envirqnmental noise.
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Electronic Cho55is bolted to reloy rack
System Grounding
System grounding csn be one of the most annoying and frustrating problems in S/N improvement that the chemist can face. I n almost every laboratory there are several electrical grounds available: the water or gas pipes, the third (neutral) conductor in the 60 Hz power distribution system, or a building ground bus. As far as small signals are concerned, none of t,hese grounds is any good whatsoever. If one looks between two different points in a ground system, one usually sees hundreds of millivolts-if not volls-at 60 Hz. One should in general ground the chassis of electronic equipment heing used to only one point of the distribution system ground, and refer all shields to this same point. But this is not going to be good enough for the most sensit,ive measurements. The varidus chassis grounded to the one common point are still going to have millivolts if mounted of 60 Hz between them-ven in the same relay rack! Ac magnetic fields from transformers, etc., will induce potential differences even if the chassis have zero resistance. Avoiding these ground-loop voltages is often difficult. Although more will be said on this subject when amplifiers are discussed, in general,
An additional point about 60 Ha problems should be made here. Even though a given system relies entirely on dc or high frequencies for carrying the signal information, 60 Hz problem can still be encountered. The maenit,nde of the oower
I f it can be arranged to zero the system not once an hour or once a minute, but say 100 times a second, many important advantages can be realized. First, the repetitive zero's will easily keep up with the system drift. Second, ac amplifiers and ac signal processing techniques can be rlsed, avoiding l/jnoise. Third, i t may be possible to have only the quantity of interest, (Continued a page A584)
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Chemical Instrumentation turned on and off (chopped) while interfering elements are not. This gives the wanted quantity a "signstu~.o" which can be identified by some signal processing scheme after smplifieation. In designing a system in which it is possible to repetitively zero (or chop or modulate) the quantity of interest a t some reletively high rate, i t is extremely valuable to have a second signal available, one that tells when the system is on zero and when i t is on the unknown. If this signal iis available, correlation techniques can be used to greatly improve the S/N.
Transducers I n making a measurement of the desired small-effect phenomena, t.he chemist must select s probe or transducer in order to derive s. signal that can he processed and measured. If he has a choice, he will naturally choose one which gives the largest signal with the smallest amount of interference. Thelinearity, accuracy, and stability of the transducer, of coone, are also important considerations. I f the signal is modulated or chopped or is inherently non-dc, the frequency response of the transducer is also of importance. From the standpoint of S/N it is important thst the transducer have a goad N F and a high signal level output. Let us consider two examples of transducers that are often used by chemists. The photomultiplier ( 6 )is an example of s n excellent transducer often used in photochemistry and spectroscopy. I t is efficient in converting weak light into electrical signals, has very high frequency mponse and has high inherent gain with large dynamic range. The noise properties are also very good, giving it a N F very near zero dB. When meawrinn. . verv . weak livlat q ~ g n n l the i I'll'* NF i.it,l bc lmprwrd I coding i t n, dry-wr tcrnpcr;tttlrc.?. C ~ ~ f o r t t ~ ~ rlie ~ t rgziin I y ,
