to samples taken from the same melt by the ASTM method by such a a bias may be present* The ( 5 )and bias may be large or small and positive or negative depending on the particular alloy, Table 11shows this variation for several powder coml?acts. For any type Of standard, the value assigned is immaterial as long as it accurately relates to the routine sample, while the important characteristic is repeatabillty. Standards so adjusted are used as fixed reference points to correct for instrument drift and powder compact standards are particularly useful in this’respect because of inherent uniformity.
LITERATURE CITED (1) V. V. Poliakova. Academy of Science of t h e U.S.S.R., No. 2, Vol. 19, pp 153-154, 1955. (2) H. Yakowitz, R. E. Michaelis, and D. L. Vieth, !Vat/. Bur. Stand. (U.S.),Spec. Pub/. 260-12, September 1966. (3) H. Yakowitz, R. E. Michaelis, and D. L. Veith, Natl. Bur. Stand. (U.S.),Spec. Pub/.,260-16, January 1969.
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Testing & Materials, Part 42, 1976.
RECEIVED for review
November 19,1976. Accepted January
19, 1977.
Voltage Controlled Oscillator for Signal-Proportional Data Acquisition Rates D. E. Leyden”’ and L. D. Rothman2 Department of Chemistry, University of Georgia, Athens, Ga. 30602
J. C. Lennox Department of Chemistry, University of North Carolina, Chapel Hili, N.C. 27514
Many chemical and physical instrumental methods of analysis and testing provide a voltage-time signal, a great portion of which is baseline. Computers or data storage devices frequently acquire data either under fixed frequency interrupt control, or program control in which a real-time software system evaluates the incoming data for significance. In many examples, these systems function effectively. However, a simple hardware system which may evaluate data significance and adjust the data sampling rate accordingly wouid save core and cpu time where computers are used, and permit data rate adjustments when simple hard-wired data acquisition systems are used. This report describes the application of a voltage controlled oscillator (VCO) which is substituted for a fixed frequency clock in the interrupt mode of data acquisition. The input to the VCO is the absolute value of the derivative of the signal to be acquired. Provisions are made to adjust the rate of data acquisition on the signal baseline, as well as the maximum data rate. The result is an inexpensive device which provides data at a rate proportional to the rate of change of the signal amplitude. The data and a real-time clock are read as a two-dimensional array such that the signal-time function can be displayed in its original form. Thus, no software is required for peak detection during the experiment. The circuit used is shown in Figure 1. This circuitry consists of a voltage follower (OAl), a differentiator (OA2), an absolute value circuit (OA3 and OA4), a summing amplifier (OA5), a voltage-control oscillator (VCO) and a frequency divider (IC1-IC6) . The voltage follower (OAl) is used to isolate the signal source from the differentiator. This is necessary if capacitor, 61, is large (-0.1 pf) to avoid loading the signal source. The differentiator (OA2) is the critical component in this system. Values for C1 and R1 must be chosen to provide a measureable output voltage (5-10 V) from OA2 for values of duldt corresponding to normal data ( V , = -RC dvin/dt). A feedback capacitance, in parallel with R1,may be necessary to limit the Present address, Department of Chemistry, University of Denver, Denver, Colo. 80208. Present address, Dow Chemical Company, Midland, Mich. 48640.
response of the differentiator to noise. It is important to choose a capacitor with very low leakage current characteristics, such as metallized polypropylene. Values of RC tend to be large, generally on the order of 1 to 10 or even greater. I t is generally preferable (and cheaper) to use as large a resistance as possible, therefore minimizing the necessary capacitance. This also helps to avoid loading OAl during signal voltage changes. This is important if equal data densities on both sides of a peak are desired. The absolute value circuit (OA3 and OA4) is used to make the differentiator output compatible with the VCO. Since a VCO generally operates with unipolar input only, it is necesl sary to change the bipolar differentiator output to a unipolar signal. The absolute value circuit shown here always maintains a negative output polarity with a gain of 1,regardless of the polarity of the input voltage. The output polarity may be reversed by reversing the four diodes. The summing amplifier (OA5) allows an offset to be added to the signal from the absolute value circuit. This offset control (Pl) allows adjustment of the minimum data acquisition rate for zero change in the input signal (“baseline”). The gain of OA5 (determined in part by R9) allows control of the maximum output voltage of the amplifier and, therefore, the maximum data acquisition rate. The output of OA5 is input to the VCO, which has a transfer function of 1 kHz/V and a useful input voltage range of 0 to +5 V. The output of the VCO is TTL compatible. The output frequency is divided by flip-flops (ICl; IC2) and decade counting units (IC3-IC5) until the desired range of frequencies is obtained. A monostable (IC6) is used to set the output pulse width of the divider circuit to 1ms. This pulse is used to provide an interrupt signal to the computer. The circuit in Figure 1 was used to monitor the detector signal of a liquid chromatograph. The output of the circuit was tied to the interrupt (Bus Request) line of a PDP-11/20 minicomputer (Digital Equipment Corp., Maynard, Mass.) programmed in BASIC with a real-time overlay ( I ) . The data acquisition program performed one analog-to-digital conversion on every interrupt signal and stored the result in an array. Immediately afterwards, a 1-kHz elapsed time clock was read and the result stored in a second, parallel array. Each data point, therefore, consists of a voltage reading and a time ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
681
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of which 21 min were baseline. A total of 40 points define the baseline, while 160 points were taken across the peaks. The data density across the peaks is therefore on the order of 20 times as great as the baseline data density. Further, when operating properly, the data density will be highest when the signal is changing most quickly and, therefore, when the most information is desired. It is relatively easy to write computer software to perform the same task as this circuit. However, for rapidly changing data, computer systems programmed in relatively slow languages, such as BASIC, may not be able to execute such software rapidly enough to be practical. In such cases, this circuit may provide a significant advantage for data collection. Data storage space may also be reduced by the use of this circuit. The storage space required for the chromatogram in Figure 2 is 400 points (200 for the data and 200 for the times). To record the same chromatogram with the same data density across the peak (40 pointdmin average) at a constant data rate, would require 1000 data points. This method of data acquisition is not directly applicable to situations in which moment analysis is performed because of the unequal data spacing on the time axis. However, it may be possible to use data collected in this manner to calculate by interpolation, a set of data with equal spacing.
ACKNOWLEDGMENT The authors thank H. C. Acree for his assistance. measurement. The chromatograms were plotted on an X-Y recorder interfaced to the minicomputer. Figure 2 shows a point plot of a liquid chromatogram taken using the circuit described in this report. The sample contained two components which were not fully resolved. The total time for the chromatogram was approximately 25 min,
682
ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977
LITERATURE CITED (1) J. E. Davis and L. 6. Rogers, Decus Program Library.
RECEIVEDfor review October 28,1976. Accepted December 13,1976. This work was supported in part by Research Grant MPS 78-08675 A02 from the National Science Foundation.
