High-Precision Digital Acquisition Using a Low-Resolution Analog-to-Digital Converter Stanley N. Deming and Harry L. Pardue Department of Chemistry, Purdue University, Lafayette, Ind. 47907
EFFORT WAS RECENTLY initiated in this laboratory directed at the development of a computer-controlled system for the complete automation of kinetic studies. The system is being built around a spectrometer capable of yielding data reliable to 0.01% T ( I ) . The combined requirements of 0.01% reliability and wide dynamic range suggested an analog-todigital converter (ADC) with a minimum resolution of 14 bits. The computer to be used was equipped with a 10-bit ADC. The choice was to replace the ADC with one of higher resolution or to use the available ADC in a manner to yield the required resolution and dynamic range. This report describes a technique developed to utilize this low-resolution ADC in a manner such that it yields data equivalent to an ADC with resolution between 14 and 15 bits. The technique makes use of a digital-to-analog converter (DAC) to suppress unwanted portions of the spectrometer signal. The DAC is computer controlled to provide automatic real-time ranging of a zero suppression voltage. The computer senses when the amplified difference signal is approaching either limit of the ADC and changes the digital signal to the DAC to move the analog signal near the other limit of the ADC. (1) H. L. Pardue and S . N. Deming, ANAL.CHEM., 41,986 (1969).
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Figure 1. Digital-to-analog converter response to reacting chemical system
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Figure 3. Analog-to-digital converter response to reacting chemical system
EXPERIMENTAL
The spectrometer ( I ) employs a current-to-voltage converter to provide an output signal of 0 V at 0 ’% T and - 1 V at 100% T . The current-to-voltage converter is followed by a gain of ten ( X 10) amplifier to provide an output of 10 V at 100 T . The output from the X 10 amplifier is summed algebraically with the output from a computer-controlled DAC. The output from the summing amplifier is passed through a multiplexing channel to the ADC. A second multiplexing channel is used to connect the output from the current-to-voltage converter into the ADC. The summing amplifier (Model 141B, Analog Devices, Cambridge, Mass.) employs 10 K input resistors and a 250 K feedback resistor to provide a gain of -25. The feedback resistor is shunted by 470 pF capacitance and a pair of opposed Zener diodes (1N4105 at 11 V and 1N4099 at 6.8 V) to restrict the output of the amplifier to - 11 V and +6.8 V and prevent it from limiting. Resistors used for amplifier gain are carbon film resistors with low temperature coefficients. The input resistors are matched to within 0.01 %. The ADC used is a 10-bit converter (Model C002, Digital Equipment Corporation, Waltham, Mass.) with a full scale range of 0 to - 10.23 V. It is accurate to 0.1 of full scale and has an aperture time of 33 psec. The ADC is operated at 1 KHz for most of this work. The DAC is a 16-bit (15 data bits plus sign) bipolar converter (Model 6933A, Hewlett-Packard, Palo Alto, Calif.). It is accurate to better than 1 mV and has a resolution of 0.5 mV. In this work only the sign bit and 6 most-significant bits are used. In effect, it is used as a 7-bit bipolar converter accurate to better than 1 mV and with a resolution of 256 mV. The computer used is a 16-bit machine with 8 K of core memory (Model 211 5A, Hewlett-Packard, Palo Alto, Calif.). The data acquisition subroutine is written in assembly language and is called from Hewlett-Packard BASIC. At the beginning of any run, the amplified difference signal between the DAC and the X10 amplifier may exceed the range of the ADC. Therefore a “chase” routine is used to quickly bring the amplified difference signal within the range of the ADC. In this routine, the unamplified photometer signal (0 to -1 V) is multiplexed into the ADC, a conversion value is read into the computer, and the DAC is set at a value equivalent to 10 times the result (to account
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
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for the gain of 10 amplifier). At this point the program exits the chase routine and enters the normal data acquisition routine which accepts the low-resolution chase value as the first data point, stores it appropriately, and continues with normal high-resolution data acquisition. The chase routine requires 1 ADC conversion to bring the amplified difference signal within the range of the ADC. Switching levels and the change in DAC output have been programmed to prevent the expanded difference signal from exceeding the range of the ADC and to provide 0.5% full scale noise immunity when changing ranges. If the signal to the ADC exceeds the bounds of the switching levels, that is, if less than 1.28 V separates it from either limit of the ADC, then the DAC is changed 1 2 5 6 mV to return the signal to within bounds. When returned, the signal is 2.56 V from the opposite limit of the ADC. This provides (2.56-1.28)/25 or 51 mV noise immunity on the unexpanded signal. RESULTS AND DISCUSSION
The system was evaluated for chemical applications by preliminary kinetic measurements on the reaction between semicarbazide and 2,6-dichlorophenolindophenolat pH 7.0. The reaction was followed spectrophotometrically at 522 nm. Data were taken using the data acquisition system described above which was programmed to take one data point every 1000 conversions (1 data point/sec). Figures 1, 2, and 3 are computer plots of acquired data. Figure 1 represents the DAC output as a function of time. As expected, it is stepwise in nature with each step representing about 2.56% T. The sharp rise at about 10 sec represents the change in transmittance when the dye is added to initiate the reaction. A smooth curve drawn through the bottom-most point in each transition represents the inverse of the amplified photometer signal displaced +0.102 V (2.56 V return level + 25) on the vertical axis. Figure 2 represents the combined DAC and ADC values for
this run. These data demonstrate that the data acquisition technique described yields the smooth response curve expected. Figure 3 is a plot of the ADC values. As mentioned previously, the rate of acquiring data points is one point per second. However, the output from the summing amplifier was reexamined for DAC updating at a 1 KHz rate. Therefore, the maxima and minima on this plot do not represent the switching and return levels, but rather the last and first points before and after switching. Although the plot is irregular at the transition points there are no data lost at either end. This is exemplified by the smooth plot of these data in Figure 2. This point was confirmed further by examination of numerical data across the transition point. In summary, the ten-bit ADC, as used here, can resolve 10 mV of a signal that has been expanded to an equivalent of 250 V full scale, corresponding to a resolution of 1 part in 25,000 or 0.004%. The price paid for this increased resolution is some reduction in conversion speed and usage of some computer space and time. The system described here allows data acquisition rates as fast as 10 KHz to be realized. The theoretical maximum rate-of-change of signal which can be followed accurately is 0.510 V/msec. Most chemical applications do not require expensive ultrahigh-accuracy converters. A high-speed, medium-resolution ADC will usually be adequate for most applications. The principal advantage of the approach reported here is that a relatively inexpensive ADC can be modified to provide ultra-high accuracy conversions at relatively low cost. RECEIVED for review May 4, 1970. Accepted July 1, 1970. This work supported in part by PHS Research Grant No. G.M. 13326-04 from National Institutes of Health and by an Ethyl Corporation predoctoral fellowship to S.N.D. for 1969-70.
Cobalt(l1) as a Developing Agent in Thin-Layer Chromatography of Amine Derivatives Shaul M . Aharoni and Morton H. Litt Diuision of Macromolecular Science, Case Western Reserve University , Clecelmd, Ohio 44106 EVENTHOUGH it has been long known that Co(I1) can form highly colored salts and complexes with nitrogen-containing organic compounds (1-6), its capacity as a qualitative analysis tool for such compounds has not been exploited extensively. Some methods for the detection of nitrogen-con.
taining compounds (7, 8), imidazoles and benzimidazole derivatives (9-12) have appeared in the literature. The procedure we suggest seems to us, however, to be the simplest of themall. We have found that a concentrated solution of CoC12.
(1) F. Feigl and H. Gleich, M o m s h . 49, 385 (1928). (2) P. C. Daidone, ANAL.CHEW,27, 103 (1955). (3) L. Cambi and L. Canonica, A f t i Accad. Nnz. Lirtcei, CI. Sci. Fis., Mar. Natur. Rerid., 20, 17 (1956). (4) S. P. Ghosh and H. M. Ghose, J . Iridian Clzem. SOC.,33, 899 ( 1956). (.5,) M. Goodaame and F. A. Cotton, J . Amer. Chem. SOC.,84, 1543 (1962). (6) G . Schwarzenbach and H. Flaschka, “Comulexometric Titra’tions,” Methuen and Co., Ltd., London, 1969, pp 242-245.
(7) F. Feigl, “Specific, Selective, Sensitive Reactions,” Academic Press, New York, 1949, p 233 ff. (8) F. Feigl, R. Belcher, and W. I. Stephen, Adcar?. Arial. Chem. Instrum., 2, l(1963). (9) K . K. Koessler and M. T. Haney, J . Biol. Cliem., 39, 497 (19 19). (10) K. Bottcher, German Patent 531,297 (1927). (11) H. Frey, Compt. Rerid., 209,759 (1939). (12) H. Frey, Ann. Chim., 18, 5 (1943).
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