Table 11. Removal of Bromide Interference by Addition of the LIPB Total concn Electrode potential, mV of Cu(II), Ma 1.0x -367.6 (-367.1)b 1.0x
1.0 x
lo-4
1.0x 10-5
-401.6 -433.2 -455.2 -459.8
(-400.9) (-433.0) (-455.4) (-457.1)
1.0x a All concentrations were final ones after addition of the LIPB. The concentration of sodium bromide was 1.0 X M. Figures in parentheses are potentials for standards.
various ligands by addition of the LIPB. Anions such as halide ions, which form sparingly soluble silver salts, interfere greatly with the activity measurement of copper(I1). These kinds of anions convert silver sulfide at the membrane surface to silver salts of corresponding anions ( 2 , 3 ) . Since solubilities of the silver salts increase in the presence of polyamines, such conversions of the membrane surface may be depressed by addition of the LIPB. It is thus expected that the LIPB may also be effective in elimination of this kind of interference. In the presence of 1.0 X lo-* M of sodium bromide, the electrode did not show any sensitivity to cupric ion, but responded to bromide ion in weakly acidic media. As shown in Table 11, the interference of bromide
was effectively eliminated by the addition of the LIPB. Thus, using the LIPB designed in this work, one can determine copper(I1) in the presence of complexing agents and halide ions without any pretreatment of samples. The principle of the LIPB is seemingly applicable to the case of lead(I1) or cadmium(I1) electrode, by selecting an appropriate ligand.
ACKNOWLEDGMENT The authors extend their thanks to K. Abe and K. Imamura for their help in some of measurements.
LITERATURE CITED (1) G. Nakagawa, H. Wada, and T. Hayakawa, Bull. Chem. SOC.Jpn.. 49, 424 (1975). (2) R. A. Durst, “Ion Selective Electrodes’’, Marl. Bur. Stand. ( U . S . ) . Spec. Pub/. 314, Washington, D.C., 1969 (3) D. J. Crombie, G. J. Moody, and J. D. R. Thomas, Tabnta, 21, 1094 (1974). (4) M. S.Frant and J. W. Ross, J r . , Anal. Chem., 40, 1169 (1968).
Akinori Jyo Takao Hashizume Nobuhiko Ishibashi* Department of Applied Analytical Chemistry Faculty of Engineering 36, Kyushu University Fukuoka 812, Japan
RECEIVED for review June 20, 1977. Accepted July 19, 1977. This work was supported by the Japanese Ministry of Education (Grant No. 011911).
AIDS FOR ANALYTICAL CHEMISTS Integrating Analog-to-Digital Converter Displaying High Signal Fidelity and Noise Immunity J. W. Frazer,’ G. M. Hleftje,” L. R. Layman,* and J. T. Sinnamon Department of Chemistry, Indiana University, Bioomington, Indiana 4740 1
In a conventional analog-to-digital conversion, the analog signal to be converted is sampled at appropriate intervals and the samples are encoded in a digital form. For this procedure to provide an accurate digital representation of the original waveform, the sampled increment should ideally be infinitely narrow in time, Le., approach a delta function. Furthermore, the analog signal must be sampled at intervals that are no more than half the period of the highest frequency component present in the signal, in order to avoid distortion ( I ) . Although these sampling criteria can be practically met by a large number of converters now commercially available, the criteria are not sufficient to ensure that the principal signals of interest will be converted with greatest noise immunity and accuracy. Electrical noise, present on all signals, can often obscure a digitized waveform of interest because of the potentially high frequency response of sampling circuitry. This situation is aggravated by the likelihood that high-frequency noise will be “aliased” ( 2 ) down into the signal frequency region. Fortunately the effect of noise on a digitized waveform can be minimized by judicious use of analog or digital filtering. Present address, Department of Chemistry, Lawrence Livermore Laboratory, Livermore, Calif. 94550. Present address, Pacific Lutheran University, Tacoma, Wash.
98444.
In particular, a hybrid system consisting of an analog filter followed by analog-to-digital conversion provides the advantages of effectiveness and programming simplicity. Unfortunately, simple analog filters can often distort a waveform to be digitized by attenuating high frequency signal components and can also alter somewhat the sampling frequency considerations. To improve this situation, the filtering action could be limited to the time between sampling operations, with each successive sample being independent of all others. Such an operation can be accomplished simply by integrating the sampled waveform for a time equal to the sampling period, after which the integral could be digitized, the integrator reset, and the process repeated. This procedure would provide maximum immunity from noise and greatest signal reconstruction capability while requiring minimal software for operation. Of course, it will be necessary to sample at a rate twice that of the signal frequency component of interest. In this paper, we will describe a new analog-to-digital conversion device which embodies the foregoing characteristics. The system, termed a variable-aperture integrating converter (VAIC) is capable of digitizing waveforms a t a maximum rate of 1000 samples per second so that most common laboratory signals can be properly digitized. T h e sample waveform is integrated between conversions for a ANALYTICAL CHEMISTRY, VOL. 49, NO. li!, OCTOBER 1977
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EXTERNAL CONTROL
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selectable time interval which can be equal to or less than the sampling period, and which is variable over the range of 1 ms t o 10 s. In its present configuration, the VAIC is capable of 12-bit digitization, and has correspondingly precise input circuitry. The device has conveniently selectable gain and is equipped with appropriate input/output hardware to enable simple interfacing to most laboratory computers. At present, the VAIC integration time is selected by front panel thumbwheel switches, calibrated in milliseconds, although it is planned to incorporate computer programmability of this variable in a future system. The sampling rate can be chosen either manually, again using thumbwheel switches, or can be governed by synchronizing pulses from an external source such as a computer, real-time clock, or signal modulator. In the following discussion, the effectiveness of the VAIC will be demonstrated in a number of applications involving the digitization of both synthesized waveforms and those obtained from real laboratory instruments. Signals from spectroscopic measurement devices and reaction-rate analysis instruments have been used to illustrate the latter. In these studies, the VAIC has demonstrated the ability to reliably digitize a chosen signal and to provide noise immunity far surpassing that of a conventional analog-to-digital converter, without the introduction of distortion into the digitized 1870
ANALYTICAL CHEMISTRY, VOL. 49. NO. 12, OCTOBER 1977
waveform. In addition, the VAIC was found to be useful in rejecting specific noise-rich frequencies merely by appropriate choice of the integrating time aperture. The device could also serve as a synchronous detector simply by locking its sampling rate in time with the digitized signal. Finally, in some applications in which the time integral of the signal is the final information desired, the VAIC was shown to enable significant reductions in software complexity, a feature of considerable importance in small computer applications.
DEVICE DESCRIPTION The nature of the variable-aperture integrating converter can be understood from the schematic block diagram shown in Figure 1 and from the timing plan in Figure 2. The heart of the converter is a programmable analog integrator and 12-bit A / D converter, which combine upon command to integrate the analog input signal for a selected time and to encode the resulting integral in digital form. The integration period is controlled in the present system by front panel thumbwheel switches which count a precise (0.01 9'0), stable (1part in I O 6 per day) 1-MHz crystal clock for the desired time period. In this operation, the number in the thumbwheel switches is entered into a down-counter upon receipt of a conversion command; the integrator is simultaneously enabled.
Passage of the chosen number of pulses then clears the down counter, which in turn stops the integration and initiates the A/D conversion. Upon completion of the A/D conversion, the 12-bit digital word is gated into a buffered output register where it is available as a binary output and is simultaneously displayed on a front panel 12-bit light-emitting diode (LED) array. At the same time, the integrator is reset in preparation for another conversion cycle. The logic functions involved in determining the integration period, in initiating and terminating a conversion, and in gating out the digital word are performed by the control logic system. For maximum flexibility, the control logic system can operate in three modes: manual, automatic, and external. In the manual mode, a front panel pushbutton is used to initiate an integrate/conversion cycle, while in the external mode an input logical “1“ t o “0” (TTL-compatible) transition causes a cycle to begin. Under automatic operation, the VAIC operates continuously, with each integration/conversion operation being automatically triggered by completion of the preceding one. Under external triggering, a 0.2-ps delay exists between application of the negative-going input signal and the start of integration. The gain of the VAIC is selectable by means of front panel switches which vary the time constant of the analog integrator. Found in the gain normalization logic section of the instrument, these resistor-capacitor combinations govern the rate of integration of the analog input signal and thereby directly determine the integrator gain. These time constant combinations, like those for the integration period, are designed to read directly in milliseconds (settable in 100 ps, l-ms or 10-ms steps) so that the gain of the integrator can be conveniently calculated as the ratio of the integration period thumbwheel settings to those of the gain normalization. As indicated in Figure 1,the various instrument functions are isolated from each other by suitable transformers or relay systems to provide the greatest possible freedom from noise and crosstalk. Timing of the VAIC functions, indicated in Figure 2, is determined by the control logic system. The timing train shown in Figure 2 is for an externally triggered integrate/conversion cycle, although essentially identical timing is employed in the auto or manual mode. Upon receipt of a command trigger (external, manual or internal/auto), a short internal delay occurs (between 0.2-1 p s , depending on trigger mode), after which the analog integrator is enabled and integration of the input signal begins. At the same time the integrator is enabled, a “busy” flag is set to indicate a conversion is under way. The “busy“ bit is available at a rear panel connector and is also coupled to a front panel LED indicator. When the integration is complete, as determined by the integration period logic system, A/D conversion occurs, and the output registers are cleared in anticipation of the digitized integral value. The entire conversion and clearing operation takes about 30 ps. If the integrator has been overloaded or has been limited, this fact will be detected during A/D conversion as an overflow, and will cause an “overload” flag to be set. This flag bit is available on a back panel card-edge connector and also appears on a front panel LED indicator. At the time of conversion, the sign of the integral is also determined and reflected as a rear-panel logic level or a front panel indicator. After the digitization process is complete, the binary data word is loaded serially into the output data registers, which are in turn connected to a rear panel connector for transmission to an external device (e.g., computer). As stated before, the digitized data word is also displayed in binary form on a front panel LED array. T o signal that the data word has been completely loaded into the buffer register, a flag is set
Table I. Specifications of VAIC Not Discussed in Text Input: * 2 0 volts peak Input impedance: 1 0 k n Gain: t2djustable from to l o 2 by selection of integration period and gain normalization Linearity: 0.1% of full range Integration period accuracy: ~ ( 0 . 0 1 %- 2 ,us) Integration period stability: ~ 0 . 0 0 2 %per‘ C ; 0.0001% per day Gain normalization accuracy: 0.4% Gain normalization stability: 0.05% per C; 0.005% per day 0.5 p s later; this flag, like those used to indicate “busy” and “integrate“ conditions, is sent both to a n edge connector for remote sensing and to an LED indicator. The final operation performed during a conversion cycle is the resetting to zero of the integrator, in preparation for another trigger. When this operation is complete, 120 ,us later, the “busy” flag is reset to zero and the VAIC is ready for another data sample. As indicated in the foregoing discussion, the input/output features of the VAIC are designed for maximum user convenience under either manual or computer-controlled operation, with most control and command functions operating in parallel. This latter characteristic enables an operator to monitor and interrupt or override the VAIC when it is operating in the automatic mode. Other characteristics of the present instrument which are important to specific applications are listed in Table I.
DEVICE OPERATION The operation of the VAIC was demonstrated in four different applications, each emphasizing one aspect of its unique characteristics. These four applications include its use as a high noise immunity synchronized detector of a modulated signal, its use as a notch or frequency rejection filter, its ability to reduce high-frequency noise without distorting the time waveform of a signal, and its ability to collect integrated analog data with less software complexity than required for digital integration of the data. The experimental arrangement consisted of a signal source (several different sources used) fed directly into the VAIC and an auxiliary conventional A/D converter equipped with a 150 ns aperture sample-and-hold amplifier. Both converters were triggered on command from the computer, a PDP-12/40. A program was written to take data simultaneously from both converters and to store it on magnetic tape for later analysis and comparison. The first set of experiments employed the VAIC as a synchronized detector for a square-wave-modulated optical signal and compared its performance with the sample-and-hold detector. The modulated signal was derived from a n electronically chopped hollow cathode lamp whose radiation was selected by a monochromator and detected by a photomultiplier tube. By using very narrow monochromator slits, and running the photomultiplier tube at high gain, the signal was made artificially noisy. The lamp was modulated at 50 Hz, and the VAIC and sample-and-hold detectors were synchronized at 100 Hz, taking one point each half cycle of the lamp waveform. Thus, each pair of points consisted of one with the lamp on and one with it off, and their difference was proportional to the lamp intensity. The results of this study show a significant improvement in S / N ratio to be obtained by the VAIC over the sampleand-hold data acquisition system. Figure 3 shows ( A ) the sample-and-hold data and ( B )the data from the VAIC taken simultaneously with ( A ) . The actual improvement in S / N ratio was a factor of 4.1 using a VAIC integration period of 4 ms per point, and a factor of 4.8 using a 9 ms VAIC inteANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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