Multichannel, rapid recording spectrometerC. R. Dunnam, NS. Chiu, and S. H. Bauer Citation: Review of Scientific Instruments 57, 384 (1986); doi: 10.1063/1.1138951 View online: http://dx.doi.org/10.1063/1.1138951 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/57/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Multichannel soundscape recording techniques. J. Acoust. Soc. Am. 127, 1744 (2010); 10.1121/1.3383519 Modular multichannel surface plasmon spectrometer Rev. Sci. Instrum. 76, 054303 (2005); 10.1063/1.1899503 Multichannel spectrometer for plasma diagnostics Rev. Sci. Instrum. 57, 552 (1986); 10.1063/1.1138870 Multichannel grating spectrometer for millimeter waves Rev. Sci. Instrum. 48, 1355 (1977); 10.1063/1.1134890 A compact multichannel spectrometer for field use Rev. Sci. Instrum. 45, 1349 (1974); 10.1063/1.1686498

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Multichannel, rapid recording spectrometer c. R. Dunnam,S) N-S. Chiu,b) and S. H. 8auerbl

Cornell University. Ithaca. New York 14853

(Received 8 January 1985; accepted for publication 4 October 1985)

A microcomputer data-collection system is described based on a 20-element linear photodiode array which is positioned in the focal plane of the exit slit of a grating monochromator. Time­dependent, spectrally resolved signals developed by individual diodes in the array are preamplified and directed to a fast multichannel sampling digitizer subsystem. The maximum throughput is 20 parallel channels per 20-,us conversion period; the resulting single byte data words are temporarily buffered in a fast memory for eventual host CPU read in. The digitizer creates a data file consisting of 20 channels by eight bits by a selectable file length of up to 1024 words. Samples of data collected with this equipment of time-dependent absorption spectra of shock-heated toluene are presented.

INTRODUCTION

Records of rapidly changing absorption or emission spectra of shock-heated materials, or of plasmas immediately subse­quent to pulsed electrical discharges, or of gases subjected to flash photolysis provide excellent diagnostics of the chemi­cal dynamics of such rapidly evolving systems. For many decades it has been possible to record the time dependence of absorption or emission of radiation at a single wavelength, utilizing photomultipliers and oscilloscopes, and currently with the more sophisticated digital correlators. I There re­mained a trade-off between the recorded bandwidth (t.A) and the time response of the detector-recorder system (1'). It has also been possible to photograph an extended absorp­tion spectrum at a selected time interval by using a bright, pulsed white light source of duration at triggered at the time t, by appropriate delay circuitry. Current technology per­mits one to record such a spectrum during a slice of time as smail as 10 ns, over a substantial portion of the visible range, using a gated vidicon tube/silicon array with subsequent buffered readout in analog or digital form. OMA III (PAR 2

) operates with a linear array of 512 (or 1024) ele­ments, each 25 ,um wide. Selection of the time interval is achieved by gating the bias on the microchannel image inten­sifier. Serial readout permits recording of successive spectra at 8.3-ms (16.6 ms) intervals. Ostertag3 described an inter­face for synchronizing such an optical multichannel analyz­er for picosecond spectroscopy. The Hewlett-Packard4

# 8451 spectrophotometer records a spectrum over the range 190-820 nm in 100 ms but the repetition interval is limited to several seconds. This instrument also operates with serial readout.

While these devices have been actively utilized in many laboratories, one can generate a spectral histogram of a ra­pidly evolving phenomenon only by repeating the experi­ment many times; then a two-dimensionall(}.; t) function can be synthesized by combining many records. Obviously such a procedure has drawbacks for phenomena induced by shock waves or electrical discharges, which are inherently difficult to reproduce time after time, as is required to deve­lop an adequately sampled record. Hansen et aU described an electro-optical multichannel spectrometer with an image

converter streak unit for recording transient Raman and ab­sorption spectra. In this report we briefly describe the con­struction of a sensing, digitizing, and readout unit of suffi­cient rapidity to permit recording spectral intensities J(ntJ..A; m1') with n = 20 channels; l' = 20, 50, or 100 f-Ls; m = 64, 128, 256, 512, or 1024 samples. The magnitude of tJ..A is de­termined by the dispersion of the spectrometer (in our case: tJ..A = 1.0 nm); the central wavelength is set by the grating tilt. We show how the various system components were inte­grated into a functioning unit. Rapidly changing absorption spectra of gaseous toluene while undergoing pyrolysis in a shock tube are presented to illustrate the application of this unit in our laboratory.

As recently summarized by Compton and Landon,6

who cited the pertinent literature, the advantages of parallel detection systems in spectroscopy are many. By implemen­tation of many inexpensive parallel channels for analog-to­digital conversion, our system retains the advantages of si­multaneous event capture while minimizing the throughput delay inherent in seriaily read CCO designs.

!. SPECIFICATIONS AND SYSTEM DESIGN

Figure 1 is a schematic of our current experimental con­figuration. Specifications of the multichannel spectrometer digitizer (MSD) processing subsection of the system (Fig. 2) were determined by the proposed use of a 20-diode photo­detector array (Centronic LD20-5),7 the expected output levels of this array with the available light flux from the spec­trometer optics, and the total period required for real-time accumulation of information resulting from a single run. The overall cost effectiveness of various designs were evalu­ated to arrive at the maximum conversion throughput and digital resolution which could be developed for a per-chan­nel ADC section components cost of approximately $100.

A. Preamplifier

Under typical spectrometer illumination conditions, a Centronic LO 20-5 diode generates an output current in the range of 50 nA. Low-noise, high-gain local preamplification was therefore essential to avoid difficulties with wideband

384 Rev. Scl.lnstrum. 57 (3), March 1986 0034-6748/86/030384-06$01.30 @ 1986 American Institute of Physics 384 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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I I I I

IOc}

9~

II

7b 7a 17

17

0 16

14

15

5

, , , , Quartz Plate I 0 4 I , 8 Heated to 150 C 6 :

~-----------------------------------~

(to 50 kHz) transmission of a good fidelity signal ::::: 4m feet to the MSD unit. Fortunately, newer FET-input precision op-amp devices, such as the PMI-OP-15,8 facilitate the de­sign of a simply realized high-gain block. Each of the 20 discrete preamplifiers (Fig. 3) in the current system pro­vides an effective transform resistance of 1 M!l while con­tributing less than 30 pA equivalent input noise current over a bandwidth of 50 kHz. The measured noise at a preamplifier output is 400 ft V peak to peak, referenced to a typical (in our experiment) full-scale output of 50 m V. Since the preampli­fier contribution is ::::: 40 ft V, sensor diode dark current noise is the dominant component.

SENSOR,

INCIOCNT­LIGHT -

MSD,

FROM PREAMP ANALOG

T.P.

TRIGGER IN

BUSY OUT

AC LINE INPUT

CSM WE AODR

PWR

FIG. 2. Block diagram for MSD.

OIR

385 Rev. Setlnstrum., Vol. 57, No.3, March 1986

MBUS

CPU CPU DATA CONTROL

IN 6 DATA OUT

-6--@ 2

FIG. 1. Schematic of experimental arrange­ment for recording rapidly changing absorp­tion spectra of shock-heated gases. 1. lOO-W Xe lamp; 2. short! I. lens; 3. plastic backplate; 4. diaphragm location; 5. quench tank; 6. lead to vac lines; 7. piezogauges (a,b); 8. sampling port; 9. front surface mirror; lO.long! Liens; 11. ~ m Spex monochromator; 12. 20-element Si diode array; 13. analog amplifiers; 14. NC converters; sample and hold; 15. computer (disk record); control and test; 16. Tek scope monitor; 17. trigger from s.t. (piezoelectric button).

B. Analog conditioning

Prior to conversion to digitized form, further preampli­fication of the signal voltages is necessary. The input stages of the MSD must provide a commonly selectable gain factor without introducing a simultaneous change of the overall system bandwidth or dc offset. For this reason, a "cold switched" attenuator section was inserted in each of the 20 channels (Fig. 4). Use of analog switch sections within a suitable resistance network provides the specified perfor­mance inexpensively. This type of attenuator design allows calibrated gain steps in a 1-2-5 sequence over a range of 50 to 1 without adverse effects on bandwidth or baseline offset. Amplification was divided around the attenuator in a man­ner which limited the contribution of noise from the active devices while maintaining adequate dynamic range at the input. Overall, the input stages generated a full-scale level of 10 V to the ADC for a wide range of expected diode array illuminations.

C. Sample/hold and ADC Samplelhold (SIH) and analog-to-digital conversion

(ADC) functions were implemented by advantageous use of relatively low-cost monolithic devices. A performance trade-off is most evident here. Review of available ADC units indicated a sharp cost threshold at a sampling period of 15 fts for 8-bit resolution. We felt that for the application at hand, a monolithic conversion device (Analog Devices AD7574)9 capable of digitizing eight bits, to a resolution of ± 112 least-significant bit, in slightly less than 15 fts offered

CENTRONIC 020-5 r-------, I I

: I J I I I , I' .... "2.1 I I I I L ______ .J

FIG. 3. Analog preamplifiers.

Spectrometer

lOOn

OP-IS

>-- CHANa 'I" CHAN!

I I >-- CHANI9

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lOOK 1%

¢ OFFSET NULL

1.5K(I"!.) IK(I%)

OP-15 G=2.5

1/2 LM412A G=20

02.5

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·OG202 Rs= 120ll 10K(! "10)

-15

SHM-IC-l G= 10

FIG. 4. Step attenuator and samplelhold-schematic.

82.5K 1"1.

To ~DC

10V F.S.

the best cost-performance ratio. A companion S/H unit of 4 J-ls to 0.5% accuracy was selected (Datel SHM-IC-l), 10 for a conservative total sample-and-convert time of 20 J-ls. We also found that the microprocessor bus compatible output lines of the AD7574 ADC made possible a significant reduc­tion in complexity and cost in the overall MSD architecture.

O. Data manipulation

The ADC output data are routed by a central control and timing section, which supplies all appropriate timing signals required for transmission of the data over the internal bus pathways (Fig. 5). In the data-taking mode, 8-bit data words produced by each of the 20 parallel converters are sequentially stored in dedicated Iocall-kbyte memory parti­tions. Under direct host control, data from anyone of the channels may be read out from that particular RAM to the machine bus (MBUS) and on to the CPU interface input port at a maximum rate of 500 kilowords per second.

For test purposes, provision was made for the host CPU to overwrite the memory in one of two alternating polarity bit patterns. Readout to the CPU of memory contents in each test polarity then allows software verification of for correct RAM operation.

A "status" register is available for readout of essential information relating the functional status of the MSD con­troller. Three encoded bits indicate the selected file length, while individual bits flag perturbing conditions, such as one or more power supplies out of range, stale data, a general error condition, or a busy controller (actively converting data).

E. Input/output

Codes directed from the CPU to the MSD are necessary for selection of the operating mode, and for a memory read-

386 Rev. ScI. Instrum., Vol. 57, No.3, March 1986

FROM CPU

OP CODE

DATA

I/O XFER

[

FRONT ] PANEL

,CONTROLS

GAIN

SAMP. PERIOD

FILE LENGTH

[EXT.]

TRIG

BUSY

TIMING

a CONTROL

GAIN

SAMP

CS

RD

10 ADDR CSM WE

ANALOG INPUT

TO OTHER CHANNELS

M BUS

FIG, 5. Analog processor and data control (block); timing diagram.

out sequence for specification of which channel file is to be transferred. Three bits of the 8-bit output word are decoded to determine the operational mode, with five bits of data available when needed for indicating the channel number.

Of the eight OP code possibilities, five designate a unila­teral mode for the digitizer, such as even or odd memory test, arming for the advent of a trigger, initiating a triggered event, or initializing to the powerup state. Two of the re­maining OP codes are allocated for initiation of read-in ac­tivity with the host computer. These codes enable a read-in sequence which transfers either an 8-bit status word or a series of data bytes which comprise one channel's record. The eighth OP code is presently not used.

Read in of a data record is performed on a per channel basis. A suitable op code with identification of the desired channel (0-1910

) is first read to the MSD from the CPU. Within approximately 500 ns, the initial word of a record appears at the MSD "DIN" data port for CPU read in. Through succeeding "read" operations the MSD responds in a "slave" mode; a read-in operation from the CPU inter­face to CPU internal memory generates a signal which indi­cates that each read cycle was completed. Upon receipt of the "data-transferred" strobe, the MSD then fetches a new data word from the next buffer memory address of the desig­nated channel.

Connection from the host CPU interface to the MSD chassis is made via a 20 twisted pair flat cable which may be up to 100 ft in length. The permissible cable length is other­wise dependent on the type of driver circuitry present in the CPU interface.

F. Manual controls and adjustments

Provision was made for manual. selection of various op­erating parameters by means of front-panel switches. The settable parameters are analog gain (volts full scale), sam-

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pIing period ( J.ls per data word), and file length (number of words per channel buffer). An analog test connector was provided for sampling anyone of the 20 signals in analog form at a 0-1 V -full-scale level. At the front panel, a 21 posi­tion switch selects the appropriate channel; one switch posi­tion is used to select a comparative reference ground level. Preliminary adjustments of light intensities, wavelength calibration and band selection, amplification level, etc. can be readily made without engaging the CPU.

G. Mechanical

All analog processing and conversion elements (with the exception of the diode array preamplifiers) are located with the digital systems in the main chassis. Careful segrega­tion of analog and digital sections was essential to prevent

(a) CHANNEL 12 100-.--------------------------------~

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20-

I I I I I

0 20 40 60 80 100 120

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100 (C) A:: 5458 4000 MICROSEC

80

60

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0

0+ I I J

0 4 8 12 16 20 24 Channel :#

deleterious coupling oflogic signals into the low-level analog input stages. Although wire wrap techniques were used in construction of the two printed circuit boards comprising this unit, no cross talk was evident during performance test­ing. Use of ground plane prototype boards is essential in achieving satisfactory isolation. Approximately 300 inte­grated circuits and component headers were incorporated into the main chassis. with an additional 40 devices in the preamplifier enclosure. The MSD chassis also houses several power supplies which are required for system operation. The overall dimensions of the MSD enclosure are 11.5 cm (4.5 in.) X 41.0 em (16in.) X 30.7 cm (12 in.). A separate 12.8 (5 in.) X 12.8 (5 in.) X ll.S-cm (4.5 in.) lightproof enclosure (attached to the spectrometer) houses the diode array and associated preamplifiers.

(b) CHANNEL 12 100

80

~ 60 en c:: C1)

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20

0 40 80 120 160 200 240

Microsec 10 2

(d) 80

A = 5788 7000 M I CROSEC

60

I 40

20

0 0 4 8 12 16 20 24

Channel 41:

FIG. 6. (a) Hg I emission (547.07-nm line) recorded at channel # 12 with MSDset to read for 100flsat each point. for a total of 1024 points (no smoothing). (b) As above, on an expanded scale. The noise probably originates in the Hg lamp. (c) Hg I emission (dial set at 545.8 nm) recorded by all 20 channels. with MSD set to read 20 fls/point; the top curve was read 4000 flS from t = 0; the lower curve was read at 1000 flS from I = 0 (intensity change at peak. due to lamp modulation). (d) Hg I emission (dial set at 578.8 nm); 20 fls/point; reading 7000 Jl.S from t = O. This spectrum demonstrates the resolution of the yellow doublet at 577.0 (channel #12) and 579.0 (channel #14).

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II. HOST CPU INTERFACE

An 8085/88 dual CPU (CompuPro) is the remote host microcomputer. The 8088 microprocessor (8-MHz clock) manages the CPU RAM only; an the other operations are handled by the 8085. A parallell /OcontroJ port (Godbout) interfaces the MSD and CPU at a speed of approximately 20 kilowords per second, for which an assembly subroutine was written. All MSD operations are controlled by the host CPU with exception of the analog gain, sample period, and the sample file length. For our application, manual control of these functions was prefered. To accomodate other applica­tions such as software control of the gain, sample period, and sample file length could be readily introduced.

Typical CPU commands issued to the digitizer subsys­tem in the course of data-taking includes a memory test of each polarity followed by the ARM command. At that point, the unit is ready for the trigger signal from the experi­ment. Immediately following the trigger, the MSD asserts its BUSY signal until a complete file of the selected length is converted. On completion. the CPU reads the current MSD "status" word to determine that files had been successfully recorded. Then the accumulated data may be transferred to the host CPU.

Read-in operations are slave driven by the host CPU. The file of a particular channel is vectored as part of an OP3 (READ) command, and the MSD places the first file word on the eight parallel DIN lines. When the CPU transfers this datum to its memory, a DTRANS signal is automatically generated by its interface card. This "data-transferred" ac­knowledgment is processed by the MSD controller as a cue to set up the next sequential data word. The procedure is repeated until the end of the file is reached, as indicated by the file length bits from the previously read status word. An attempt to read buffer locations past the end of the file gener­ates a front-panel ERROR, and flags the status word ER­ROR bit. The remaining channel data files are read in by individually specifying a new OP3 vector, and the above steps are repeated. Although the CPU data transfer rate is 20 kHz, the time required by our CPU is typically about 7 s for readin and storage on disk of a full buffer (20 kiJowords).

For viewing data, one has the option of plotting the in­tensity curves or displaying them on a VDT. Several other options are available for manipulating and cross plotting the data, i.e., I(t; specified channel); I(channel #; specified time), (lo /1) and log (l 0 /1), where lois a complete data set when no absorber is interposed between the lamp and spec­trometer. Also, a smoothing routinell (5-25 pts.) may be applied to I (t) for aU channels.

m. PERFORMANCE

The data obtained with this unit are illustrated in Figs. 6-8. The wavelength dial of the SPEX spectrometer was ini­tiaHy calibrated by locating the diode array so that one diode gave the maximum response to a low-pressure mercury source at 546.07 nm. Figures 6(a) and 6(b) show the time­dependent intensities recorded at channel # 12; the modula­tion of the mercury lamp is presented on two time scales. Figure 6 ( c) shows the absence of" cross talk" between chan-

388 Rev. Scl.lnstrum., Vol. 57, No.3, March 1986

II),;!)

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t (in units of 60 p..s) -+

FIG. 7. Absorption spectra of shock-heated toluene (2% in Ar) in the re­Hected shock regime. All spectra were recorded (Fig. 1) with 20-)ls time resolution. The spectrometer dial was set at 400.0 nm; t, = 0 indicates the time when the shock was reflected from the quartz end plate. The shock temperature was high (incident shock speed. u, = 1.031 mm/)ls). The de­cline in I(A; t = 0) with increasing channel number is due to the lower sen­sitivity of the Si diodes and reduced lamp output with increased frequency. Careful inspection shows that the decay is somewhat faster at #20 than at #1.

nels, and Fig. 6(d) illustrates the spectral resolution avail­able with the SPEX monochromator; the MSD recorded these signals at a conversion rate of 20 fLs per point. Figure 7 is an I(A.; t) 2D plot of light transmission by shock-heated toluene, recorded with 20-fLS resolution. The delay time for

;:-::--__ . ___ . ___ ---=I IX it)

I I i 3 4 5 6 7 8 t (in units of 120 /-LsI -

FIG. 8. The spectrometer dial was set at 800.0 run; t, = 0 indicates when the shock was reflected from the quartz end plate. Channels # 1-#20 show indistinguishable time dependencies. The incident shock speed was u, = 1.020 nun/)ls.

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onset of strong absorption in the reflected shock regime is measured from the vertical stroke at t, = O. Over this range of 20 nm (392-412 nm) there is a significant change in the sensitivity of the Si diodes and lamp output, and there is a very small change in absorption (between 200-300 J-ls) by the shock generated species. As expected, the evolution of absorbing species is temperature dependent. At the other end of the recorded spectrum (Fig. 8) there is no significant wavelength dependence. There is a marked delay in the onset of absorption. Note a preliminary saturation at ::::: 480 J-lS and the onset of a second decay at ::::: 840 J-ls.

IV. DISCUSSION

Experience with this system demonstrates that the origi­nal specifications were met. Improvements could be achieved in sampling rate and digital resolution at a signifi­cantly higher cost per channel, although newer monolithic devices are constantly pushing down the cost/performance ratio. Analog performance could also be upgraded through the use of more recent, lower tempco FET op amps. A digi­tizer capable of 12-bit accuracy at lOJ-ls per conversion could now be built for a components cost of approximately $250 per channel. Expansion up to 32 paranel channels is possible for accomodation of a higher resolution diode array. 7 Other system changes for improved design would eliminate all manual front-panel adjustments in favor of! /0 loadable val­ues, and provide automated offset nulling for the analog pro­cessing sections.

We have described a high-quality, relatively low-cost multichannel digitizer capable of creating and storing a (20X 1) kbyte data record per trigger event. Use ofthis de-

389 Rev. ScI. Instrum., Vol. 57, No.3, March 1986

vice has increased the overall experimental efficiency by a factor of at least two orders of magnitude when compared with previous data-taking methods.

ACKNOWLEDGMENTS

This program was supported by the U. S. Department of Energy under contract DE-ACOl-80 ER 10661.AOO4. We thank Professor P. M. Jeffers and W. Ausserer for recording some of the time-dependent spectra of shock heated toluene, and Chris Meier for his technical assistance in construction of the MSD prototype.

a) Laboratory of Nuclear Studies. h) Baker Chemical Laboratory, Department of Chemistry. 'J. C. Thomas, Y. T. Lum, and D. Kennett, Rev. Sci. Instrom. 54, 1346 (1983).

2Princeton Applied Research, P. O. Box 2565, Princeton, NJ 08545. 'E. Ostertag, Rev. Sci. Instrom. 48, 18 (1977). 4Hewlett-Packard Corp., 1601 California Avenue, Palo Alto, CA 94304. 'K. B. Hansen, R. Wilbrandt, and P. Pagsberg, Rev. Sci. Instrorn. SO, 1532 ( 1979).

6R. D. Compton and D. O. Landon, Lasers and Applications, August 1984, pp. 65-68; D. Landon and R. D. Compton, Laser Focus, August 1982, p. 47ft'.

7Hamamatsu (420 South Avenue, Middlesex, NJ 08846) markets a 35-element linear array detector, with a spectral response range 190-1100 nm (SI592-01). United Detector Technology also manufactures a variety of multielement photodiode arrays.

8Precision Monolithics, Inc., 1500 Space Park Drive, Santa Clara, CA 95050.

9Analog Devices, Inc., Route I, Norwood, MA 02062. IODatel, Inc., 11 Cabot Blvd., Mansfield, MA 02048. IIA. Savitzky and M. J. E. Golay, Anal. Chern. 36,1627 (1964). To remove

high-frequency noise an autocorrelation smoothing routine has also been programmed.

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