ANALYTICAL
BIOCHEMISTRY
190,134-140
(1990)
Construction of a Fast, Inexpensive Rapid-Scanning Diode-Array Detector and Spectrometer’ Thomas Institute
Received
P. Carter,
Haesun
K. Baek,
Lee Bonninghausen,
Roger
J. Morris,
and Harold
E. Van Wart2
of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee,Florida 32306-3006
November
29,1989
A 512-element diode-array spectroscopic detection system capable of acquiring multiple spectra at a rate of 5 ms per spectrum with an effective scan rate of 102.9 kHz has been constructed. Spectra with fewer diode elements can also be acquired at scan rates up to 128 kHz. The detector utilizes a Hamamatsu silicon photodiodearray sensor that is interfaced to Hamamatsu driver1 amplifier and clock generator boards and a DRA laboratories 12-bit 160-kHz analog-to-digital converter. These are standard, commercially available devices which cost approximately $3.500. The system is interfaced to and controlled by an IBM XT microcomputer. Detailed descriptions of the home-built detector housing and control/interface circuitry are presented and its application to the study of the reaction of horseradish peroxidase with hydrogen peroxide is demonstrated. 0 1990 Academic Press, Inc. _
Rapid-scan spectroscopy (l-13) is a valuable technique for the study of biochemical reactions involving chromophoric species. The sequential time-resolved acquisition of spectra can provide information on the number of elementary steps in a reaction and directly visualize the transient intermediates and products that are formed. Such studies also allow the quantitation of the rates of formation and decay of these species. The most widely applicable rapid-scan technique is time-resolved optical spectroscopy. There has been a recent increase in the use of this technique that has resulted from the development of commercially available self-scanningsilicon diode-array detectors (7,8,10). These detectors are well suited for time-resolved optical experiments bei This work was supported by Research Grants DMB 85-20068 from the National Science Foundation and GM27276 from the National Institutes of Health. * To whom correspondence should be addressed.
cause of their wide spectral sensitivity, large dynamic range, linearity of response, and temporal stability. Because of the current high level of interest in rapidscan diode-array spectroscopy, a need has developed for alternatives to the commercially available equipment (4,6,11-13). In particular, the relatively high cost of commercial systems has prevented the widest possible application of this technique to biochemical kinetics. Thus, a design for a low-cost rapid-scan detector constructed from commercially available components would be welcome. The construction of a home-built instrument also permits design flexibility that can give features not available in commercial instruments. In this paper, the construction and performance testing of a flexible diode-array detector assembled from commercially available components for a cost (not including the computer) of approximately $3500 is described. The instrument uses a diode array with 512 elements and a 160-kHz A/D3 converter that enables the user to acquire consecutive 128-element spectra in 1 ms or 256-element spectra in 2 ms for an effective scan rate of 128 kHz. Alternatively, 512-element spectra can be acquired over selectable time intervals as short as 5 ms at an effective scan rate of 102.9 kHz. The instrument can also be operated in a single-channel mode using a phototube as a detector at the maximum data acquisition rate of 160 kHz. The construction is straightforward and represents an alternative to commercial instruments. GENERAL
DESIGN
CONSIDERATIONS
An important consideration in the design of the diode-array detector was that it be constructed from commercially available components that required the minimum amount of interfacing. A second design goal was to produce a device whose effective scan speed is comparable to that of standard commercial models. 3 Abbreviations used: A/D, analog to digital, PCD, device; EOS, end of scan; TTL, transistor-transistor
134 All
Copyright 0 1990 rights of reproduction
plasma-coupled logic.
0003-2697190 $3.00 by Academic Press, Inc. in any form reserved.
RAPID-SCAN TABLE
135
ARRAY DETECTOR
1
Components of Diode-Array Detector Component Hamamatsu Model S2304-5126 512-element PCD linear diode array Hamamatsu Model C2324-228 driver/amplifier board Hamamatsu Model C2335 clock generator board DRA Laboratories Model A2D-160 160-kHz analog/digital converter board Melcor Model CP LO-127-05L Peltier cooler Sola Model 85-05-210 modular +5-V, 1-A power supply Semiconductor Circuits Model SP 6231 modular +15-V, 200-mA power supply Hewlett-Packard Model 6281A 37-W power supply (optional) a June,
cost= $ 615 400 142 1,295 30 141 25 920
1989.
Third, the design of the detector head which houses the array should incorporate the best features of commercial detectors. Specifically, the size should be no larger than a photomultiplier tube housing, but still large enough to contain critical electronics which need to be kept close to the array to reduce signal loss and radio frequency interference. The detector head should also be designed to allow interactive focusing when used in conjunction with a spectrograph, a feature lacking in many commercial detector-head designs. A fourth consideration was that the data acquisition system be built around an IBM PC-compatible microcomputer, since such computers are widely available. Last, the choice of analog-to-digital (A/D) converter board was limited to those which had a data acquisition rate high enough to keep up with the diode array output at its fastest scan rate. RESULTS
FIG. 1. Photograph of the partially disassembled inner sleeve assembly of the diode-array detector head showing the sensor on the extreme right and the driver/amplifier board in the center.
AND
DISCUSSION
Choice of components and overall design. The sensor chosen for this detector was the Hamamatsu S2304512Q plasma-coupled device (PCD) linear image sensor. This device consists of 512 contiguous silicon photodiode elements with a 25pm center-to-center spacing and a height of 2.5 mm. It is fitted with a quartz window to enhance the ultraviolet light sensitivity. This sensor works in conjunction with the Hamamatsu Model C2324-228 diode-array driver/amplifier and the Hamamatsu Model C2335 clock generator boards. The first of these boards contains the diode-array driver and circuits to amplify and condition the array output. The latter board provides clock and timing pulses to control the diode scanning and TTL output signals to synchronize external devices. The maximum scan rate of the diode array using these devices is 128 kHz. These compo-
nents were selected because of their low cost and easeof integration. The only other items required to get these Hamamatsu components to work together as a scanning detector are +5- and f15-V power supplies. A Melcor Model CPl.O-127-05L Peltier cooler was chosen to cool the diode-array for reduced dark noise, although in most rapid-scan experiments there is no need for this capability. The data acquisition system consists of a DRA Laboratories Model A2D-160 12-bit A/D converter PC board that has a maximum sampling rate of 160 kHz, some simple homemade control, timing, and interface electronics, and an IBM XT microcomputer. The operation of the diode array is controlled by a menu-driven program of our own creation. The program allows for acquisition, display, and manipulation of up to eight spectra at a time (limited by the software), as well as provisions for permanent disk storage and hard copy (plotter or printer) output. A listing of these components and their cost as of June, 1989, is given in Table 1. Detector head. The diode array is mounted in a homemade housing, shown partially disassembled in
FIG. mounted
2.
Photograph showing the on the spectrograph and next
diode-array detector to the controller box.
head
136
CARTER Outer
b]
FIG. 3. parts
Mounting
Components are aluminum;
ring
cylinder
cl
End
cap
of the home-built diode array all dimensions are given in inches.
housing.
All
Fig. 1. The fully assembled detector head is shown in Fig. 2 mounted on a spectrograph and next to the control box (see below). Figures 3 and 4 are schematic diagrams of the detector head components showing some critical dimensions and assembly details. The detector head contains the Melcor Model CP l.O-127-05L Peltier (thermoelectric) cooler that is capable of cooling the array to lower than -20°C if dark noise reduction is desired. The cold stage of the cooler is in thermal contact with a copper cold finger which extends through a slot (provided for this purpose) in the small circuit board on which the diode-array is mounted, and cools the back of the array. The hot side of the cooler is in thermal contact with another copper block which contains channels to accommodate the flow of a heatexchange fluid (e.g., chilled water). If only modest temperature reduction is needed (say to 5”C), the heatexchange fluid is not needed. Due to the high power required by the cooler (up to -30 W at 15 V), an external power supply (Hewlett-Packard Model 6281A or equivalent) is used when operating at low temperatures. (It should be noted that when high source intensities are available, cooling is not necessary.) The driver/amplifier is also mounted as close to the diode array as possible in the detector head to minimize external electrical interferences and losses from corrupting the high-frequency signals which are passed between the array and the driver/amplifier. The detector housing consists of Detector housing. four parts machined from aluminum: an outer cylinder, an inner sleeve, an end cap, and a mounting ring. The outer cylinder (Fig. 3a) has an outside diameter of approximately 3.25 in. with a wall thickness of i in. and is
ET
AL.
open at both ends. The mounting ring (Fig. 3b) attaches to the outside of the outer cylinder with set screws, and has a flange to allow attachment to a spectrograph. This arrangement allows the outer cylinder to be rotated within the mounting ring to orient the diode axis of the array to be horizontal. The other end of the outer cylinder is closed by the end cap (Fig. 3~). This is shaped like a jar cap and fits over the end of the outer cylinder to make an effective light-tight seal, and is held in place by thumb screws. The end cap has a standard DB-25 bulkhead connector mounted in it to accommodate a cable for transferring electrical signals between the detector assembly and the control box. The inner sleeve (Fig. 4) is machined to fit inside the outer cylinder and serves as a mount for the array, the cooler assembly, and the driver/amplifier board. A longitudinal slot has been machined through the outer cylinder and parallel to its axis to accommodate a bolt that is screwed into the inner sleeve. Sliding the bolt along the slot adjusts the position of the inner sleeve in the outer cylinder. This allows the array to be positioned at the focal plane of the spectrograph. When the array is correctly positioned, this bolt is used to lock the inner sleeve in place. Driver/amplifier. The driver/amplifier actually consists of two boards: a small board on which the array is mounted and a larger board that contains the driver/ amplifier electronics. This two-board layout allows for greater flexibility in the design of the detector head geometry. The driver/amplifier has a twofold purpose, as its name implies. Its first duty is to provide an interface between the diode-array and the clock generator, relaying the START pulses from the clock generator to the array and producing a three-phase clock output for the Inner sleeve assembly Peltier
cooler driver/amplifier
7
_--.-
FIG. 4.
Three views housing. The electrical clarity. This assembly
coolant
tubes
of the inner sleeve assembly of the diode-array cables and connectors have been omitted for is designed to fit inside the outer cylinder.
RAPID-SCAN
ARRAY
array from the single-phase CLOCK output of the clock generator. Its second duty is to amplify and buffer the data output from the diode-array. The array and driver/amplifier interact as follows. The array needs a START pulse and pulses from the three-phase (&-&) clock to read the diodes. After the array receives a START pulse, it stops integrating the light, sets its EOS (end-of-scan) signal low (to 0 V), and then waits for pulses on its clock inputs. A & pulse causes the intensity of the first diode to be read out, the following & pulse causes the second diode intensity to be read out, and so on, continuing sequentially along the array until the last element is reached. The array signals the end of its scan by setting its EOS output high (to 5 V), and begins integrating the light intensity until the next START pulse arrives. During the scan, the video output from the array (a train of voltage pulses, one for each diode in sequence, whose amplitude is proportional to the accumulated charge on the corresponding diode) is sent to the driver/amplifier, which captures each pulse briefly in a sample-and-hold buffer to eliminate switching transients associated with the diode scanning electronics. The output from the sample-and-hold buffer is inverted and amplified by the driver/amplifier. The EOS signal of the array is also inverted by the driver/amplifier for use as a not-ready signal to synchronize external devices to the start of an array scan. In addition, the driver/amplifier produces a TRIGGER output which is used to synchronize external events (e.g., A/D conversions) to the video output rate from the sample-and-hold buffer. The TRIGGER output provides a pulse whenever the sample-and-hold buffer of the driver/amplifier has a valid reading ready for the A/D converter. A/D conuerter. The DRA Laboratories Model A2D160 A/D converter is a high-speed, dual-channel, 12-bit (4096 levels) sampling converter on a circuit board that can be used with an IBM PC, XT, or AT compatible computer. The manufacturer provides a library of software subroutines which performs all of the necessary hardware manipulations and greatly simplifies its utilization. In our application, the board is configured to operate in a mode where the two A/D channels sample the same input at a faster rate than when it is operating in its ordinary single-channel sampling mode. Operating in this mode and utilizing direct memory access for transferring the data to the memory of the computer yields an overall A/D conversion and storage rate of up to 166 kHz. This is a good match to the maximum scan rate of the diode array which is 128 kHz (the readout of 512 diode values in 4 ms). The A/D board also has a single g-bit D/A converter which is utilized as described below. Control and interface circuits. A small control box is used to house the +5- and +15-V power supplies, the
137
DETECTOR
(k, 7400 Ext. Trigger Free run START
(from
To A/D
ClK bd.1
-
EOS (from >
INITIATE
To A/D Somp4e Clock
a
wroyl b 7
7473
5
+,,d r^ 411
IO
To A/D
GND
FIG. 5. Circuit diagram for synchronizing the D/A board to the diode array. The four D-shaped devices are NAND gates contained in a single 7400 quad-NAND chip. The numbers around the 74121 and the 7473 are the pin numbers for the two devices.
clock generator board, some timing and control electronics (vide infra), control switches, and the input/output connectors to the computer, detector head, and oscilloscope. The clock generator board produces the master CLOCK and START pulses which control the diode scanning rate and the start of a scan, respectively. The board comes with switches mounted on it which control the signal integration time by adjusting the clock frequency and the interval between START pulses. For convenience, we replaced the original START timing control interval switches on the clock generator board with ones mounted on the front panel of the control box (Fig. 2). This gives the user easy access to adjustment of the signal integration time, which can have values from
BIOCHEMISTRY
190,134-140
(1990)
Construction of a Fast, Inexpensive Rapid-Scanning Diode-Array Detector and Spectrometer’ Thomas Institute
Received
P. Carter,
Haesun
K. Baek,
Lee Bonninghausen,
Roger
J. Morris,
and Harold
E. Van Wart2
of Molecular Biophysics and Department of Chemistry, Florida State University, Tallahassee,Florida 32306-3006
November
29,1989
A 512-element diode-array spectroscopic detection system capable of acquiring multiple spectra at a rate of 5 ms per spectrum with an effective scan rate of 102.9 kHz has been constructed. Spectra with fewer diode elements can also be acquired at scan rates up to 128 kHz. The detector utilizes a Hamamatsu silicon photodiodearray sensor that is interfaced to Hamamatsu driver1 amplifier and clock generator boards and a DRA laboratories 12-bit 160-kHz analog-to-digital converter. These are standard, commercially available devices which cost approximately $3.500. The system is interfaced to and controlled by an IBM XT microcomputer. Detailed descriptions of the home-built detector housing and control/interface circuitry are presented and its application to the study of the reaction of horseradish peroxidase with hydrogen peroxide is demonstrated. 0 1990 Academic Press, Inc. _
Rapid-scan spectroscopy (l-13) is a valuable technique for the study of biochemical reactions involving chromophoric species. The sequential time-resolved acquisition of spectra can provide information on the number of elementary steps in a reaction and directly visualize the transient intermediates and products that are formed. Such studies also allow the quantitation of the rates of formation and decay of these species. The most widely applicable rapid-scan technique is time-resolved optical spectroscopy. There has been a recent increase in the use of this technique that has resulted from the development of commercially available self-scanningsilicon diode-array detectors (7,8,10). These detectors are well suited for time-resolved optical experiments bei This work was supported by Research Grants DMB 85-20068 from the National Science Foundation and GM27276 from the National Institutes of Health. * To whom correspondence should be addressed.
cause of their wide spectral sensitivity, large dynamic range, linearity of response, and temporal stability. Because of the current high level of interest in rapidscan diode-array spectroscopy, a need has developed for alternatives to the commercially available equipment (4,6,11-13). In particular, the relatively high cost of commercial systems has prevented the widest possible application of this technique to biochemical kinetics. Thus, a design for a low-cost rapid-scan detector constructed from commercially available components would be welcome. The construction of a home-built instrument also permits design flexibility that can give features not available in commercial instruments. In this paper, the construction and performance testing of a flexible diode-array detector assembled from commercially available components for a cost (not including the computer) of approximately $3500 is described. The instrument uses a diode array with 512 elements and a 160-kHz A/D3 converter that enables the user to acquire consecutive 128-element spectra in 1 ms or 256-element spectra in 2 ms for an effective scan rate of 128 kHz. Alternatively, 512-element spectra can be acquired over selectable time intervals as short as 5 ms at an effective scan rate of 102.9 kHz. The instrument can also be operated in a single-channel mode using a phototube as a detector at the maximum data acquisition rate of 160 kHz. The construction is straightforward and represents an alternative to commercial instruments. GENERAL
DESIGN
CONSIDERATIONS
An important consideration in the design of the diode-array detector was that it be constructed from commercially available components that required the minimum amount of interfacing. A second design goal was to produce a device whose effective scan speed is comparable to that of standard commercial models. 3 Abbreviations used: A/D, analog to digital, PCD, device; EOS, end of scan; TTL, transistor-transistor
134 All
Copyright 0 1990 rights of reproduction
plasma-coupled logic.
0003-2697190 $3.00 by Academic Press, Inc. in any form reserved.
RAPID-SCAN TABLE
135
ARRAY DETECTOR
1
Components of Diode-Array Detector Component Hamamatsu Model S2304-5126 512-element PCD linear diode array Hamamatsu Model C2324-228 driver/amplifier board Hamamatsu Model C2335 clock generator board DRA Laboratories Model A2D-160 160-kHz analog/digital converter board Melcor Model CP LO-127-05L Peltier cooler Sola Model 85-05-210 modular +5-V, 1-A power supply Semiconductor Circuits Model SP 6231 modular +15-V, 200-mA power supply Hewlett-Packard Model 6281A 37-W power supply (optional) a June,
cost= $ 615 400 142 1,295 30 141 25 920
1989.
Third, the design of the detector head which houses the array should incorporate the best features of commercial detectors. Specifically, the size should be no larger than a photomultiplier tube housing, but still large enough to contain critical electronics which need to be kept close to the array to reduce signal loss and radio frequency interference. The detector head should also be designed to allow interactive focusing when used in conjunction with a spectrograph, a feature lacking in many commercial detector-head designs. A fourth consideration was that the data acquisition system be built around an IBM PC-compatible microcomputer, since such computers are widely available. Last, the choice of analog-to-digital (A/D) converter board was limited to those which had a data acquisition rate high enough to keep up with the diode array output at its fastest scan rate. RESULTS
FIG. 1. Photograph of the partially disassembled inner sleeve assembly of the diode-array detector head showing the sensor on the extreme right and the driver/amplifier board in the center.
AND
DISCUSSION
Choice of components and overall design. The sensor chosen for this detector was the Hamamatsu S2304512Q plasma-coupled device (PCD) linear image sensor. This device consists of 512 contiguous silicon photodiode elements with a 25pm center-to-center spacing and a height of 2.5 mm. It is fitted with a quartz window to enhance the ultraviolet light sensitivity. This sensor works in conjunction with the Hamamatsu Model C2324-228 diode-array driver/amplifier and the Hamamatsu Model C2335 clock generator boards. The first of these boards contains the diode-array driver and circuits to amplify and condition the array output. The latter board provides clock and timing pulses to control the diode scanning and TTL output signals to synchronize external devices. The maximum scan rate of the diode array using these devices is 128 kHz. These compo-
nents were selected because of their low cost and easeof integration. The only other items required to get these Hamamatsu components to work together as a scanning detector are +5- and f15-V power supplies. A Melcor Model CPl.O-127-05L Peltier cooler was chosen to cool the diode-array for reduced dark noise, although in most rapid-scan experiments there is no need for this capability. The data acquisition system consists of a DRA Laboratories Model A2D-160 12-bit A/D converter PC board that has a maximum sampling rate of 160 kHz, some simple homemade control, timing, and interface electronics, and an IBM XT microcomputer. The operation of the diode array is controlled by a menu-driven program of our own creation. The program allows for acquisition, display, and manipulation of up to eight spectra at a time (limited by the software), as well as provisions for permanent disk storage and hard copy (plotter or printer) output. A listing of these components and their cost as of June, 1989, is given in Table 1. Detector head. The diode array is mounted in a homemade housing, shown partially disassembled in
FIG. mounted
2.
Photograph showing the on the spectrograph and next
diode-array detector to the controller box.
head
136
CARTER Outer
b]
FIG. 3. parts
Mounting
Components are aluminum;
ring
cylinder
cl
End
cap
of the home-built diode array all dimensions are given in inches.
housing.
All
Fig. 1. The fully assembled detector head is shown in Fig. 2 mounted on a spectrograph and next to the control box (see below). Figures 3 and 4 are schematic diagrams of the detector head components showing some critical dimensions and assembly details. The detector head contains the Melcor Model CP l.O-127-05L Peltier (thermoelectric) cooler that is capable of cooling the array to lower than -20°C if dark noise reduction is desired. The cold stage of the cooler is in thermal contact with a copper cold finger which extends through a slot (provided for this purpose) in the small circuit board on which the diode-array is mounted, and cools the back of the array. The hot side of the cooler is in thermal contact with another copper block which contains channels to accommodate the flow of a heatexchange fluid (e.g., chilled water). If only modest temperature reduction is needed (say to 5”C), the heatexchange fluid is not needed. Due to the high power required by the cooler (up to -30 W at 15 V), an external power supply (Hewlett-Packard Model 6281A or equivalent) is used when operating at low temperatures. (It should be noted that when high source intensities are available, cooling is not necessary.) The driver/amplifier is also mounted as close to the diode array as possible in the detector head to minimize external electrical interferences and losses from corrupting the high-frequency signals which are passed between the array and the driver/amplifier. The detector housing consists of Detector housing. four parts machined from aluminum: an outer cylinder, an inner sleeve, an end cap, and a mounting ring. The outer cylinder (Fig. 3a) has an outside diameter of approximately 3.25 in. with a wall thickness of i in. and is
ET
AL.
open at both ends. The mounting ring (Fig. 3b) attaches to the outside of the outer cylinder with set screws, and has a flange to allow attachment to a spectrograph. This arrangement allows the outer cylinder to be rotated within the mounting ring to orient the diode axis of the array to be horizontal. The other end of the outer cylinder is closed by the end cap (Fig. 3~). This is shaped like a jar cap and fits over the end of the outer cylinder to make an effective light-tight seal, and is held in place by thumb screws. The end cap has a standard DB-25 bulkhead connector mounted in it to accommodate a cable for transferring electrical signals between the detector assembly and the control box. The inner sleeve (Fig. 4) is machined to fit inside the outer cylinder and serves as a mount for the array, the cooler assembly, and the driver/amplifier board. A longitudinal slot has been machined through the outer cylinder and parallel to its axis to accommodate a bolt that is screwed into the inner sleeve. Sliding the bolt along the slot adjusts the position of the inner sleeve in the outer cylinder. This allows the array to be positioned at the focal plane of the spectrograph. When the array is correctly positioned, this bolt is used to lock the inner sleeve in place. Driver/amplifier. The driver/amplifier actually consists of two boards: a small board on which the array is mounted and a larger board that contains the driver/ amplifier electronics. This two-board layout allows for greater flexibility in the design of the detector head geometry. The driver/amplifier has a twofold purpose, as its name implies. Its first duty is to provide an interface between the diode-array and the clock generator, relaying the START pulses from the clock generator to the array and producing a three-phase clock output for the Inner sleeve assembly Peltier
cooler driver/amplifier
7
_--.-
FIG. 4.
Three views housing. The electrical clarity. This assembly
coolant
tubes
of the inner sleeve assembly of the diode-array cables and connectors have been omitted for is designed to fit inside the outer cylinder.
RAPID-SCAN
ARRAY
array from the single-phase CLOCK output of the clock generator. Its second duty is to amplify and buffer the data output from the diode-array. The array and driver/amplifier interact as follows. The array needs a START pulse and pulses from the three-phase (&-&) clock to read the diodes. After the array receives a START pulse, it stops integrating the light, sets its EOS (end-of-scan) signal low (to 0 V), and then waits for pulses on its clock inputs. A & pulse causes the intensity of the first diode to be read out, the following & pulse causes the second diode intensity to be read out, and so on, continuing sequentially along the array until the last element is reached. The array signals the end of its scan by setting its EOS output high (to 5 V), and begins integrating the light intensity until the next START pulse arrives. During the scan, the video output from the array (a train of voltage pulses, one for each diode in sequence, whose amplitude is proportional to the accumulated charge on the corresponding diode) is sent to the driver/amplifier, which captures each pulse briefly in a sample-and-hold buffer to eliminate switching transients associated with the diode scanning electronics. The output from the sample-and-hold buffer is inverted and amplified by the driver/amplifier. The EOS signal of the array is also inverted by the driver/amplifier for use as a not-ready signal to synchronize external devices to the start of an array scan. In addition, the driver/amplifier produces a TRIGGER output which is used to synchronize external events (e.g., A/D conversions) to the video output rate from the sample-and-hold buffer. The TRIGGER output provides a pulse whenever the sample-and-hold buffer of the driver/amplifier has a valid reading ready for the A/D converter. A/D conuerter. The DRA Laboratories Model A2D160 A/D converter is a high-speed, dual-channel, 12-bit (4096 levels) sampling converter on a circuit board that can be used with an IBM PC, XT, or AT compatible computer. The manufacturer provides a library of software subroutines which performs all of the necessary hardware manipulations and greatly simplifies its utilization. In our application, the board is configured to operate in a mode where the two A/D channels sample the same input at a faster rate than when it is operating in its ordinary single-channel sampling mode. Operating in this mode and utilizing direct memory access for transferring the data to the memory of the computer yields an overall A/D conversion and storage rate of up to 166 kHz. This is a good match to the maximum scan rate of the diode array which is 128 kHz (the readout of 512 diode values in 4 ms). The A/D board also has a single g-bit D/A converter which is utilized as described below. Control and interface circuits. A small control box is used to house the +5- and +15-V power supplies, the
137
DETECTOR
(k, 7400 Ext. Trigger Free run START
(from
To A/D
ClK bd.1
-
EOS (from >
INITIATE
To A/D Somp4e Clock
a
wroyl b 7
7473
5
+,,d r^ 411
IO
To A/D
GND
FIG. 5. Circuit diagram for synchronizing the D/A board to the diode array. The four D-shaped devices are NAND gates contained in a single 7400 quad-NAND chip. The numbers around the 74121 and the 7473 are the pin numbers for the two devices.
clock generator board, some timing and control electronics (vide infra), control switches, and the input/output connectors to the computer, detector head, and oscilloscope. The clock generator board produces the master CLOCK and START pulses which control the diode scanning rate and the start of a scan, respectively. The board comes with switches mounted on it which control the signal integration time by adjusting the clock frequency and the interval between START pulses. For convenience, we replaced the original START timing control interval switches on the clock generator board with ones mounted on the front panel of the control box (Fig. 2). This gives the user easy access to adjustment of the signal integration time, which can have values from
