Abstract:
Isolation circuits are shown which can be implemented using low efficiency LEDs such as those that can be made directly on silicon. Using silicon based LEDs results in reduced cost and facilitates the incorporation of many optically isolated channels using silicon chips separated with a transparent insulator. These circuits include a linear isolation device for analog signals, an isolated A to D converter, an isolated D to A converter, isolated buffer/driver circuits, isolated multiplexers, and isolated switches. Also, a microprocessor is shown with optically isolated I/O ports, A to D and D to A converters, a multiplexer, and solid state switches.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of United States Provisional Application No. 60/101,442, filed Sep. 21, 1998. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention. 
     This invention relates to applications of optically coupled electronic integrated circuits, and more particularly to applications in which two physically and electrically isolated silicon integrated circuits may each contain either a single LED or a plurality of LEDs and also either a single corresponding light detector or a plurality of corresponding light detectors. The LEDs are integrated onto the silicon substrate and may be fabricated by any number of means such as porous silicon, avalanching silicon PN junction, forward biased silicon PN junction, deposited silicon carbide junction, light emitting polymer, or deposited GaAs. Furthermore, these applications can be realized using low efficiency, silicon based LEDs 
     2. Prior Art. 
     Traditional opto-couplers are made using an GaAs LED and a silicon detector. In the simplest opto-couplers the detector is a single device such as a diode, a bipolar transistor, an SCR, or a Triac. Detector chips may also include circuits such as amplifiers and various types of output buffer/drivers. Moreover, an additional silicon chip can be added as a input buffer/driver for the LED. The input signal may be, for example, a TTL type which can not directly drive the LED. Since the LED diver chip must be isolated from the detector chip, three separate chips are thus required in this case: the silicon LED driver, the LED, and the detector chips. 
     Linear opto couplers have also been made which can transmit a voltage or a current level to an output from an isolated input. 
     The simple LED-detector opto-couplers require a reasonably efficient LED since the light must activate the detector which is also a switch without powered amplification. For example, the detector/switch may be a floating base bipolar transistor or a floating gate SCR. Light from the LED must provide enough photo generated base current to turn the bipolar transistor “On”. In another application involving a simple switch a MOSFET is turned “On” by applying to the gate a photo voltage generated by a series of diodes illuminated by the LED. Unfortunately, these applications are not well suited for low efficiency, “on” silicon chip LEDs. 
     Some opto-coupler applications can make use of LEDs with much less efficiency if powered amplification is available for the detector output. Thus, low efficiency on silicon chip LEDs can find useful applications if the lower speed and increased amplification can be tolerated. There can be inherent cost savings in using on silicon chip LEDs if opto-coupler applications require circuits both on the LED side as well as on the detector side since only 2 and not 3 chips are needed. The biggest cost savings are applications which require multiple optical channels between two silicon chips. In these applications putting several discrete GaAs LEDs as well as at least two silicon chips in a package is not as cost effective as putting just two silicon chips in a package with on silicon LEDs. 
     Some potential applications for the on silicon LED include isolated linear amplifiers, isolated line drivers such as an RS232 driver, microprocessors with isolated I/Os, and isolated switches and switch arrays. Because of the low quantum efficiency of on chip silicon LEDs, to date none of these applications have been realized by industry in spite of the physical ability to do so. 
     SUMMARY OF THE INSTANT INVENTION 
     It is the objective of this invention to show how low efficiency, on chip silicon LEDs can be used to realize various types of optically isolated circuits. Specifically, these circuits include a linear isolation device for analog signals, an isolated A to D converter, an isolated D to A converter, isolated buffer/driver circuits, isolated multiplexers, and isolated switches. Also, a microprocessor is proposed with optically isolated I/O ports, A to D and D to A converters, a multiplexer, and solid state switches. These functions can be achieved using two silicon chips which optically communicate data back and forth. Isolation is achieved by placing a transparent, insulating barrier between the two chips through which light is transmitted. 
     PRIOR ART STATEMENT U.S. Pat. No. 5,049,527. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a schematic diagram of a optically isolated D to A converter. 
     FIG. 1B shows a diagram of an optically isolated A to D converter. 
     FIG. 2 shows a diagram of an optically isolated linear amplifier using D to A and A to D converters. 
     FIG. 3 shows a diagram of an optically isolated bidirectional logic buffer/line driver. 
     FIG. 4 shows a multiplexer whose control inputs are optically isolated. 
     FIG. 5 shows a microcontroller with an optically isolated I/O, an optically isolated D to A converter, an optically isolated A to D converter, and an optically isolated multiplexer. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1A shows an example of an optically isolated D to A converter which an be realized using low efficiency on chip silicon LEDs. It should be noted that the circuitry to the left of the transparent insulating barrier  10  exists on a first silicon chip  24  or integrated circuit and the circuitry to right of the barrier  10  exists on a second silicon chip  25 . The first silicon chip  24  contains two integrated, on chip LEDs  6  and  7  while the second chip  25  contains two integrated light detectors  11  and  12 . 
     A digital data word  8  is input to the first chip  24  which takes the data word  8  and puts it into a serial format via data formatter  1  using the clock  5  to time the operation. The enable input  4  is used to signal when a new data word is available for transmission. The serial data output from I is fed into a buffer  2  which is used to drive the on chip silicon LED  6 . The clock signal  5  is also input to a buffer  9  which is used to drive a second on chip silicon LED  7 . 
     A transparent insulating barrier  10  is used to electrically isolate chips  24  and  25  while allowing light to be transmitted. Light detector  11  receives the light emitted by LED  6  and outputs a signal corresponding to the serial data to amplifier  14 . Amplifier  14  converts the weak signal from detector  11  into an appropriate logic signal which is input to the data formatter  15 . Data formatter  15  converts the serial data into a word  26  which corresponds to the input word  8 . The data word  26  is then converted into an analog signal  20  via the D to A converter  16 . 
     The timing clock  5  is transmitted from chip  24  to chip  25  via LED  7  and is received by the light detector  12 . The weak clock signal from detector  12  is then amplified to a logic level signal  17  and is then fed to the data formatter and the D to A converter for timing control. A scaling operational amplifier  18  is provided for conditioning the D to A output  20 . The amplifier  21  has two differential inputs, In+ 23  and In— 22 , and an output  21 . The terminals of the operational amplifier are accessed externally so that appropriate feedback elements can be added. 
     Note the positive power terminals,  3  and  19 , of chips  24  and  25  respectively are isolated from each other. Not shown are the ground terminals of chips  24  and  25  which are also isolated from each other. 
     Thus, the circuit of FIG. 1A can accept a digital word and transmit a corresponding analog level to an output while being electrical isolated from each other. Note that the weak binary encoded light signal can be more easily detected than an analog signal since only two levels are required instead of a continuum of levels. 
     It should be noted that there can be variations of the circuit of FIG.  1 . For example, only one LED-detector pair,  6  and  11 , can be used if the serial data is transmitted asynchronously. Also, the data can be transmitted as a word of parallel bits if more LED-detector pairs are used. For example, if a word size is 8 bits then 8 LED-detector pairs can be used to transmit the word across the insulating barrier  10  and fed directly into the A to D converter  16 . This parallel configuration results in greater bandwidth at the expense of more power and LED-detector pairs. 
     FIG. 1B shows an example of an optically isolated analog to digital converter which is comprised of two silicon chips,  125  and  126 . Chip  125  accepts an analog signal and then converts the signal to a digital number and subsequently transmits the number optically to the second chip  126  which outputs the digital number. 
     Chip  125  has an operational amplifier  101  which is used for scaling the analog signal input. It has differential inputs, In+ 105  and In− 104 , and an output  106 . The output  106  of the operational amplifier  101  feeds into the A to D converter  108  which outputs a logic word to the Data Formatter  109 . The Data Formatter  109  then converts the parallel word into serial data. The output of the Data Formatter  109  connects to buffer  111  which drives LED  112 . 
     A clock signal  102  is also input to chip  125  and controls the timing for the A to D converter  108  and the Data Formatter  109 . An input buffer  103  is used to take the external clock signal  102  and distribute the clock signal  110  to the converter  108  and the Data Formatter  109 . Buffer  124  is used to drive LED  113  which transmits the clock signal to the second chip  126 . 
     A transparent insulating barrier  114  is used to electrically isolate the chips  125  and  126  from one another. A light detector  115  on chip  126  is used to receive the data signal from chip  125 . Amplifier  122  is used to amplify the weak signal from detector  115  and convert it into a logic level signal. The Data Formatter  119  then converts the serial data into a parallel word  120  which is output from chip  126 . An output  123  from the Data Formatter  119  is used to signal when the output data  120  is stable. 
     Light detector  116  is used to receive the clock signal from chip  125 . Amplifier  117  is used to ampify the weak signal from detector  116  and present a logic level signal to the internal clock bus  118  of chip  126 . 
     Note the positive power terminals,  107  and  121 , of chips  125  and  126  respectively are isolated from each other. Not shown are the ground terminals of chips  125  and  126  which are also isolated from each other. 
     It should be noted that there can be variations of the circuit of FIG.  1 B. For example, only one LED-detector pair,  112  and  115 , can be used if the serial data is transmitted asynchronously as can be appreciated by one normally skilled in the art. Also, the data can be transmitted as a word of parallel bits if more LED-detector pairs are used. Again, transmitting bits in parallel results in greater bandwidth at the expense of more power and LED-detector pairs. 
     FIG. 2 show an example of an optically isolated circuit in which both the input and the output are analog signals. A to D and D to A circuits are used so that the light signal is transmitted digitally which will improve dynamic range and linearity. 
     Two chips,  230  and  231 , are used. The input chip  230  has an operational amplifier  201  which is used for conditioning the input analog signal It has differential inputs, In+ 205  and In− 204 , and an output  206 . The output  206  of the operational amplifier  201  feeds into the A to D converter  208  which in turn outputs a logic parallel word to the Data Formatter  209 . The Data Formatter  209  then converts the parallel word into serial data The output of the Data Formatter  209  connects to buffer  211  which drives LED  212 . 
     A clock signal  202  is also input to chip  225  and controls the timing for the A to D converter  208  and the Data Formatter  209 . An input buffer  203  is used to distribute the clock signal  210  to the converter  208  and the Data Formatter  209 . Buffer  224  is used to drive LED  213  which transmits the clock signal to the second chip  226 . 
     A transparent insulating barrier  214  is used to electrically isolate the chips  230  and  231  from one another. A light detector  215  on chip  226  is used to receive the data signal from chip  230 . Amplifier  222  is used to amplify the weak signal from detector  215  and convert it into a logic level signal. The Data Formatter  219  then converts the serial data into a word  220  which is output to a D to A converter  223 . The analog output  225  of the D to A converter is then sent out from chip  231 . 
     A scaling operational amplifier  226  is also provided with differential inputs  229  and  228  and an output  227 . 
     Light detector  216  is used to receive the clock signal from chip  230 . Amplifier  217  is used to amplify the weak signal from detector  216  and present a logic level signal to the internal clock bus  218  of chip  231 . 
     As with the previous examples, the clock portion of the opto coupler can be eliminated if asynchronous transmission techniques are used. Also, as before, data can be optically transmitted as a group of parallel bits at the expense of power and additional LED-Detector pairs. Also, an internal oscillator of appropriate frequency can be placed onto chip  230  for generation of the timing clock  210 . 
     FIG. 3 shows an example of a optically coupled bidirectional digital data transceiver. This arrangement allows digital data to be sent and received while electrically isolating the signal paths. Applications include standard data buses such as the RS232. 
     The bus driver of FIG. 3 includes two chips,  317  and  318 , and a transparent insulating barrier,  308 . For chip  317  data is input to buffer  300  from lead  302 . Buffer  302  then drives LED  307  which transmits a light signal across the transparent isolation barrier  308 . This light is received by light detector  309  of chip  318  which produces a weak signal. The signal is amplified by  311  which outputs a logic level signal on lead  316 . 
     In the other direction data is input on lead  313  which is the input for buffer  312 . Buffer  312  drives LED  310  which sends a light signal across the barrier  308 . Light detector  306  of chip  317  receives the signal and sends it to amplifier  304  which outputs a logic level signal on lead  303 . 
     Positive power for chip  317  is input on lead  301  and negative power on lead  305 . For chip  318  positive power is input on lead  315  and negative power on  314 . Note that the power supply voltage levels for each chip can be at different magnitudes depending on the bus logic level requirements of each interface. 
     For one normally skilled in the art, the circuit of FIG. 3 can be also configured as an tri-state, optically isolated I/O interface driver. 
     FIG. 4 is an example of a two chip optically isolated multiplexer. In this case a digital word is used to control a multi-channel multiplexer. 
     The digital word  400  is input to the Data Formatter  405  of chip  427 . The Data Formatter  405  converts the data word  400  into a serial format for transmission. The enable control  402  of the Data Formatter is used to signal that the data is stable and is ready for transmission. The serial output of Data Formatter  405  is fed into a buffer  406  which turn drives LED  407 . The light signal from LED  407  crosses the transparent insulating barrier  409  and is received by light detector  410  of chip  428 . The weak signal from  410  is amplified by  411  which outputs a logic level signal to the data input  416  of register  425 . This data is used to turn on one switch of an N input multiplexer. The multiplexer is capable of steering one of the N input signals to an output  426 . The first multiplexer input is terminal  421  and the Nth multiplexer input is terminal  423 . The multiplexing device is a switch such as  422  with a control lead such as  424  which comes from the register  425 . Register  425  stores the data associated with which switch position of the N switches is enabled or turned “on”. 
     To control timing a clock signal  403  is used. The clock signal  403  sequences the data flow of the Data Formatter  405 . Clock signal  403  is also fed to buffer  404  which in turn drives LED  408 . LED  408  sends the clock signal across the barrier  409  to light detector  412 . The weak signal from detector  412  is then amplified by amplifier  413  which outputs a logic level signal  414 . The clock signal  414  is then used to sequence the register  425 . 
     A signal conditioning amplifier  418  is also present which can be hooked up to the output  426  of the multiplexer if analog signals are being input to the multiplexer. The amplifier  418  consists of differential inputs,  419  and  420 , and an output  417 . Appropriate external feedback components can be added to the amplifier for the required signal conditioning. 
     Power for chip  427  is provided by lead  401  and for chip  428  by lead  415 . Ground power is not shown but is also separate for each chip. 
     Other applications using the basic configuration shown in FIG. 4 are also possible as can be appreciated by one normally skilled in the art. For example, the output  426  can be grounded and the multiplex switches such as  422  can be power switches. Thus, the multiplex inputs can function as “On-Off” power switches to ground for the purpose of actuating electromechanical devices. The output  426  can also be hooked to a power node for the purpose of switching power to devices hooked to any of the inputs such as  421  or  423 . In these applications note that more than one switch such as  422  can be turned “on” at any given time. 
     FIG. 5 shows an application in which a microcontroller chip,  500 , controls optically isolated interface circuits such as logic I/O  513 , a D to A converter  514 , an A to D converter  515 , and a multiplexer  516 . All of the aforementioned optically isolated interface circuits reside on a single silicon chip,  502 . The operation of these interface circuits is similar to that associated with FIGS. 1A,  1 B,  3 , and  4 . Normal, non isolated I/O associated with the microcontroller such as memory addresses, data I/O ports, control lines, etc. are represented by  518 . 
     The microcontroller chip  500  includes integrated optical communication devices such LEDs and light detectors. As mentioned before, the integrated LEDs can be porous silicon devices, avalanching silicon PN junction diodes, forward biased silicon PN junction diodes, deposited silicon carbide junction diodes, light emitting polymer, or deposited GaAs diode. The light detectors can be PN diodes, Schottky diodes, or photoconductors. Thus, the system depicted in FIG. 5 consists of two silicon chips,  500  and  502 , and a transparent insulating barrier  501 . 
     In FIG. 5, the microcontroller circuit  517  within chip  500  drives a buffer  504  which drives an on chip LED  503  which sends a light data signal across the transparent barrier  501  to the logic I/O module  513 . The I/O module  513  receives the light signal and converts it into a stream of digital data. Conversely, the I/O module  513  can send data back to the microcontroller  517  via detector  505  and amplifier  506 . 
     To transmit an optically isolated analog signal from the microcontroller  517  a digital word corresponding to the desired analog level is sent to module  514  of chip  502  via the buffer  508  and the LED  507 . Also provided in module  514  is a scaling operational amplifier/buffer  522  which can scale the voltage output from the D to A converter of module  514 . 
     Correspondingly, the microcontroller can receive an optically isolated analog signal via the A to D module  515 . The A to D module  515  converts the analog signal level into a digital word. Detector  509  and amplifier  510  are used to receive the light signal from module  515  and input the A to D data into the microcontroller  517 . The operational amplifier  523  is used to scale the analog signal for the A to D converter of module  515 . 
     Module  516  is an analog multiplexer and can used to select one of several analog voltage sources for input to the A to D module  515 . Buffer  512  and LED  511  are used to send multiplexer addressing data from the microcontroller  517  to the data register  524  which stores the multiplexer address data. 
     Not shown in the diagram is the clock signal which is optically fed from the microcontroller to the various circuits of chip  502 . The clock is used to time or sequence data communication between the various modules of chip  502  and the microcontroller. 
     Power for the microcontroller chip  500  is provided via terminal  518  and  519  and power for chip  502  is provided via terminals  520  and  521 . Note that additional power supply terminals may be required for chip  502  depending on what power levels are required to operated the various interface modules. 
     To reduce the number of LED-detector pairs, it is possible to use serial registers to receive and transmit data between the modules of chip  502  and the microcontroller. The serial registers will require both an address word for the target module and a data word for the target module. Reducing the LED-detector pairs in this manor reduces the bandwidth of data transmission between the microcontroller and the modules of chip  502 . 
     FIG. 5 is only an example of the various type of interface modules that can be optically interfaced to a microcontroller  517 . For example, addressable power switches can be added as appreciated by one normally skilled in the art. 
     It is noted that the two chip optically coupled devices illustrated here can be reduced to a single integrated chip using Silicon-On-Insulator (SOI) technology given that the devices on SOI are dialectically insulated. Light coupling from an “on chip” LED to a detector can be achieved by building wave guides on the surface of the SOI chip using a deposited and patterned transparent dielectric as the wave guide. A deposited coating can be applied over the dielectric wave guide with the optical diffraction constant of the coating being different from that of the wave guide so that the light is confined to the wave guide.