# Patent

## Title
RF isolator with differential input/output

## Abstract

A system for providing voltage isolation includes the first and second chips, each containing functional circuitry. The chips are interconnected via at least one RF isolation link that provides voltage isolation between the first chip and the second chip and limits common mode signals therebetween.

## Background
RELATED APPLICATIONS
This application is a continuation-in-part of co-pending U.S. application Ser. No. 10/860,399 filed on Jun. 3, 2004 entitled Transformer Isolator for Digital Power Supply and co-pending U.S. application Ser. No. 10/860,519 filed on Jun. 3, 2004 entitled On-Chip Transformer Isolator.


TECHNICAL FIELD OF THE INVENTION
The present invention relates to digital isolators, and more particularly, to digital isolators having differential inputs/outputs.
BACKGROUND OF THE INVENTION
Within power conversion products, there is a need for high speed digital links that provide high isolation at a low cost. Typical digital links within power conversion products require a speed of 50-100 megabits per second. Isolation between the input and output of power conversion products is required in the range of 2,500-5,000 volts. Existing solutions for providing a high speed digital isolation link have focused on the use of magnetic pulse couplers, magnetic resistive couplers, capacitive couplers and optical couplers.
Referring now to FIG. 1, there is illustrated the general block diagram of a system using a magnetic pulse coupler to isolate a digital link 102 between a driver 104 and a detector 106. The driver 104 resides upon one side of the digital link 102 and transmits information over the digital link 102 to the detector 106 residing on the other side of the digital link. Resting between the driver 104 and detector 106 is a pulse transformer 108. The pulse transformer 108 provides a electromagnetically coupled transformer between the driver 104 and detector 106. The pulse transformer 108 generates a pulse output in response to a provided input from the driver as illustrated in FIG. 2. The input from the driver 104 consists of the two pulses 202 and 204. Each pulse 202, 204 consists of a rising edge 206 and a falling edge 208. In response to a rising edge 206, the output of the pulse transformer 108 generates a positive pulse 210. The falling edge 208 of a pulse generates a negative pulse 212. The pulse transformer circuit illustrated with respect to FIGS. 1 and 2 suffers from a number of deficiencies. These include start-up where the detector 106 will not know at what point the input from the driver has begun, whether high or low until a first edge is detected. Additionally, should any error occur in the pulse output of the pulse transformer 108, the detector 106 would have a difficult time determining when to return to a proper state since there may be a long period of time between pulses.
Referring now to FIG. 2, there is illustrated an alternative prior art solution making use of a magneto resistive coupler. The magneto resistive coupler 302 consists of a resistor 304 and associated transformer 306. The resistor 304 has a resistance value that changes responsive to the magnetic flux about the resistor. The transformer detector 306 utilizes a wheatstone bridge to detect the magnetic flux of the resistor and determined transmitted data.
Another method of isolation between a driver 404 and a detector 406 is illustrated in FIG. 4. The driver 404 and the detector 406 are isolated on opposite sides of a digital link 402 by a capacitor 408. The capacitor 408 capacitive couples the driver 404 and detector 406 together to achieve a level of isolation. A problem with the use of capacitive coupling to isolate digital links is that capacitive coupling provides no common mode rejection.
An additional problem with some isolator designs involves the reception of RF interference from nearby transmitting GSM, DCS and CDMA cellular telephones. The problem is caused by the application printed circuit board acting as a dipole antennae at GHz frequencies. This results in large common mode signals being seen at the isolator at RF frequencies. Some manner for minimizing these large common mode signals at GHz frequencies would be highly desirable.
Thus, an improved method for providing isolation over high speed digital links within power supply components would be greatly desirable.
SUMMARY OF THE INVENTION
The present invention disclosed and claimed herein, in one aspect thereof, comprises a system for providing voltage isolation and for limiting the common mode signals passing through an RF isolation link. First and second chips or dies contain functional circuitry. Between the first and second chips or dies, an RF isolation link interconnects the chips or dies. The RF isolation link provides voltage isolation between the chips or dies and limits common mode signals passing through the link.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying Drawings in which:
FIG. 1 illustrates a block diagram of a prior art magnetic pulse coupler isolator;
FIG. 2 illustrates the input and output signals of the prior art magnetic pulse transformer of FIG. 1;
FIG. 3 illustrates a prior art magneto resistive coupler;
FIG. 4 illustrates a prior art capacitive coupler;
FIG. 5 illustrates a switched power supply including isolation circuitry;
FIG. 6 illustrates an RF isolation link of the present disclosure;
FIG. 6a illustrates a schematic block diagram of a circuit for providing the RF isolation link using frequency modulation;
FIG. 7 illustrates a schematic diagram of the circuitry for providing the RF isolation link using amplitude modulation;
FIG. 8 illustrates the waveforms present on the transmit side of the RF isolation link of FIG. 7;
FIG. 9 illustrates the waveforms present on the receiving side of the RF isolation link of FIG. 7;
FIG. 10 illustrates the frequency response of the RF isolation link;
FIG. 11 illustrates a model of one of the transformers included within the RF isolation link;
FIG. 12 illustrates the frequency response of one transformer of the RF isolation link;
FIG. 13 illustrates the voltages across each transformer included within an RF isolation link and across the entire RF isolation link;
FIG. 14a is a block diagram illustrating the circuitry included within a chip on one side of an RF isolation link for providing multiple isolation link channels;
FIG. 14b is a schematic diagram of an oscillator circuit;
FIG. 14c is a block diagram of the logic circuit of FIG. 17a; 
FIG. 15 illustrates a pair of chips within a single package including four separate channels for providing four isolated digital data links;
FIG. 15a illustrates the RF isolation link within a chip package;
FIG. 15b illustrates a side view of a bond wire;
FIG. 16 illustrates an integrated RF isolation link in a single package including two dies;
FIG. 16a illustrates an integrated RF isolation link in a single package having a digital input and a digital output;
FIG. 16b illustrates an integrated RF isolation link in a single package including a digital input/output and an analog input/output;
FIG. 16c illustrates an integrated RF isolation link in a single package including an analog input/output and an analog input/output;
FIG. 17a illustrates an RF isolation link integrated with a microcontroller;
FIG. 17b illustrates an RF isolation link integrated with a microcontroller interconnected to a second chip providing both analog input and analog output;
FIG. 18a illustrates one coil of a transformer of the RF isolation link;
FIG. 18b illustrates a second coil of a transformer of the RF isolation link;
FIG. 19 illustrates an overlapping view of the transformers described in FIGS. 21a and 21b; 
FIG. 20 is a side view of the coils forming a transformer of the RF isolation link;
FIG. 21 illustrates an offset between metal layers to increase breakdown voltages within a transformer;
FIG. 22 illustrates a perspective cutaway view of the coil construction;
FIG. 23 illustrates the separate areas in which the transformer coils and the circuitry would be implemented on a chip utilizing the RF isolation circuit; and
FIG. 24 illustrates the structure of the RF isolation link integrated on a single chip.
FIG. 25 illustrates an isolator chip having two printed circuit boards which may act as a dipole antenna at higher frequencies;
FIG. 26 illustrates the parasitic capacitance between windings at higher frequencies;
FIG. 27 illustrates how RF signals can be passed through the transformer as common mode signals;
FIG. 28 illustrates a schematic diagram of an RF isolator including a differential output;
FIG. 29 is a more detailed schematic view of the RF isolator of FIG. 28;
FIG. 30 is an illustration of the transformer coils of the RF isolator including a center tap;
FIG. 31 illustrates the manner in which voltage may be altered to maintain optimized receiver/transmitter gain;
FIG. 32 is a schematic diagram illustrating a prior art method for generating the band gap reference voltage;
FIG. 33 is a schematic diagram of the manner for generating the band gap reference voltage according to the present disclosure;
FIG. 34 illustrates a prior art method for generating a reference voltage; and
FIG. 35 illustrates a modified method for generating a band gap reference voltage.



DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, and more particularly to FIG. 5, there is illustrated a block diagram of a DC-DC switching power supply utilizing an RF isolation link. Switching power supplies utilize a plurality of switches which are turned on and off to switch an input DC voltage across a transformer to a load, the output voltage at a different DC voltage level. By switching the current inductively coupled through the transformer to the load in a particular manner, a DC output voltage at a different voltage level than the input DC voltage can be provided to the load. The controlled switching is typically facilitated with some type of control circuit. This control circuit can be an analog control circuit formed from a plurality of analog discrete devices, or it can be a digital circuit. In digital control circuits, digital signal processors (DSPs) and microcontroller units (MCU) have been utilized. The DSPs control the duty cycle and relative timing of the switches such that the edges of each control pulse to the various transistor switches controlling power delivery to the load is varied. In order to perform this operation in the digital domain, the DSP must perform a large number of calculations, which requires a fairly significant amount of code to be generated to support a specific power supply topology, operating frequency, component characteristics and performance requirements. For example, inductor size decreases with increasing POM frequency, dead times increase with increasing transistor turn-off times, and so on. Although DSPs can handle the regulation tasks, they are fairly complex and expensive and code changes in power supply applications are difficult.
Referring further to FIG. 5, the power supply includes a primary switch group 502 that is operable to receive an input voltage on a node 504, this being a DC voltage, and ground on a node 506. The primary switch group 502 is coupled through an isolation transformer 508 to a secondary switch group 510. The secondary switch group 510 is operable to drive an input voltage node 512 that is connected to one terminal of a load 514, the secondary switch group 510 also having a ground connection on a node 516, the load 514 disposed between the node 512 and the node 516. The two switch groups 502 and 510 are operable to operate in conjunction with various pulse inputs on a control bus 518 associated with the primary switch group 502 and with various pulse inputs on a control bus 526 associated with the secondary switch group 510.
A digital control circuit 524 is provided for controlling the operation of the primary switch group 502 and the secondary switch group 510. The voltages on nodes 504 and 506 are provided as inputs to the digital control circuit 524 for sensing the voltage and current on the primary side, the digital control circuit 524 generating the information on the bus 518 for control of the primary switch group 502. The control circuit 524 must be isolated from the secondary group switch 510, since there can be a significant DC voltage difference therebetween. This is facilitated by driving the bus 526 through an isolation circuit 528, such as the RF isolation circuit which will be discussed herein below, to drive the bus 520. Similarly, the control circuit 524 is operable to sense the voltage and current levels on the output node 512 through sense lines 530 which are also connected through an isolation circuit 532 to the digital control circuit 524. The digital control circuit 524 is also interfaced to a bus 536 to receive external control/configuration information. This can be facilitated with a serial databus such as an SMB serial databus.
Referring now to FIG. 6, there is illustrated the RF isolation link of the present disclosure. The RF isolation link 600 of the present disclosure is implemented by integrating a portion of the link in two chips or dies between which a high rate data link with voltage isolation is required. Each chip 602 includes a transformer 604 and transmit and receive circuitry 606 for providing the RF isolation link 600 between the chips. Alternatively, the chip 602 could include only transmit circuitry or receive circuitry with the partnered chip, including a corresponding receiver or transmitter. The RF signals are generated within the transmit/receive circuitry 606 on one side of the RF isolation link, and the RF signals are transmitted between the chips 602 utilizing the transformers 604 in each chip and the magnetic coupling effect therebetween.
Once the RF signals are received at the receiving side, the transmit and receive circuitry 606 detects the data contained within the transmission from the first chip and utilizes the data as appropriate. While the description with respect to FIG. 6 only illustrates the transformer 604 and transmit and receive circuitry 606 within each chip 602, additional circuitry will be implemented on the chips 602 for performing processing functions associated with the data transmitted over the RF isolation link 600. The data transmitted over the RF isolation link 600 may be transmitted using either frequency modulation techniques or amplitude modulation techniques. In the preferred embodiment of the disclosure, discussed with respect to FIG. 7 herein below, AM modulation is used for transmitting the data.
In operation, each of the transmit/receive circuits 606 operates in either transmit or receive mode. In the transmit mode, digital data received on a digital bus 603 is serially transmitted from one of the transmit/receive circuit 606 to the other one on the other of the dies 602. This is facilitated by driving the transformer 606 with a signal such that energy is coupled from the primary to the secondary thereof. This will allow energy to be transmitted on transmission lines 605 that couple the transformers 604 together. Each of the transformers is comprised of a primary 607 and a secondary 609. The primary 607 is driven with the input signal and energy associated therewith is coupled across the high voltage isolation boundary from the primary 607 to the secondary 609 and onto the transmission line 605. As will be described herein below, both of the transmit/receive circuits 606 and transformers 604 are all fabricated on an integrated circuit such that the primary 607 and secondary 609 are both formed thereon utilizing conventional processing techniques and available conductive layers that are shared with the transmit/receive circuits. There will be a loss associated with the coupling coefficient between the primary and secondary such that the amount of energy that can be delivered from the transmit/receive circuit 606 to the transmission line 605 is reduced and, further, there will be more loss at certain frequencies than others. As such, the transformer 604 will have a unique frequency response where the loss will be greater at some frequencies than others. To accommodate this, the transmit/receive circuit 606 has contained therein a transmitter operating at a defined frequency that is within the lowest loss portion of the frequency response of the transformer 604. By utilizing various modulation schemes, data can be transmitted on this carrier to the transmission line 605. The operation of the transmitter/receiver circuit 606 will be described in more detail herein below.
Referring now to FIG. 6a, there is illustrated an alternate embodiment of the switching power supply utilizing frequency modulation to transmit data between a pair of chips over an RF isolation link 600. The description with respect to FIG. 6a is merely provided as an illustration of one potential embodiment of an FM circuit used for creating an RF isolation link, and one skilled in the art would realize the possibility of numerous additional embodiments. The data is input on a data bus 610 into a Manchester encoding circuit 612, a conventional data encoding circuit. Also input to the Manchester encoding circuit 612 is a clock signal. The clock signal is also input to a voltage controlled oscillator 614. Data is output from the Manchester encoding circuit 612 and applied to a divide circuit 616. A second input of the divide circuit 616 is connected to the output of the voltage controlled oscillator 614. The output of the divide circuit 616 is connected to a second input of the voltage controlled oscillator 614 to allow modulation thereof with the Manchester encoding circuit 616. The voltage controlled oscillator 614 outputs a frequency modulated signal representing the received data on bus 610 to a driver 618. The signal is filtered by a capacitor 620 before being applied to a transformer 622. The FM modulated signal is coupled by transformer 622 onto transmission lines 624 passing across an interface 626 between either a first and second chip that are to be voltage isolated from each other.
The received data signal is electromagnetically coupled onto the receiver circuitry by a second transformer 628. The received signal passes through a limiter circuit 630 whose output is applied to a Divide-by-N circuit 632 and a discriminater circuit 634. The output of the Divide-by-N circuit 632 is applied to the input of a PFD (phase/frequency detector) circuit 636. A second input to the PFD circuit 636 is provided by a second Divide-by-N circuit 638 having its input connected to the output of the voltage controlled oscillator 640. The input of the voltage controlled oscillator 640 is connected to the output of the PFD circuit 636. The output of the voltage controlled oscillator 640 is connected to a second input of the discriminater 634, this being a phase locked output phase locked to the data clock. The discriminater circuit 634 determines the data contained within the received signal responsive to the output of the voltage controlled oscillator 640 and the limiter 630. This data is provided to a latch circuit 636 having its clock input connected to the output of the Divide-by-N circuit 638. The data output of the receiver is provided from the latch circuit 642.
Referring now to FIG. 7, there is illustrated the preferred embodiment of the RF isolation link 600 of the present disclosure wherein amplitude modulation is used to transmit data over the link. The RF isolation link 600 consists of transmitter circuitry 702 and receiver circuitry 704. The transmitter circuitry 702 consists of a NAND gate 708 having a first input connected to receive the data to be transmitted over the RF isolation link 600 and a second input connected to receive the RF carrier signal. The RF carrier in the preferred embodiment comprises a 2 GHz signal. The data input to the first input of the NAND gate 708 consists of either a logical “1” or “0” which will selectively gate the RF carrier signal to the output of NAND gate 708 in the presence of a logical “1.” This causes the output 709 of the NAND gate 708 to either provide the RF carrier signal when the data bit is “1” or not provide the RF signal when the data bit is “0.” The output of the NAND gate 709 is connected to the gate of a p-channel transistor 710. The drain-source path of the p-channel resistor 710 is connected between VDD and ground through a resistor 712 and a first transformer 714. The transformer 714 electromagnetically couples the RF carrier signal to transformer 718 via lines 716. This links the data represented by the RF carrier signal between the first chip 602a and the second chip 602b while providing voltage isolation between the chips 602 via the first and second transformers 714, 718. Each of the transformers 714 and 718 are associated with a particular chip 602 on opposite sides of interface 720. Thus, wherein previous systems required a separate chip to provide an isolation link between two separate chips, the present disclosed device integrates the RF isolation link 600 onto the chips 602.
The receiver circuitry 704 receives the signal which has been electromagnetically coupled via transformer 714 onto the transmission lines 716 to transformer 718. The receiver circuit 704 consists of an amplifier 705 and a detector 706. The amplifier 705 provides two stages of amplification consisting of a first amplification stage including a capacitor 722 in series with an amplifier 724 and a feedback resistor 726. The second amplifier stage is similar to the first amplifier stage and includes a capacitor 728 in series with an amplifier 730 and a feedback resistor 732. These two stages amplify the received signal from the transformer 718.
The detector 706 detects the presence or absence of the RF carrier signal within the amplified received signal to determine the data being transmitted from the first chip 602a. The amplified signal from the amplifier 705 is first filtered by a capacitor 734. N-channel transistor 736 has the gate thereof connected to capacitor 734 and has the source-drain path thereof connected to one side of a current mirror comprised of p-channel transistors 738 and 740. The source-drain path of transistor 738 is connected between VDD and node 742, the gate thereof connected to the gate of transistor 740. The source-drain path of transistor 740 is connected between VDD and a node 743, the gate thereof connected to node 743 to provide a diode connected configuration. The output of the detector 706 is provided from node 742 at which the source-drain path of the n-channel transistor 736 is connected to the p-channel transistor 738 of the current mirror. A bias network is provided by n-channel transistors 744 and 746 which have the source-drain paths thereof connected between node 743 and ground and the gates thereof connected to a node 745 through a resistor 748, with a capacitor 750 connected between node 745 and ground. Biasing is also provided by resistor 752 connected between node 745 and the gate of transistor 736, a diode connected p-channel transistor 754 connected between node 745 and ground and a current source 756 for driving node 745. When no RF signal is detected by the receiver, the Data Out from node 742 of the detector circuit 706 will be equal to VDD since the PMOS current is greater than 1.33 times the NMOS current and a logical “0” is detected. In the presence of the RF signal, the Data Out from node 742 will vary in response to the variation of the detected RF carrier signal and a logical “1.” The detector 706 outputs a low voltage when RF is present and a high voltage when RF is absent relying on the nonlinear (square root) behavior of the MOS device directed by the alternating current.
Referring now to FIGS. 8 and 9, there are illustrated the waveforms and data provided at the transmission side (FIG. 8) of an RF isolation link 600 and the receive side (FIG. 9) of the RF isolation link. On the transmission side illustrated in FIG. 8, the data 800 is either transmitted as a one bit (high) or zero bit (low). A one bit pulse is indicated at 802, 804 and 806. A zero bit pulse is indicated at 808 and 810. The transmit data provided to the transformer 714 is illustrated by the waveform 812. The transmit data waveform represents the 2 GHz RF carrier signal. When a logical “1” data bit is being transmitted and the data signal is high, the presence of the 2 GHz RF carrier is provided at the transmit data output. When a logical “0” bit is being transmitted, the signal is virtually zero at the transmit data output. Thus, whether a logical “1” bit or a logical “0” bit is transmitted is indicated either by the presence or absence of the 2 GHz RF carrier signal.
FIG. 9 illustrates the waveforms associated with the receiver 704. The received data for the logic “1” bit is represented at points 902, 904 and 906 and indicates the three 2.5 GHz RF carrier pulses transmitted from the transmitter 702 of the RF isolation link 600. The received pulses are amplified by the amplifier 705 such that when the signal is input to the detector circuit 706, the pulses are represented by the amplified waveform pulses 908, 910 and 912. As discussed previously, the detector data output rises to VDD at points 916, 918 when no RF carrier signal is detected by the detector 706 indicating a logical “0.” When an RF carrier signal is detected, the output of the detector 706 begins to vary and drops low at points 920, 922 and 924 indicating a logical “1,” this being the result of an increase in the NMOS current in transistor 736.
Referring now to FIG. 10, there is illustrated the frequency response of a channel having the RF isolation circuit 600 described in FIG. 7.
Referring now to FIG. 11, there is illustrated a model for the transformers (714, 718) illustrated in FIG. 7. The input of the transformer consists of nodes 1002 and 1100. Node 1002 is connected to ground through capacitor 1104 and resistor 1106. Node 1100 is connected to ground through capacitor 1116 and resistor 1118. Node 1102 interconnects with node 1100 via a parallel connection of capacitor 1108 in series with resistor 1110 and inductor 1112 in series with resistor 1114. The output of the transformer consists of nodes 1122 and 1124. Node 1122 is connected to ground through capacitor 1126 and resistor 1128. Node 1124 is connected to ground through capacitor 1130 and resistor 1132. Node 1122 interconnects with node 1124 via a parallel connection of capacitor 1134 in series with resistor 1136 and inductor 1138 in series with resistor 1140. Nodes 1102 and 1122 are interconnected via a capacitor 1142 with a value of approximately 125 Ff. Nodes 1100 and 1124 are interconnected via a capacitor 1144 with a value of approximately 125 Ff.
With specific reference to FIG. 13, it can seen that the low frequency response of the transformers is relatively lossy whereas the peak of the response occurs around 2.5 GHz. This is due to the manner in which the transformer was fabricated. Each side of the transformer is comprised of an inductive element, each inductive element on either side of the transformer coupled together through a layer of dielectric material, as will be described herein below. The series inductance value will result in an effect on the frequency response that will somewhat narrow the frequency response thereof. The amount of energy that is coupled from the output is a function of the coupling coefficient. The two sides of the transformers are disposed on a substrate, as will be described herein below, such that one element is disposed over the other element and separated therefrom by a high voltage dielectric to increase the effective breakdown voltage. This will allow high frequency energy to be coupled from one conductive element to the other. The voltage breakdown is a function of the properties of the material disposed between the two conductors at DC and the distance by which the two are separated. If the transformer were fabricated on a single layer of material in the semiconductor substrate, then the distances between the edges thereof would define the voltage breakdown. For example, the transformer device could be fabricated with the use of a directional coupler, which would provide a more broadband response. However, the area for such a design could be significant.
It can be seen that, due to the low frequency attenuation of the transformer, it would be difficult to couple through energy from a DC pulse, since only the high frequency energy would be passed there through. As such, the spectral energy that is coupled through the transformer of the present disclosure is concentrated therein with the use of a high frequency carrier that is disposed substantially within the center of the frequency response of the transformer. This will allow a large portion of the energy generated to be coupled across the transformer.
Using the RF isolation links 600 described above, voltage isolation of up to 5,000 volts may be achieved, 2,500 volts for each side. Thus, as illustrated in FIG. 16, the RF isolation circuit 602 may provide 5,000 volts of isolation between a first chip 602a and a second chip 602b. While the voltage between the input terminals of the chip 602a will be zero volts, and the voltage between the input terminals of the chip 602b will also be zero volts, the total voltage difference between the two chips may be 5,000 volts with a 2,500 voltage difference across each of the transformers 714, 718 associated with the interfaces to the RF isolation circuit on each chip 602.
Referring now to FIG. 14a, there is illustrated a block diagram of the structure of an interface of a single chip 602 including a portion of a plurality of channels 1402 including the RF isolation link of the present disclosure. Each channel 1402 consists of the transformer 1406 and transmit and/or receive circuitry described with respect to FIG. 7. Data may be either input or received at the interface 1404 of transformer 1406. Each channel 1402 is interconnected with a pad driver 1408 that either drives transmitted data from the pad driver over channel 1402 to be output over the interface 1404 or drives received data to the associated pad of the chip 602. The manner in which data can be either transmitted or received over a particular channel 1402a is controlled on the chip 602 by logic circuitry 1410 providing control over various control lines 1412. The manner in which the logic control 1410 controls whether a channel is used for transmitting or receiving is set by input bond pad options 1414. Thus, in this embodiment, data is received as either a logic “1” or a logic “0” and the associated transformer is driven, when a pad is configured as a transmitter, (or not driven) accordingly. For received data on the associated transformer, when configured to receive data, the output of the pad is either high or low.
An oscillator circuit 1430 is also associated with all of the channels of the interface. A band gap generator 1420 is provided on-chip and connected to VDD to provide a band gap reference voltage to a regulator circuit 1422. While the description with respect to FIG. 14a only illustrates a single voltage regulator 1422, it will be noted that a separate voltage regulator 1422 will be associated with each of the channels of the interface for noise purposes. The voltage regulator 1422 consists of an amplifier 1424 having one input connected to the output of the band gap generator 1420. The output of the amplifier 1424 is connected to the gate of a transistor 1426. The drain-source path of the transistor 1426 is connected between VDD and a node 1427. Node 1427 is also connected to the second input of the differential amplifier 1424. A capacitor 1428 is connected between node 1422 and ground. Each of the channels 1402a, 1402b, 1402c and 1402d has a regulator 1422 associated therewith. Connected to node 1427 is an oscillator circuit 1430.
FIG. 14b illustrates the oscillator circuit 1430 of FIG. 14a. The output 1435 is connected to node 1437 between transistor 1436 and transistor 1438. The drain-source path of transistor 1436 is connected between VDD and node 1437. The drain-source path of transistor 1438 is connected between node 1437 and ground. The gates of transistor 1436 and 1438 are connected to each other through a node 1439. A transistor 1440 has its gate connected to ground and its drain-source path connected between VDD and the gate of transistor 1440. Node 1439 also interconnects transistor 1442 and transistor 1444. The drain-source path of transistor 1442 is connected between VDD and node 1439. The drain-source path of transistor 1444 is connected between node 1439 and ground. The gates of transistors 1442 and 1444 are interconnected with each other via node 1445. A capacitor 1446 is connected between node 1445 and ground. Node 1445 is connected to a first terminal of coil 1450. The second terminal of coil 1450 interconnects with the circuit via node 1460. Transistors 1452 and 1454 are interconnected via node 1445. The drain-source path of transistor 1452 is connected between VDD and node 1445. The drain-source path of transistor 1454 is connected between node 1445 and ground. The gates of both transistor 1452 and 1454 connect to node 1460. Transistors 1458 and 1456 are interconnected via node 1460. The drain-source path of transistor 1458 is connected between VDD and node 1460. The drain-source path of transistor 1456 is connected between node 1460 and ground. The gates of transistors 1458 and 1456 connect to node 1445. The capacitor 1462 is connected between node 1460 and ground. Also connected to node 1460 are the gates of transistors 1464 and 1466. The drain-source pathway of transistor 1464 is connected between VDD and node 1465, and the drain-source pathway of transistor 1466 is connected between node 1465 and ground. This oscillator therefore comprises a conventional LC oscillator.
Referring now to FIG. 14c, there is illustrated one embodiment of the circuitry which might be incorporated within the logic circuit 1410. In this embodiment, the logic circuit 1410 includes of a decoder 1432. The decoder has a total of three bond pad inputs B0, B1 and B2 for receiving the indication of the version of the chip being implemented. The outputs 1434 of the decoder are input to the appropriate channels such that the channel may be configured in either a transmission or reception mode.
Referring now also to FIG. 15, there is illustrated the manner in which the single chip design described in FIG. 16 can be used to facilitate an entire RF isolation circuit including four separate RF isolated channels. A first chip 1502 is reversed such that the output channels 1402 between the first chip 1502 and the second chip 1504 are merely reversed. Thus, when viewing the chip 1502 from top to bottom of chip one, channel one is at the top, channel two is second, channel three is third and channel four is last. For the second chip 1504, the channels run in the opposite direction with channel one beginning at the bottom and channel four being at the top. The physical design of chip 1502 and chip 1504 are the same. Chip 1504 is merely reversed to facilitate the three versions of the chip as described below. Three different bond option versions may be selected for input to the logic circuit 1410 of the package containing the first chip 1502 and the second chip 1504 utilizing the decoder circuit 1432. Referring now to the Table 1, there are illustrated the three separate versions of operation for both the first chip 1502 and the second chip 1504 and the indication of whether the channel comprises a transmit or receive channel in the associated version.














TABLE 1







Version
Chip
Ch. 1
Ch. 2
Ch. 3
Ch. 4























1
1
Tx
Tx
Tx
Tx



2
1
Tx
Tx
Rx
Rx



3
1
Tx
Rx
Rx
Rx



1
2
Rx
Rx
Rx
Rx



2
2
Tx
Tx
Rx
Rx



3
2
Rx
Tx
Tx
Tx










As can be seen, the associated chips 602 and 604 channels correspond, such that when a channel on one chip is transmitting or receiving, the corresponding channel on the other chip is doing the opposite.
Referring now to FIG. 15a, there is illustrated the RF isolation link 600 within a chip package. As discussed previously in FIG. 15, there are illustrated chips 1602a and 1602b interconnected by four separate channels 604. Each channel 604 is represented by two bond wires interconnecting the transformers (not shown) within each of chips 1602a and 1602b. Each of chips 1602a and 1602b are also connected to various bond pads 1504 within the package by a connection line 1542 that provide connections to the other electronic circuitry.
The embodiment of FIG. 15a is referred to as a “split lead-frame” package. This is facilitated with the use of a lead frame 1550 on one side thereof and a lead frame 1552 on the other side thereof. Lead frame 1550 is interfaced with terminals 1554 and lead frame 1550 is interfaced with terminals 1556. During fabrication, the lead frames 1550 and 1556, which are not electrically connected to each other, provide support for the chips 602a and 602b, respectively. When the chips 602a and 602b are bonded onto their respective portions of the lead frame, they are then bonded to the appropriate terminals 1554 and 1556 and then the bond wires 604 disposed therebetween. The entire package is then encapsulated in a conventional encapsulate. Thus, the bond wires 604 each comprise a high frequency transmission line disposed between the two chips, each transformer associated with two band wires that provide a “two-wire” transmission line.
Referring now to FIG. 15b, there is illustrated a side view of one of the bond wires 604. It can be seen that the substrate associated with the die 602a has disposed thereon a bonding pad 1560 and the die 602b has disposed thereon a bonding pad 1562. The bond wire 604 is bonded to the pad 1516 on one side with a conventional bond and also to the pad 1562 on the die 602b. The length of the bond wire 604 is a fraction of a wavelength at the 2.4 GHz frequency. However, it will be inductive in nature and will have distributed inductance and capacitance associated therewith. As such, the transmission characteristics of the bond wire can affect the transmission of information between the two dies 602a and 602b. As noted herein above, the input impedance to each of the pads 1560 is on the range of 500 ohms. Thus, for ideal transmission of the information, there might be some matching circuitry required in addition to just the bond wires 604 forming the two-wire transmission line, although that has not been set forth herein.
Referring now to FIG. 16, there is illustrated the manner in which the RF isolation link 600 represented as RF isolation circuitry 1602 may be integrated into two separate multi-functional dies, 1604 and 1606, within a single package 1608. The RF isolation circuitry 1602 may provide isolation between components on two separate dies 1604 and 1606. Associated with one or both of the dies could be additional circuitry 1610 such as a microcontroller or other electronic component. This additional circuitry would be isolated from components in the other die via the RF isolation link 1602.
Referring now also to FIG. 16a, when an RF isolation link 600 is integrated onto two separate dies 1604 and 1606 in a single package 1608. The isolation interface 1602, which includes the transceivers 1612 and the transformers 1614, may be used to provide simply a digital IN, digital OUT package 1608. In this embodiment, the digital input 1620 is applied to a first transceiver 1612a. Alternatively, the digital input 1620 could be applied to digital circuitry connected to the transceiver 1612a. The isolation circuit operates in the manner described herein above and a second digital output 1622 is provided from transceiver 1612b or associated digital circuitry.
Referring now to FIG. 16b, rather than providing a solely digital input/digital output circuit, a single package 1608, including first and second dies 1604, 1606 implementing the RF isolation circuit described herein above, may provide a circuit with a digital input/output and an analog input/output. In this case, a digital input/output 1924 would connect with transceiver 1612a or digital circuitry of a first die 1604. The first die 1604 is coupled with the second die 1606 via the described RF isolation link, and the transceiver 1612b is coupled to an analog input/analog output 1626 through a data converter, either an ADC 1614 or a DAC 1616, depending upon the direction.
Referring now to FIG. 16c, a single package 1908, including first and second dies 1604, 1606 implementing the RF isolation circuit described herein above, may provide a circuit with an analog input/output and on one side and an analog input/output on the other side. In this case, an analog input/output 1640 would connect to an A-D converter 1642 and a D-A converter 1644 and then to the transceiver 1612a or digital circuitry of a first die 1604. The first die 1604 is coupled with the second die 1606 via the described RF isolation link, and the transceiver 1612b is coupled to an analog input/output 1646 via an A-D converter 1648 and D-A converter 1650. In this way, analog signals may be transmitted in either direction across the single package 1608.
Referring now to FIG. 17a, there is illustrated a chip 1702 including a portion of the RF isolation link described herein above. The chip 2002 includes a single transformer 1704 and the transmit and receive circuitry 1706 of the RF isolation link 600. The RF isolation link 600 consisting of the transceiver 1706 and the transformer 1704 are integrated with a microcontroller unit 1708 through a digital input/output 1710. A memory 1712 stores operating instructions and data needed by the microcontroller unit 1708. The chip 1702 would be able to interconnect with a second chip that included an interface consisting of a transformer 1704 and transceiver 1706 similar to that included within the chip 1702. By interconnecting to such chips, the microcontroller 1708 and the interconnected chip would be voltage isolated from each other via the complete RF isolation link between them.
The transmit and receive circuitry 1706 is part of the I/O interface for the integrated circuit. One type of integrated circuit that provides the overall functionality of that illustrated in FIG. 17a is a conventional microcontroller unit of the type C8051FXXX, manufactured by the present Assignee. This chip provides onboard processing through the MCU 1708, interface to the analog domain and interface to the digital domain. This integrated circuit also has the ability to configure the various outputs and, as such, a digital output could be provided on a serial interface for driving the transmit/receive circuitry 1706 or receiving the serial data therefrom.
The process of fabricating the MCU 1708, memory 2012 and the digital I/O 1710, in addition to the various analog-to-digital data converters or digital-to-analog data converters is fairly complex. As such, the transmit and receive circuitry 1706 and the transformer 1704 must be compatible with the process rather than making the process compatible with the transformer. As will be described herein below, there are a plurality of metal layers utilized to fabricate various interconnects associated with fabrication of the integrated circuit. By utilizing the various metal layers that are already present in the fabrication process, the two sides of the transformer 1704 can be fabricated and isolated from one another with sufficient isolation to provide adequate over voltage protection. Additionally, due to the high voltages and the high frequency of the transformer, the transformer 1704 is actually disposed in a separate portion of the chip surface area such that it does not overlie any of the circuitry associated with the digital operation or the analog operation, since this is a mixed-signal integrated circuit.
One example of this is illustrated in FIG. 17b, wherein the chip 1702 including an RF isolation link consisting of transformer 1704 and transceiver 1706 is integrated with a microcontroller unit 1708 through a digital input/output 1710. The MCU 1708 also includes an associated memory 1712. In this case, the first portion of the RF isolation link consisting of a transformer 1704 and transceiver 1706 is interconnected with a second portion of the RF isolation link consisting of transformer 1714 and transceiver 1716. In this case, the chip 1718 including the second portion of the RF isolation link includes a digital-to-analog converter 1720 and an analog-to-digital converter 1722 for converting the digital output of the transceiver 1716 of the RF isolation link into an analog output and for converting received analog inputs into digital inputs. The chip 1718 enables both the output of an analog signal at analog output 1724 and the input of analog signals at analog input 1726. These analog signals may then be used in any desired fashion by a circuit designer.
Referring now to FIGS. 18a, 18b, 19 and 20, there is illustrated the structure of the transformer coils of transformer 714 or 718 (FIG. 7) integrally formed on a CMOS device. Each transformer 714 and 718 is integrated as a part of one of the chips or dies including the RF isolation link. Referring more particularly to FIGS. 18a and 18b, there are illustrated the two coils included in each of transformers 714 or 718. A first coil 1802 consists of a first terminal 1804 and a second terminal 1806 formed in the metal layer of a chip referred to as the “Metal 1” layer. Each of the terminals in the Metal 1 layer are connected to the transformer coil 1808 which resides on a second metal layer of a chip referred to as the “Metal 2” layer. A conductive via 1810 interconnects the coil 1808 with terminal 1804. A second connective via 1812 interconnects the coil 1808 with the second terminal 1806. A second coil resides upon a fifth metal layer referred to as the “Metal 5” layer. This coil consists of a first bonding pad 1814 and a second bonding pad 1816. Each of the first and second conductive pads 1814, 1816 are interconnected by a second coil 1818 encircling pad 1816 and interconnecting with pad 1814. Unlike the coil described in FIG. 18a, coil 1818 includes both the bonding pads 1814, 1816 and the coil 1818 on the same metal layer (Metal 5).
Typically, the Metal 5 layer is the uppermost layer. Referring now also to FIG. 19, there is illustrated the overlapping view of the first and second coils of a transformer on a chip. It can be seen that the pad 1816 is dimensioned such that it is 1/32μ×94 μn. The entire coil is dimensioned to be 268 μm by 205 μm. The pad 1814 is dimensioned to the 70 μm×80 μm. The two coils 1818 and 1808 are similar in their configuration and are oriented such that they are substantially “non-overlapping.” However, they could overlap.
Referring now to FIG. 20, there is illustrated a side view of a chip 602 containing a transformer structure as described with respect to FIGS. 18a, 18b and 19. The chip 602 includes a substrate layer 2002 containing the transceiver circuitry of the RF isolation link and any electronic circuitry integrated with the RF isolation link as discussed previously. The Metal 1 layer 2004 resides upon the substrate 2002 and includes the first and second terminals 1804, 1806 of the first transformer coil. On top of the Metal 1 layer is the Metal 2 layer 2006 containing the first coil 1808 interconnected by vias to the first and second terminals 1804 and 1806 (not shown). Finally, the Metal 5 layer resides over the Metal 2 layer 2008. The Metal 5 layer 2010 contains the other portion of the transformer, including the bond conduction pads 1816 and the bond pad 1814 (not shown) and the coil 1818 interconnecting the bond pad 1816 with the bond pad 1814. The Metal 1 layer for the transformer is utilized primarily to provide interconnects to the remaining of the circuits for the terminals 1804 and 1806. However, the process utilizes all five metal layers for the various interconnects. For the purposes of over voltage protection, it is desirable to separate the coil 1818 from the coil 1808 by as much distance as possible, realizing that the material disposed therebetween is silicon dioxide, a dielectric. An additional concern is the capacitor loading on the coil 1818 to ground, the substrate 2002 typically being disposed at ground. The high voltage will be present on the coil 1818 and, therefore, it is separated from both the substrate and the coil 1818 by as much distance as possible. Although the coil 1818 could have been fabricated in the Metal 1 layer, there would then have been a requirement to provide an interconnection from the ends of the coil to the circuitry. This would have required a “run” to be provided beneath the Metal 1 layer, which would required utilization of a polycrystalline layer. Even siliciding of the poly layer would not provide as good a conductive layer as that associated with a metal layer. As such, the configuration utilizes the Metal 1 layer for the interconnects and the Metal 2 layer for the coil.
Although it would be desirable to provide an even additional metal layer to further separate the coil 1818 from the coil 1808, it is not feasible to complicate a process with a special additional layer. The only reason that an additional layer would be utilized would be for the purpose of fabricating other circuitry on the integrated circuit. The reason for this is that, once a process is defined as being able to utilize multiple metal layers, substantially all circuits run through that process will use the multiple layers. It would be difficult to dedicate a process for a single integrated circuit that only used that additional metal layer and, therefore, the coil is fabricated from already existing metal layers in an existing process. However, if an additional metal layer were utilized in an existing process in the future, then it is possible that the coil 1818 would be disposed in an even higher layer than Metal 5.
Referring now to FIG. 21, there is illustrated the offset used between metal runs 2102 of the coil 1818 on the Metal 5 layer and metal runs 2104 of the coil 1808 on the Metal 2 layer. Rather than having metal runs 2104 on the Metal 2 layer disposed directly below a metal run 2102 on the Metal 5 layer, they are offset diagonally from each other in order to increase the breakdown voltage between the components by increasing the distance. In the disclosed embodiment, the total distance between the Metal 5 layer run 2102 and the Metal 2 layer run 2404 is 3.63 μm. The Metal 2 layer run 2104 is vertically displaced from the Metal 5 layer run 2102 by 3.54 μms and horizontally displaced by 0.8 μm. The Metal 5 run layer 2102 is vertically separated from the silicon layer by 5.24 μm. This structure should provide a breakdown voltage between the Metal 5 and Metal 2 layers according to the equation 3.63×10−6 m*8×108 v/m=2904 v of breakdown voltage isolation. The breakdown voltage between the Metal 5 layer 2402 and the silicon layer 2406 can be determined according to the equation 5.24×10−6 m*8×108 v/m=4192 v.
Referring now to FIG. 22, there is illustrated a cutaway perspective view of the coils 1818 and 1808 illustrated in FIG. 21. It can be seen that the metal runs 2104 are substantially the same shape as the metal runs 2102 but they are non-overlapping and separated by a dielectric layer. This illustration illustrates only a single corner of the coils.
Referring now to FIG. 23, there is illustrated a chip 602 including an RF isolation link according to the present disclosure. The area of the chip 602 would be divided into at least two sections. A first section 2302 would contain the circuitry for providing the transformer for electromagnetically coupling with a transformer on another chip to provide the voltage isolation link between the chips. The remaining electronic circuitry of the chip would be located in a separate area 2304 and would include the transmitter and receiver circuitry of the voltage isolation link associated with the transformer as well as any electronic circuitry that would be integrated with the voltage isolation link, such as a micro controller or other type of electronic device. This would be repeated for multiple voltage isolation links for additional data paths. Additionally, it is noted that the layout is such that the area 2302 that contains the transformer on the upper surface thereof will have provided the pads 2116 in the center of the coil 2118 and the pad 2114 on the exterior thereof. The pad 2114 is located proximate the edge of the chip such that the bond wire 604 can be bonded thereto. Additionally, the pad 2116 is on the same surface as the pad 2114 such that the bond wire 604 associated therewith can be connected thereto. As such, there are no runs required to connect to the pad 2116 in a coil that would be required to run through other layers and run closer to the coils therein at right angles thereto. The bond wire 604 associated therewith will actually be disposed farther away from the actual metal runs 2102 associated with the coil 1818. An additional area could be included on the chip for additional electronic circuitry to be voltage isolated via a voltage isolation link on the same chip.
FIG. 24, illustrates the overall structure of the RF isolation link implemented on a chip 2402. Four separate interface connections 2404 provide connection of each of the four channels of the RF isolation link integrated into the chip 2402. Each of the four interfaces 2404 is linked with the oscillator 2406 and coil 2408. Connected to each of the interfaces 2404 are the transformers 2410 consisting of a first coil 2412 and a second coil 2414. Coil 2414 connects with the interface 2404 to provide interconnection with an external chip via the RF isolation link. Coil 2412 interconnects to bond pads 2416. It is noted that the channel one and channel four coils 2414 each include two separate bond pads 2416. However, the channel two and three coils 2414 each have a bond pad within the interior of the coil but share the external bond pad 2416x between channels two and three. Pad circuitry 2418 is associated with the oscillator circuit 2406 and the coils 2410. The pad circuitry 2418 is interconnected with the remainder of the circuitry on a chip 2402 via a number of bond pads. The bond pads comprise a ground bond pad 2418, a VDD bond pad 2420, two enable bond pads 2422, four output bond pads 2424 and four input bond pads 2426, one for each channel.
One problem with the above-described RF isolation link design is that RF interference from nearby transmitting cellular telephones may create common mode interference that may not be filtered in the receiving portion. Referring now to FIG. 25, at GHz frequencies the application printed circuit board consisting of two separate portions 2502 create split ground planes which may act as a dipole antenna. The split ground planes may have dimensions which are close to the quarter wavelength dimension at 900 MHz. This results in very large common mode signals which may be passed through the isolator chip 2504. Measurements from a nearby transmitting GSM cell phone at maximum power can create common mode voltages of as high as 3.4V peak at 900 MHz. This would cause interference within the RF isolation link as described herein above, causing a “0” to be incorrectly detected as a “1” when a cell phone was operating nearby. One manner for reducing this problem is by adding an EMI capacitor 2506 between the isolated ground planes. Thus, at a frequency of 900 MHz, a circuit without the EMI capacitor 2506 would have a 3.4V peak common mode voltage but with a 300 pF capacitor 2506 would only have a 1.1V peak. Likewise, at 2 GHz, the circuit without an EMI capacitor 2506 would have a 0.85V peak common mode voltage and a 0.07V peak common mode voltage when a 300 picofarad EMI capacitor 2506 was included. An RF isolator as described herein above cannot handle this level of common mode interference.
The previously described single-ended design relies upon the transformer to provide all common mode rejection. While the transformer has very good common mode rejection below 100 MGz, the common mode rejection for the transformer is poor at GHz frequencies. This is due to the parasitic capacitances 2602 that are created within the transformer 2604 as illustrated in FIG. 26. This is more fully illustrated in FIG. 27 wherein the vertical axis illustrates the common mode gain and the horizontal axis illustrates frequencies. As can be seen in FIG. 27, at 100 MHz frequencies, the common mode gain is relatively minimal. However, as the GHz frequencies are approached, the common mode gain increases, thus increasing the amount of common mode interference which would be passed through the transformer circuit of the RF isolation link.
In the embodiment illustrated in FIG. 28, the problems of common mode interference are addressed by modifying the transformer 2802 to be a center tapped transformer and including a differential amplifier 2810. Use of the center tapped transformer 2802 moves out the frequency at which the circuit resonates by splitting the parasitic capacitances. The center tap of transformer 2802 on the transmitter side is connected through a capacitor 2804 to ground. The center tap of transformer 2802 on the receive side is grounded. The bandwidth of the center tap transformer is two times higher than the single ended design for common mode signals. This helps suppress 900 MHz common mode interference. The data to be transmitted is applied to a first input of NAND gate 2814 and the RF signal is applied to second input of NAND gate 2814 before being applied to the center tapped transformer 2802. A differential amplifier 2810 is used on the receive side to further suppress common mode interference. In this circuit, common mode interference is applied to the inputs of the differential amplifier 2810 as a common mode signal which is rejected by the differential amplifier 2810. The transmitted RF signal is differential and is gained up by the receiver RF amplification and applied to a detector circuit 2810, one example of which may be the detector circuits described herein above.
Referring now to FIG. 29, there is illustrated an alternative embodiment of the RF isolation link 2900 of FIG. 28 consisting of transmitter circuitry 2902 and receiver circuitry 2904. The transmitter circuitry 2902 consists of a NAND gate 2908 having a first input connected to receive the data to be transmitted over the RF isolation link 2900 and a second input connected to receive the RF carrier signal. The RF carrier in the preferred embodiment comprises a 2 GHz signal. The data input to the first input of the NAND gate 2908 consists either of a logical “1” or “0” which will selectively gate the RF carrier signal to the output of NAND gate 2908 in the presence of a logical “1.” This causes the output of the NAND gate to either provide the RF carrier signal when the data bit is “1” or not provide the RF signal when the data bit is “0.” The output of the NAND gate 2908 is connected to the gate of a p-channel transistor 2910. The drain-source paths of the p-channel transistor 2910 is connected between VDD and a first input of transformer 2912. The transformer 2912 is a center tap transformer having its center tap node 2914 connected to a transistor 2916. The drain-source path of transistor 2916 is connected between node 2914 and ground. The gate of transistor 2916 is connected to receive signal tx_ena-bar. The output of NAND gate 2908 is also connected to an input of inverter 2918. The output of inverter 2918 is connected to the gate of transistor 2920. The drain-source path of transistor 2920 is connected between transformer 2912 and ground. A receiver amplifier 2922 is connected across transformer 2912 and may be disabled by a disable input 2924 when the chip is transmitting. The transformer 2912 electromagnetically couples the RF carrier signal to transformer 2926 via bond wires 2928. This links the data represented by the RF carrier signal between the transformers and limits common mode signals while providing voltage isolation between the chips via the first and second transformers 2912 and 2926. Each of the transformers 2912 and 2926 are associated with opposite sides of the interface.
The receiver circuitry 2904 receives the signal which has been electromagnetically coupled via the center tap transformer 2912 onto the bond wires 2928 to center tap transformer 2926. Connected to a center tap node 2930 of center tap transformer 2926 is a transistor 2932. The drain-source path of the transistor 2932 is connected between center tap node 2930 and ground. The gate of transistor 2932 is connected to VDD. The outputs of center tap transformer 2926 are connected to the inputs of a differential amplifier 2934. The differential amplifier 2934 consists of a first stage 2936 and second stage 2938 providing common mode rejection and a third stage 2940 providing single ended gain.
The first stage 2936 consists of a set of two p-channel transistors 2942, 2944, and two n-channel transistors 2946 and 2948. The drain-source path of transistor 2946 is connected between node 2950 and node 2952 connected to center tap transformer 2926. The gates of transistors 2946 and 2948 are cross coupled through capacitors 2956 and 2958 to nodes 2950 and 2956, respectively. Transistor 2942 has its drain-source path connected between VDD and node 2952. Transistor 2948 has its drain-source path connected between node 2954 and node 2956. Transistor 2944 has its drain-source path connected between node VDD and node 2954. The gate of transistor 2942 is connected to node 2952. The gate of transistor 2944 is connected to node 2954. A resistor 2962 is additionally connected between the gate of transistor 2946 and a bias node 2964. A resistor 2966 is also connected between the gate of transistor 2948 and the bias node 2964.
The second stage 2938 is connected to the first stage 2936 at nodes 2952 and 2954. Transistor 2968 has its gate connected to node 2952. Transistor 2970 has its gate connected to node 2954. The drain-source path of transistor 2968 is connected between node 2972 and node 2974. Transistor 2970 has its drain-source path connected between node 2976 and node 2974. A current source 2978 is connected between node 2974 and ground. Transistor 2980 has its drain-source path connected between VDD and node 2972. The gate of transistor 2980 is connected to node 2972. Transistor 2982 has its drain-source path connected between VDD and node 2976. The gate of transistor 2982 is connected to node 2972. Transistor 2984 has its gate connected to node 2976. The drain-source path of transistor 2984 is connected between VDD and node 2976. A current source 2986 is connected between node 2976 and ground.
The third stage 2940 connects with the second stage 2938 at node 2976. A capacitor 2988 is connected between node 2976 and an input of amplifier 2990. The output of amplifier 2990 has a feedback resistor 2992 connected to its input. The output of amplifier 2990 is also connected to a detector circuit 2994 for detecting the amplified data coming from the gained amplifier. A transmitter circuit 2926 connects to the single tap transformer 2926 at node 2950. The transistor 2928 has its drain-source path connected between node 2956 and ground. The gate of transistor 2928 is also connected to ground.
Referring now to FIGS. 30a, 30b, 31 and 32, there is illustrated the structure of the transformer coils of transformer 2912 or 2926 (FIG. 7) integrally formed on a CMOS device. Each transformer 2912 and 2926 is integrated as a part of one of the chips or dies including the RF isolation link. Referring more particularly to FIGS. 30a and 30b, there are illustrated the two coils included in each of transformers 2912 or 2926. A first coil 3002 consists of a first terminal 3004 and a second terminal 3006 formed in the metal layer of a chip referred to as the “Metal 1” layer. Each of the terminals in the Metal 1 layer are connected to the transformer coil 3008 which resides on a second metal layer of a chip referred to as the “Metal 2” layer. A conductive via 3010 interconnects the coil 3008 with terminal 3004. A second connective via 3012 interconnects the coil 3008 with the second terminal 3006. A second coil resides upon a fifth metal layer referred to as the “Metal 5” layer. This coil consists of a first bonding pad 3014 and a second bonding pad 3016. Each of the first and second conductive pads 3014, 3016 are interconnected by a second coil 3018 encircling pad 3016 and interconnecting with pad 3014. Unlike the coil described in FIG. 30a, coil 3018 includes both the bonding pads 3014, 3016 and the coil 3018 on the same metal layer (Metal 5).
Typically, the Metal 5 layer is the uppermost layer. Referring now also to FIG. 31, there is illustrated the overlapping view of the first and second coils of a transformer on a chip. It can be seen that the pad 3016 is dimensioned such that it is 70 μm×70 μm. The entire coil is dimensioned to be 205 μm by 205 μm. The pad 3014 is dimensioned to the 70 μm×70 μm. The two coils 3018 and 3008 are similar in their configuration and are oriented such that they are substantially “non-overlapping.” However, they could overlap. The center tap is provided on the M1 layer with a strip 3104 extending all the way across coils of the transformer and including a conductive via 3102 providing the center tap interconnecting the Metal 1 layer to the Metal 2 layer in coil 3008.
Referring now to FIG. 32, there is illustrated a side view of a chip 3200 containing a transformer structure as described with respect to FIGS. 30a, 30b and 31. The chip 3200 includes a substrate layer 3202 containing the transceiver circuitry of the RF isolation link and any electronic circuitry integrated with the RF isolation link as discussed previously. The Metal 1 layer 3204 resides upon the substrate 3202 and includes the first and second terminals 3004, 3006 of the first transformer coil. On top of the Metal 1 layer is the Metal 2 layer 3206 containing the first coil 3008 interconnected by vias to the first and second terminals 3004 and 3006 (not shown). Finally, the Metal 5 layer resides over the Metal 2 layer 3008. The Metal 5 layer 3210 contains the other portion of the transformer, including the bond conduction pads 3016 and the bond pad 3014 (not shown) and the coil 3018 interconnecting the bond pad 3016 with the bond pad 3014. The Metal 1 layer for the transformer is utilized primarily to provide interconnects to the remaining circuits for the terminals 3004 and 3006. However, the process utilizes all five metal layers for the various interconnects. For the purposes of over voltage protection, it is desirable to separate the coil 3018 from the coil 3008 by as much distance as possible, realizing that the material disposed therebetween is silicon dioxide, a dielectric. An additional concern is the capacitor loading on the coil 3018 to ground, the substrate 3202 typically being disposed at ground. The high voltage will be present on the coil 3018 and, therefore, it is separated from both the substrate and the coil 3018 by as much distance as possible. Although the coil 3018 could have been fabricated in the Metal 1 layer, there would then have been a requirement to provide an interconnection from the ends of the coil to the circuitry. This would have required a “run” to be provided beneath the Metal 1 layer, which would required utilization of a polycrystalline layer. Even siliciding of the poly layer would not provide as good a conductive layer as that associated with a metal layer. As such, the configuration utilizes the Metal 1 layer for the interconnects and the Metal 2 layer for the coil. The center tap strip 3104 runs through the Metal 1 layer and connects to the coil 3008 in the Metal 2 layer using conductive via 3102.
Although it would be desirable to provide an even additional metal layer to further separate the coil 3018 from the coil 3008, it is not feasible to complicate a process with a special additional layer. The only reason that an additional layer would be utilized would be for the purpose of fabricating other circuitry on the integrated circuit. The reason for this is that, once a process is defined as being able to utilize multiple metal layers, substantially all circuits run through that process will use the multiple layers. It would be difficult to dedicate a process for a single integrated circuit that only used that additional metal layer and, therefore, the coil is fabricated from already existing metal layers in an existing process. However, if an additional metal layer were utilized in an existing process in the future, then it is possible that the coil 3018 would be disposed in an even higher layer than Metal 5.
Another concern in reducing common mode rejection is the ability to set the receiver gain and transmit power to a level to reliably pass through data but no higher. This conserves power in the transmitter and improves common mode rejection which is worse at higher receiver gains. Once this gain is established, it should remain constant over temperature and process changes to provide optimal system performance. This can be achieved by setting the power supply voltages (VDD) to the transmitter and the receiver to vary with temperature and process instead of being a constant regulated voltage. This is illustrated in FIG. 33. As can be seen, for both a slow process and fast process, the voltage VDD increases as the temperature increases. This helps to keep the RF gain of the amplifier more constant as temperature changes and allows lower supply currents at lower temperatures.
Referring now to FIG. 34, there is illustrated a prior art method for generating the reference voltage wherein the PTAT current generator 3402 is connected to the gate of transistor 3404. The drain-source path of transistor 3404 is connected between voltage and node 3406. A resistor 3408 is connected between node 3406 and transistor 3410. The emitter/collector pathway of transistor 3410 is connected between transistor 3408 and ground. The base of transistor 3410 is connected to its collector.
FIG. 35 illustrates the modified method for generating the band gap reference voltage such that the voltage will vary with respect to temperature. The PTAT current generator 3402 again provides a voltage to the gate of transistor 3404 which provides a PTAT current. The PTAT current provided by the PTAT current generator 3402 is proportional to absolute temperature. The source-drain pathway of transistor 3404 is connected between voltage and node 3406. A p-channel transistor 3502 has its source-drain pathway connected between node 3406 and node 3504. The gate of transistor 3502 is also connected to node 3504. A resistance 3506, which is larger than the resistance of resistor 3408 in FIG. 34, is connected between node 3504 and ground. By setting the size of the PMOS transistor 3502 and the resistance 3504, the reference voltage can be set to a desired level. Since the bias current provided to the receiver is a PTAT current, this keeps the receiver gain constant.
Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

## Claims

1. A system comprising:
a first chip containing functional circuitry;
a second chip containing functional circuitry; and
at least one RF isolation link interconnecting the first and the second chip, the RF isolation link providing voltage isolation between the first chip and the second chip and limiting common mode signals passing through the at least one RF isolation link, wherein the at least one RF isolation link further comprises:
a first transformer on the first chip; and
a second center tap transformer on the second chip.

2. The system of claim 1, wherein the at least one RF isolation link is integrated on the first chip and the second chip.

3. The system of claim 1, wherein the at least one RF isolation link further comprises:
a transmitter associated with the first transformer for transmitting an RF carrier onto the RF isolation link;
a receiver associated with the second center tap transformer for receiving an RF carrier onto the RF isolation link.

4. The system of claim 3, wherein the power supply voltage of the transmitter and the receiver varies with respect to temperature.

5. The system of claim 3, wherein the power supply voltage of the transmitter and the receiver varies with respect to process.

6. The system of claim 3 wherein the first transformer is a center tap transformer.

7. The system of claim 1, wherein the at least one RF isolation link further comprises:
first transceiver circuitry associated with the first transformer for transmitting and receiving an RF carrier onto the at least one RF isolation link;
second transceiver circuitry associated with the second center tap transformer for transmitting and receiving an RF carrier onto the at least one RF isolation link.

8. The system of claim 1, wherein the at least one RF isolation link further comprises:
a transmitter responsive to a logical data signal and an RF carrier signal for generating a transmitter output;
wherein the first transformer electromagnetically couples the transmitter output on to a link;
a differential amplifier for amplifying signals applied to inputs of the differential amplifier;
wherein the second center tap transformer electromagnetically couples the transmitter output on the link to the differential amplifier inputs;
a detector coupled to the output of the differential amplifier for detecting the presence or absence of the RF carrier signal in an amplified transmitter output from the amplifier stage.

9. The system of claim 8, wherein the transmitter outputs the RF carrier as the transmitter output responsive to a logical “1” logical data signal.

10. The system of claim 8 wherein the transmitter shuts down the RF carrier as the transmitter output responsive to a logical “0” logical data signal.

11. The system of claim 8, wherein the presence of the RF carrier signal indicates a received logical “1” and the absence of the RF carrier signal indicates a logical “0”.

12. The system of claim 1, wherein the at least one RF isolation link transmits data between the first and the second chip using amplitude modulation.

13. The system of claim 1, wherein the at least one RF isolation link transmits data between the first and the second chip using frequency modulation.

14. An apparatus comprising:
a chip containing functional circuitry; and
a portion of at least one RF isolation link interconnecting the chip with an external chip, the RF isolation link providing voltage isolation between the chip and the external chip and limiting common mode signals passing through the portion of at least one RF isolation links, wherein the portion of the at least one RF isolation link further comprises;
an integrated center tap transformer for electromagnetically coupling a transmitter output onto an external link to a transformer on the external chip.

15. The apparatus of claim 14, wherein the portion of the at least one RF isolation link further comprises:
a transmitter responsive to a logical data signal and an RF carrier signal for generating the transmitter output.

16. The apparatus of claim 14, wherein the portion of the at least one RF isolation link comprises:
a receiver for detecting the presence or absence of an RF carrier signal in a signal on the external link; and
an integrated center-tap transformer for electromagnetically coupling the signal on the external link to the receiver.

17. The apparatus of claim 16, wherein the receiver further comprises:
a differential amplifier for amplifying the signal on the external link;
a detector coupled to the differential amplifier for detecting the presence or absence of the RF carrier signal in an amplified transmitter output from the amplifier stage.

18. A circuit package, comprising:
a first die containing functional circuitry;
a second die containing functional circuitry; and
at least one RF isolation link interconnecting the first and the second die, the RF isolation link providing voltage isolation and limiting common mode signals between the first die and the second die, wherein the RF isolation link further comprises:
a first transformer on the first die; and
a second center-tap transformer on the second die.

19. The system of claim 18, wherein the RF isolation link further comprises:
a transmitter associated with the first transformer for transmitting an RF carrier onto the RF isolation link; and
a receiver associated with the second transformer for receiving an RF carrier onto the RF isolation link.

20. The system of claim 18, wherein the RF isolation link further comprises:
first center-tap transceiver circuitry associated with the first transformer for transmitting and receiving an RF carrier onto the at least one RF isolation link; and
second transceiver circuitry associated with the second transformer for transmitting and receiving an RF carrier onto the at least one RF isolation link.

21. The system of claim 18, wherein the at least one RF isolation link further comprises:
a transmitter responsive to a logical data signal and an RF carrier signal for generating a transmitter output;
a first transformer for electromagnetically coupling the transmitter output on to a link;
a differential amplifier for amplifying signals applied to inputs of the differential amplifier;
a second center-tap transformer for electromagnetically coupling the transmitter to the amplifier stage input;
a detector coupled to the differential amplifier for detecting the presence or absence of the RF carrier signal in an amplified transmitter output from the amplifier stage.