MONOLITHIC ANALOG-TO-DIGITAL CONVERTER

Abstract: An analog comparator compares an analog signal to be converted with an analog ramp signal. The output of the comparator enables a digital latch having a binary Gray code counter input. When the analog ramp equals the analog signal, the digital latch captures the state of the Gray code counter. Metastability in the digital latch is resolved by a latch train. The Gray coded output is then decoded by a Gray decoder to a standard binary output. An array of converters are constructed on a monolithic integrated circuit where each converter shares a single analog ramp generator, binary Gray code counter and Gray decoder. A multiplexer selects a particular converter and switches the standard binary output from the selected converter to line drivers to be used off-chip. The two least significant bits of the Gray code are generated with phase shifting circuits.

Description: MONOLITmC ANALOG-TO-DIGITAL CONVERTER Background Of The Invention 1. Field of the Invention The present invention relates generally to analog-to-digital converters and more particularly to multiple analog-to-digital converters integrated on a monolithic integrated circuit. 2. Discussion of the Related Art In conventional single slope methods of analog-to-digital conversion, a clocked comparator is employed to compare a sampled and held signal with an analog ramp. The clocked comparator in turn generates a signal that is used to clock a latch circuit that stores the state of a digital counter when the sampled and held signal is equal to the analog ramp. The stored counter value is a digital representation of the magnitude of the analog signal. Integrated circuit applications of this type of circuit provide an array of such circuits. Conventional single slope analog-to-digital converters encounter resolution and speed limitations due to limitations on maximum clock rate. One limit to the maximum clock rate with the convention single slope converter is due to metastability. Metastability is defined as the instability of a flip-flop when the clock and data inputs change simultaneously. Although the output of a flip-flop cannot, in principle, be guaranteed to have settled to a valid logic state after any given period of time, the probability that the output has not settled decreases exponentially with time. After about 69 time constants, for example, the probability of the output not settling is less than 10 "30 , which is acceptable for most applications. Due to this problem, the clock rate must be reduced substantially to allow the flip-flop, which synchronizes the comparator output to the system clock time, to recover from metastability. Conventional devices require the metastability resolution to be done at a frequency determined by the time resolution of the conversion. As a result, conventional devices are limited to clock rates much less than their circuits are capable of. The invention improves on conventional devices by removing the requirement to synchronize individual converters to the master clock and by generating a higher resolution digital code. It is therefore one object of the invention to provide an analog-to-digital converter that realizes a significant improvement in resolution and speed compared to conventional converters. Summarv Of The Invention The invention provides an apparatus to convert an analog signal to a digital signal comprising: an analog ramp generator having an analog ramp output, a digital ramp generator having a Gray coded digital ramp output, a comparator for comparing the analog signal with the analog ramp signal, wherein the comparator has a comparison output, and a metastability resolving latch for storing data having a first data input connected to the digital ramp output and an enable input connected to the comparison output, wherein the metastability resolving latch has a metastability resolved digital signal output. The invention also provides an apparatus to convert a plurality of analog signals to a plurality of digital signals comprising: an analog ramp generator having an analog ramp output, a digital ramp generator having a Gray coded digital ramp output, a plurality of comparators for comparing the analog signal with the plurality of analog ramp signals, wherein the plurality of comparators has a plurality of comparison outputs, and a plurality of metastability resolving latches for storing data having a plurality of first data inputs connected to the digital ramp output and a plurality of enable inputs connected to the comparison output, wherein the plurality of metastability resolving latches have a plurality of metastability resolved digital signal outputs. The invention also provides an analog-to-digital conversion method comprising the steps of operating an unclocked comparator to compare an input analog voltage to a voltage ramp to enable a digital latch to store a Gray coded digital timer word when the two comparator inputs are substantially equal. The features and advantages of the present invention will be more readily understood and the apparent from the following detailed description of the invention, which should be read in conjunction with the accompanying figures, and from the claims which are appended at the end of the detailed description. Brief Description Of The Drawings In the drawings, which are incorporated herein by reference and in which like elements have been given like reference characters, FIG. 1 is a schematic block diagram of an analog-to-digital converter according to the invention; FIG. 2 is a schematic block diagram of the metastability resolving circuit illustrated in FIG. 1; FIG. 2 A is a schematic block diagram of the analog waveform generator illustrated in FIG. 1; FIG. 3 is a schematic block diagram of an array of analog-to-digital converters according to the invention; FIG. 4 is a schematic block diagram of the clock multiplying phase locked loop illustrated in FIG. 3; FIG. 5 is a schematic block diagram of the 90° phase shifter illustrated in FIG. 3; FIG. 6 is a schematic block diagram of an alternate embodiment of the circuit illustrated in FIG. 3; FIG. 7 is a schematic block diagram of a two phase voltage controlled oscillator and squaring circuit used in the circuit of FIG. 6; FIG. 8 is a detailed schematic circuit diagram of the voltage controlled oscillator illustrated in FIG. 7; FIG. 9 is a detailed schematic circuit diagram of the squaring circuit illustrated in FIG. 7; and FIG. 10 is a timing diagram illustrating the temporal relationship among the least significant bits of the Gray code signal. Detailed Description For purposes of illustration only, and not to limit generality, the present invention will now be explained with reference to particular embodiments and operational parameters. One skilled in the art will appreciate, however, that the present invention is not limited to the particular operational parameters described. Reference is now made to FIG. 1, which is an overall schematic block diagram of the analog-to-digital converter of the invention. Analog input signal 15, the analog signal to be converted, is connected to one input of an unclocked analog comparator 20. The other input of the comparator 20 is connected to an analog ramp signal 18. Analog waveform generator 30 generates the analog ramp signal 18. When the analog ramp signal 18 substantially equals the analog input signal 15, the comparator generates output signal 21. The comparator output signal 21 is connected to a control input of a metastability resolving circuit 35. Synchronized with the analog waveform generator 30 is a Gray code generator 45 that generates a digital Gray code on a digital Gray code bus 62. The digital Gray code bus 62 is connected to a data input of the metastability resolving circuit 35. The metastability resolving circuit 35 stores the states of the digital Gray code on bus 62 in response to an active state of the comparator output signal 21. As a result, the digital output signal 47 of the metastability resolving circuit 35 is a digital representation of the magnitude of the analog input signal 15 when the magnitude of the analog ramp signal 18 equals the magnitude of the analog input signal 15. Reference is now made to FIG. 2, which illustrates the metastability resolving circuit 35 in more detail. Comparator output signal 21 is connected to the control input of an N-Bit data latch 11. N is the number of bits of resolution that the analog signal 15 is digitized (converted) into by the analog-to-digital converter. N can be any number and is typically between eight and sixteen for most applications. The N-bit data latch 11 data input is connected to the digital Gray code bus 62 from the Gray code generator 45. The data latched by the N-bit data latch 11 (which is a code generated by Gray code generator 45) is provided on line 17 to N-bit flip-flop 19. N-bit flip-flop 19 resolves the metastability of the system by storing the data on line 17 a predetermined time period after the N-bit data latch 11 has stored the state of the Gray code generator 45. The digital output 47 is provided as described above. Reference is now made to FIG. 2 A, which illustrates a schematic block diagram of analog waveform generator 30 illustrated in FIG. 1. Operational amplifier 32 provides the analog ramp signal 18 by providing an output signal to integration capacitor 28. RESET signal 19 is generated by timing circuit 33 and activates switch 28A to discharge capacitor 28 when a new conversion is to be initiated. One input 21 of operational amplifier 32 in connected to a RAMP_BIAS signal and a second input 23 in connected to the output of a programmable current source 31. Programmable current source 31 is controlled by operational transconductance amplifier 27. Amplifier 27 has a first input connected to the analog ramp signal 18. A second input of amplifier 27 is connected to a ramp reference voltage RAMP_REF. A third input of amplifier 27 is connected to the output of ramp adjust circuit 29. The starting voltage of the analog ramp is adjustable by changing the RAMP_BIAS voltage. The slope of the analog ramp signal 18 is controlled by amplifier 27. By changing the output of programmable current source 31 in response to a current signal 27A from transconductance amplifier 27, the slope of the analog ramp signal 18 can be changed. In response to control signal 33 A from timing circuit 33, issued just before ramp signal 18 is to terminate, ramp adjust circuit 29, via control signal 29A, turns amplifier 27 on to sample the difference between the RAMP_REF voltage and the voltage of analog ramp signal 18. Transconductance amplifier 27 converts this voltage difference to a current 27A that is used to control programmable current source 31. After programmable current source 31 has been adjusted, timing circuit 33, via control signal 33A, turns amplifier 27 off to open the feedback loop, issues RESET signal 19 to discharge capacitor 28 using switch 28 A, and then opens switch 28A to begin another integration cycle. Reference is now made to FIG. 3, which is a schematic block diagram of parallel analog- to-digital converters 10A and 10B. Only two converters are shown for clarity; there could be m number of converters in an array. In one preferred embodiment there are 328 converters in an array. Each analog-to-digital converter is connected to the digital Gray code bus 62 and an output bus 57. The digital Gray code bus 62 is connected to each data input of the m data latches. For clarity, only the connections to data latches 24A and 24B are shown. The data input of each data latch is driven by the gray code generator 45. The N-bit output bus 57 is connected to the data output of each transfer latch (latches 26A and 26B being illustrated) and is read by multiplexer readout circuit 59. The analog signal on line 15 A, the signal to be converted, is stored by capacitor 23 A until sampling switch 12A is closed, thereby transferring the charge to capacitor 16 A. Capacitor 16A integrates the analog signal 15A until switch 12A is opened. After a predetermined time interval has passed, switch 12A is opened and switch 23 A is closed, thus resetting capacitor 23 A at the start of each conversion period. Those skilled in the art will recognize that any charge transfer device or circuit may be used to transfer the signal to be compared. During the read out phase the sampled signal 14A is compared to an analog ramp signal 18 by comparator 20A. When the sampled signal 14A is equal to or at some predetermined potential with respect to the analog ramp signal 18, the output 22 A of the comparator 20 A activates latch 24 A. The output of the comparator 20A is connected to the enable input of latch 24A. The latch 24 A, connected to digital Gray code bus 62, stores the state of the gray code count at the time the analog ramp signal 18 equals the sampled signal 14A in response to comparator output signal 22 A. The output of latch 24A is provided to a transfer latch 26A. Output control shift register 54, connected to transfer latches 26 A and 26B, selects the output of a particular analog-to-digital converter from the array of converters. The output of each transfer latch is connected to sense amplifier 53 via N-bit output bus 57, part of multiplexer readout circuit 59. Only one transfer latch is active and supplying an output to bus 57 at any one time. The output control register 54 is synchronized with input clock 68. Multiplexer readout circuit 59 will now be described. One skilled in the art will appreciate that each of the circuit blocks in multiplexer readout circuit 59 is N-bits wide to accomodate the number of bits from each transfer latch. The output of sense amplifier 53 is connected to the input of input register 55 which is clocked by input clock 68. Input register 55 latches the data on N-bit output bus 57 from whichever N-bit transfer latch has been enabled by output control shift register 54. The output of register 55 is connected to the input of a metastability resolving register 36 that is also clocked by input clock 68. Metastability resolving register 36 is clocked so that one full clock cycle after the state of N-bit output bus 57 has been latched into input register 55, the data from input register 55 is provided to the input of metastability resolving register 36. Register 36 resolves the metastability of the conversion that may have arisen when the digital signal on bus 62 was latched by output signal 22 A from analog comparator 20 A. Circuit analysis of this latch train arrangement has indicated that the metastability of the system is improved by a factor of at least 2 30 by the addition of metastability resolving register 36. The output of the metastability resolving register 36 is connected to a Gray code decoder 38 that converts the Gray code signal to a standard binary signal. The Gray code decoder 38 may use an exclusive-ORing (XORing) process in which the output of each latch in the metastability resolving register 36 is exclusive-ORed (XORed) with an adjacent bit that has in turn been exclusive-ORed with another bit, and so on. The standard binary N-bit code output by Gray code decoder 38 is provided to the data input of N-bit output register 71, which latches the output value in response to input clock 68. The output of output register 71 is provided to N output drivers 73 that provide the N-bit converted binary output signal 47. Input clock 68 is also provided to a clock multiplying phase locked loop circuit 50 that generates a high speed clock 64. In one embodiment of the invention, the clock multiplier is a 12x clock multiplier. In one embodiment of the invention, for example, input clock 68 is a 7 MHz nominal clock and clock multiplier 50 increases this by a factor of 12 to 84 MHz. Gray code generator 45 will now be described. The digital Gray code on bus 62, which in one embodiment of the invention is an N-bit binary gray code, is generated by concatenation of three bit streams: a least significant bit 60, a next-to-least significant bit 58 and a N-2 bit gray code word 56. The high speed clock 64 clocks an N-2 bit synchronous binary counter 48. The N-2 bit synchronous counter 48 provides an output signal to an N-2 bit Gray code encoder 46. The Gray code encoder provides the N-2 most significant bits 56 of the digital Gray code on bus 62. Gray code encoder 46 provides a Gray code by XORing each bit output by counter 48 with an adjacent output bit. The high speed clock 64 and the N-2 bit synchronous counter's least significant bit 49 is connected to a negative edge triggered flip-flop 44. The negative edge triggered flip-flop 44 provides the next to least significant bit signal, LSB+1 58, as part of the digital Gray code on bus 62. The high speed clock 64 is also connected to a 90° analog phase shifter 42. The 90° phase shifter 42 generates the least significant bit signal, LSB 60, as part of the digital Gray code on bus 62 by shifting the high speed clock 64 by 90°. In one example embodiment, N is equal to 13 bits, synchronous counter 48 and Gray code encoder 46 provide the 11 most significant bits on Gray code bus 62. A 12th bit (LSB+1) is supplied by dividing a 75 (approximately) MHz clock by two and then latching it with the falling edge of the 75 MHz clock in flip-flop 44. The 13th bit (LSB) is generated by delaying the 75 MHz clock by precisely 90°, l Λ of a complete clock cycle, in closed loop phase shifter 42. This type of phase shifter is sometimes referred to as a delay locked loop. Reference is now made to FIG. 4, which illustrates in more detail, the clock multiplying phase locked loop 50 of FIG. 3. Clock multiplier 50 includes a phase detector 100 that detects a difference in phase between input clock 68 and a frequency divided version of high speed clock 64 on line 102. The output 104 of phase detector 100 is used to control a frequency multiplying voltage controlled oscillator (VCO) 106. VCO 106 increases the frequency of input clock 68 by a predetermined factor. In one example, VCO 106 increases the frequency of input clock 68 by a factor of 12 to produce high speed clock 64. The output 108 of VCO 106 is provided to a "squaring" circuit 110. The function of squaring circuit 110 is to shape the output signal 112 so that high speed clock 64 has a fifty percent duty cycle, i.e., a "square" output. High speed clock 64 is also provided to a divide by n circuit 114 that divides the frequency by a factor n so that the frequency of the clock signal delivered on line 102 is equal to the frequency of input clock 68. As discussed before, in one embodiment, if VCO 106 increases the clock frequency by a factor of 12, then n would be 12 so that divide by n circuit 114 reduces the frequency of high speed clock 64 by a factor of 12 before providing that signal to phase detector 100. In one embodiment, VCO 106 may include a ring oscillator. Reference is now made to FIG. 5, which FIG. is a schematic block diagram of the 90° analog phase shifter 42 illustrated in FIG. 3. High speed clock 64 and its complement from clock multiplier 50 are connected to the first and second clocking inputs of a four input exclusive or (XOR) gate 80. XOR gate 80 includes an output coupled to the inverting input of a high gain integrating amplifier 82. Amplifier 82 outputs a control signal 83 which is coupled to a control input of a voltage controlled delay circuit 78. The voltage controlled delay circuit 78 also receives a clocking signal from the high speed clock 64. The high gain of amplifier 82 ensures that the delay is always 90° even in the presence of variations in component values and clock frequency. The voltage controlled delay circuit 78 outputs a delayed signal in response to the control signal 83 and clock 64 to a "squaring" circuit 77. Squaring circuit 77 shapes the delayed signal so that it is symmetrical and has a fifty percent duty cycle (i.e., a "square" output) and outputs a signal to the input of line driver invertor 75. Squaring circuit 77 is similar to squaring circuit 110, previously described. Line driver invertor 75 outputs a first line driver invertor signal 75 A and a second line driver invertor signal 75B to third and fourth inputs of the four input exclusive OR gate 80. The first and second line driver invertor signals are also coupled to first and second inputs of a delay matching circuit 81. Signals 75 A and 75B comprise a complementary delayed clock. Delay matching circuit 81 ensures that the delay experienced by each signal 75A and 75B is the same, so that the signals remain in the proper phase relationship with each other. The delay matching circuit 81 outputs LSB 60. Reference is now made to FIG. 6, which is a schematic block diagram of an alternate embodiment of the circuit of FIG. 3. In the circuit of FIG. 6, the 90° phase shifter 42 of FIG. 3 has been eliminated. In addition, clock multiplier 50 has been modified so as to provide LSB 60 directly. In all other respects, the operation of FIG. 6 is the same as already described in connection with FIG. 3. Reference is now made to FIG. 7, which FIG. is a schematic block diagram of the clock multiplier 50 of FIG. 6. In FIG. 7, as in FIG. 4, input clock 68 is provided to a phase detector 100 that provides a control signal 104, in response to input clock 68 and signal 102, to a voltage controlled oscillator 120. VCO 120 also multiplies the output frequency provided on line 108 to squaring circuit 110 in order to generate high speed clock 64 on line 112. The output of squaring circuit 110 on line 112 is additionally provided to divide by n circuit 114 that delivers control signal 102 in the same manner as described in connection with FIG. 4. VCO 120 also provides a second output 122 that is phase shifted 90° with respect to output 108 and then provided to another squaring circuit 110. Squaring circuit 110 operates in the manner described in connection with FIG. 4 to provide a "square" output for the LSB 60 on line 124. Reference is now made to FIG. 8, which FIG. is a schematic diagram of VCO 120. VCO 120 provides two outputs 108, 122 that are 90° out of phase from each other. VCO 120 is a ring oscillator formed from an odd number of invertor stages connected in a loop. In particular, VCO 120 includes invertors 126, 128, 130, 132, and 134. The output of invertor 134 is connected via line 136 to the input of inverter 126 in order to form the ring. If t is the time delay of one of the invertors and p is the number of stages in the oscillator, then the oscillation frequency f is: (1) f= l/(2pt) Changing the frequency is accomplished by changing the power supply voltage of the invertor chain, thus changing time t. In the case of the CMOS invertor, the propagation delay increases as the supply voltage is decreased. The phase shift per stage in the ring oscillator is: (2) Phase/stage = 180/p For example, in the five stage oscillator illustrated in FIG. 8, the phase shift per stage is 36°. Thus, a tap two stages away from the main output will have a 72° phase shift, while a tap three stages away from the main output will have a 108° phase shift. If all of the invertors are identical, then a 90° phase shift is not possible. However, if the different invertors in the ring oscillator are not identically constructed, then a 90° phase shift between invertors in the ring oscillator can be obtained. In a CMOS invertor, the delay through the invertor depends upon a number of factors, including the size and shape of the component transistors and the amount of capacitive loading on its output. Adjusting any of these factors to increase the propagation delay of one of the invertors with respect to the remaining invertors in the ring can be used to provide the required 90° phase shift. In VCO 120 illustrated in FIG. 8, the propagation delay of invertor 130 is adjusted by adding two transistors 138 and 140 that are biased so as to always be in the on state. This increases the propagation delay through invertor 130 so that the total delay through invertors 134, 126, and 128 is about the same as the delay through modified invertor 130 and invertor 132. If the delay through invertors 134, 136, and 128 is the same as the delay through modified invertor 130 and invertor 132, then there is exactly a 90° phase shift between outputs 108 and 122. - 10 - Reference is now made to FIG. 9, which FIG. is a schematic circuit diagram of squaring circuit 110 illustrated in FIG.s 4 and 7. Squaring circuit 77 is FIG. 5 also operates in the same manner as squaring circuit 110. As illustrated in FIG. 9, the output of VCO 120 is provided to squaring circuit 110. Obviously, in the case of the circuit illustrated in FIG. 7, two squaring circuits are provided, one for each output of VCO 120. Typically, VCO 120 operates at a reduced voltage compared to the rest of the circuitry and therefore outputs 108 and 122 need to be translated to the higher voltage level of the rest of the circuits. In addition, the propagation delay generally will not be the same for the rising and falling edges of the output signal and therefore the output of the level translator circuit will not be symmetrical, i.e., have a "square" output or fifty percent duty cycle, even though the signals internal to the ring oscillator are symmetric. Circuit 110 thus incorporates the level translator into a closed loop feedback circuit that adjusts the input threshold as needed to maintain the symmetry of the output signal. The level shifter includes transistors 150 and 152, and invertors 154 and 156. Two current source transistors 158 and 160 are controlled by voltages VMINUS and VPLUS. The voltages VMINUS and VPLUS are supplied by current mirror 162 and control the amount of current delivered by the transistors 158 and 160. A feedback loop of signal 112 or 124 is provided through transistors 158, 160, transistor 164, to level shifting transistors 150 and 152. If the waveform of output signal 112 or 124 becomes asymmetrical, i.e., not "square", transistors 158, 160 respond by changing the gate voltage on the input stage current source transistor 150 in a direction that reestablishes symmetry of the output. Additionally, transistor 164, used as a capacitor, filters out any ripple voltage and sets the response time of the feedback loop. Reference is now made to FIG. 10, which is a timing diagram of the LSB 60 and LSB+1 58. The timing diagram of FIG. 10 illustrates the operation of either the circuit of FIG. 3 or the circuit of FIG. 6. The high speed clock 64 transitions from low to high at time 3. The least significant bit 4 of the N-2 bit binary counter 48 transitions on the low to high transition of the high speed clock 64. The LSB 60, derived from the high speed clock 64, transitions high at time 5 and low at time 9. The LSB+1 58, derived from the counter least significant bit 4, transitions high at time 7 and low at time 2. The N-2 most significant bits 56 of the Gray encoded signal transition only at time 3 while the LSB 60 and LSB+1 58 signals do not change at time 3. At times 2, 5, 7, and 9 only one of these signals changes at a time, thus meeting the Gray code requirement of having only a single bit change when there is a change in the count. One skilled in the art will appreciate that rather than using an input clock having a frequency that is multiplied in order to provide a high speed clock, an external high speed clock could be used to control counter 48, flip-flop 44, and 90° phase shifter 42. One advantage of the present invention is that the Gray code least significant bit frequency may be equal to the frequency of the clock that is used to control the circuit. This means that the least significant bit frequency may be equal to the maximum toggle frequency for the flip-flop. Conventionally, for a typical Gray code, the master clock frequency is four times the frequency of the least significant bit of the Gray code. In the present invention, by contrast, the frequency of the least significant bit of the Gray code can be equal to the clock frequency. Therefore, the clock frequency is only limited by the inherent frequency limitations of the clock counter circuitry itself. This allows for higher conversion rates then conventionally achievable. For a typical 2 micron CMOS process at room temperature, this frequency limit is about 150 MHz, and about 500 MHz at 80°K. For a typical 1 micron CMOS process, this frequency limit is about 500 MHz at room temperature, and may be more than 1 Ghz at 80°K. In one embodiment of the invention, a 72 MHz master clock generates a Gray code with 3.5 ns resolution which allows a 13 bit conversion in 30 μs. A 500 MHz master clock generates a Gray code with 500 ps resolution, allowing a 16 bit conversion in 33 μs or 12 bits in 2 μs. With several hundred of these converters on one chip, the total conversion rate may be on the order of 100 MHz. The estimated power is less than 50 μw per channel. As a result, the present invention allows the relatively slow single slope method of analog-to-digital conversion, when a array of such converters are used on a single chip, to provide relatively high conversion rates, while consuming low amounts of power. Furthermore, the simple design of single slope analog- to-digital converters saves power and allows integration of a large number of these converters on a single integrated circuit, particularly when using CMOS technology. A Gray code count is used as the digital signal to be stored when the comparator is activated because, by definition, only one bit changes for each increment of the code. Since only one of the Gray coded bits can be in the process of changing when the latch is enabled, only one of the sampled bits can exhibit metastability, and the resultant code will be uncertain by only one least significant bit. This is in contrast to the case when a standard binary code is used as the digital signal to be stored. Since more than one bit may be changing for each increment of the code, a number of the sampled bits can exhibit metastability. The use of a Gray code count also advantageously allows the metastability resolution to be determined at a point in the circuit where there is more time to complete it, thus reducing the power and speed requirements of the circuitry. As a result, in the present invention, the metastability resolution can be postponed until after the multiplexing of the data when the data rate is considerably lower than the rate at which the data is provided by each analog-to-digital converter. In particular, in conventional circuits, the metastability resolution might typically be provided when the binary code from a counter is clocked into the N-bit data latches. This might require the metastability resolution to be performed in a very short time interval at a relatively high clock rate. As noted, by contrast, the present invention can accomplish this function using a significantly lower clock rate, which reduces the power and speed requirements of the circuitry. For example, a 75 MHz (approximately) clock may be used to generate the Gray code. This 75 MHz clock is generated from a 6 MHz (approximately) input clock. The 75 MHz clock is used only for the analog-to-digital conversion; the 6 MHz clock is used for all other functions of the integrated circuit. By using an array of 328 converters, a conversion is completed in approximately 30 microseconds with a resolution of 13 bits. A conventional approach requires a master clock frequency of approximately 300 MHz, which is higher than the capability of, for example, a conventional 2 micron CMOS process. The use of a gray code allows the metastability resolution to be performed at a rate of 6 MHz on 13 bits rather than 300 MHz on 328 comparators. The invention also provides a method of converting the analog signals from an array of analog-to-digital converters with a high effective clock rate, and increased resolution. A multitude of input signals, one for each converter, are sampled and held. Signals are formed by integrated the current from an analog source. This signal is held constant on a capacitor for the duration of the conversion process. Next, an analog ramp and a digital counter are simultaneously started. A comparator circuit compares the voltage of the ramp with the sampled and held voltage. When the two are equal, the output of the comparator changes state and causes the value of the digital counter to be stored in an N bit latch. The values stored in the array of latches, which are a digital representation of the various input voltages, are transferred in parallel to another array of latches. Then a new set of conversions can be performed while the results of the previous conversions are multiplexed to form a digital output signal. For an array of converters, the digital counter and ramp generator are common to all converters. Each converter itself needs only a sample and hold, a comparator, and an array of digital latches. The circuits of the present invention may be monolithically integrated in semiconductor form using convention CMOS technology. Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.

Claims: CLAIMS 1. An apparatus to convert an analog signal input to a digital signal output comprising: (a) means for generating an analog ramp having an analog ramp output; (b) means for generating a Gray encoded digital ramp having a digital ramp output; (c) means for comparing the analog signal input with the analog ramp signal, wherein the means for comparing has a comparison output; and (d) means for storing data comprising a first data input connected to the digital ramp output and an enable input connected to the comparison output, comparison output, wherein the means for storing data has a metastability resolved digital signal output. 2. The apparatus of claim 1 wherein the means for storing data further comprises: (a) a data latch connected to the digital ramp output and an enable input connected to the comparison output, wherein the data latch has a data output; and (b) a flip-flop connected to the data output, wherein the flip-flop outputs the metastability resolved digital signal output. 3. The apparatus of claim 1 further comprising means for sampling the analog signal input. 4. The apparatus of claim 1 further comprising means for decoding the metastability resolved digital signal output. 5. The apparatus of claim 4 wherein the means for decoding the metastability resolved digital signal output further comprises a plurality of XOR gates. 6. The apparatus of claim 1 wherein the means for comparing further comprises an analog comparator. 7. The apparatus of claim 1 wherein the digital ramp output has a least significant bit, a next-to-least significant bit and most significant bits, and wherein the means for generating a Gray encoded digital ramp comprises: (a) a digital clock with a clock output; (b) a synchronous counter having a synchronous counter output with a counter least significant bit; (c) an XOR Gray encoder connected to the synchronous counter output, wherein the XOR Gray encoder has a Gray code output connected to the most significant bits; (d) a first phase shifter connected to the clock output, wherein the first phase shifter has a first phase shifted output connected to the least significant bit; and (e) a second phase shifter connected to the counter least significant bit, wherein the second phase shifter has a second phase shifted output connected to the next-to-least significant bit. 8. The apparatus of claim 7 wherein the first phase shifter comprises: (a) a voltage controlled delay having a voltage control delay output; (b) a phase detector having a first input connected to a reference clock and a second input connected to the voltage control delay output, wherein the phase detector has a phase output; and (c) an operational amplifier connected to the phase output, wherein the operational amplifier has an amplifier output connected to control the voltage controlled delay. 9. The apparatus of claim 1 wherein the means for generating an analog ramp, means for generating a Gray encoded digital ramp, means for comparing, and the means for storing data is fabricated on a monolithic semiconductor chip. 10. The apparatus of claim 9 wherein the monolithic semiconductor chip comprises CMOS. 11. An apparatus to convert a plurality of analog signal inputs to a plurality of digital signal outputs comprising: (a) at least one analog ramp generator means for generating an analog ramp having at least one analog ramp output; (b) at least one digital ramp generator means for generating a Gray encoded digital ramp having at least one digital ramp output; (c) a plurality of comparator means for comparing one of the plurality of analog signal inputs with at least one analog ramp signal, wherein the plurality of comparator means has a plurality of comparison outputs; and (d) at least one storage means for storing data having at least one first data input connected to the at least one digital ramp output and a plurality of enable inputs connected to the plurality of comparison outputs, wherein the at least one storage means has at least one metastability resolved digital signal output. 12. An analog-to-digital converter apparatus, wherein there are a plurality of analog signals, the analog-to-digital converter comprising: (a) means for generating an analog ramp having an analog ramp output; (b) means for generating an encoded digital ramp having a digital ramp output; (c) a plurality of means for selecting one of the plurality of analog signals, having a plurality of selected signal outputs; (d) a plurality of means for sampling the plurality of selected signal outputs having a plurality of sample outputs; (e) a plurality of means for comparing the plurality of sample outputs with the analog ramp signal having a plurality of analog comparison outputs; (f) a plurality of first means for latching data having a first data input connected to the digital ramp output and a latch enable input connected to the plurality of comparison outputs having a plurality of first latched outputs; (g) a plurality of second means for latching data each having a second data input connected to one of the plurality of first latched outputs, the second means for latching having a plurality of second latch outputs; and (h) means for multiplexing the plurality of second latch outputs having a Gray digital output. 13. The apparatus of claim 12 further comprising decoder means for decoding the Gray digital output into a decoded digital signal output, wherein the decoder means has an input connected to the Gray digital output and wherein the decoder means has a digital signal output. 14. The apparatus of claim 12 wherein the means for generating an encoded digital ramp further comprises a Gray encoder. 15. The apparatus of claim 12 wherein a single analog waveform generator and digital counter are shared. 16. The apparatus of claim 12 wherein the digital ramp output has a least significant bit, a next-to-least significant big and most significant bits, and wherein the means for generating an encoded digital ramp comprises: (a) a digital clock with a clock output; (b) a synchronous counter having a synchronous counter output with a counter least significant bit; (c) an XOR Gray encoder connected to the synchronous counter output, wherein the XOR Gray encoder has a Gray code output connector to the most significant bits; (d) a first phase shifter connected to the clock output, wherein the first phase shifter has a first phase shifted output connected to the least significant bit; and (e) a second phase shifter connected to the counter least significant bit, wherein the second phase shifter has a second phase shifted output connected to the next-to-least significant bit. 17. The apparatus of claim 16 wherein the first phase shifter comprises: (a) a voltage controlled delay having a voltage control delay output; (b) a phase detector having a first input connected to a reference clock and a second input connected to the voltage controlled delay output, wherein the phase detector has a phase output; and (c) an operational amplifier connected to the phase output, wherein the operational amplifier has an amplifier output connected to control the voltage controlled delay. 18. The apparatus of claim 12 wherein each of the plurality of means for comparing further comprises an analog comparator. 19. The apparatus of claim 12 further comprising a phase locked loop clock generator coupled so as to clock at least the means for generating an encoded digital ramp. 20. The apparatus of claim 12 further comprising a state machine sequencer coupled to control the means for generating an analog ramp. 21. An analog-to-digital conversion method comprising the steps of operating an unclocked comparator, where the unclocked comparator includes comparator inputs, to compare an input analog voltage to a voltage ramp; and enabling a digital latch to store a Gray coded digital timer word when the comparator inputs are substantially equal. 22. The analog-to-digital conversion method of claim 21 further including the step of transferring the Gray encoded digital timer word stored in the digital latch to a second latch to resolve metastable states. 23. The analog-to-digital conversion method of claim 21 further including the step of decoding the Gray coded digital timer word to a standard binary code. 24. The analog-to-digital conversion method of claim 21 further including the step of generating the Gray coded digital timer word with a clock signal that is a multiple of an externally supplied clock signal. 25. The analog-to-digital conversion method of claim 21 further including the step of generating the Gray coded digital timer word, wherein the digital timer word further comprises a plurality of bits, wherein one or more bits are generated directly from a master clock by analog phase shifting.

Inventors: BUTLER NEAL R, BUTLER, NEAL, R.
Applicants: BUTLER NEAL R, LOCKHEED MARTIN IR IMAGING SYS, BUTLER, NEAL, R., LOCKHEED-MARTIN IR IMAGING SYSTEMS
Application year: 1996
Main classification: H03M1
IPC codes: H03K23, H03M1