RECURSIVE MULTI-BIT ADC WITH PREDICTOR

Abstract: A flash converter (104) is preceded by an accurate continuous-time error amplifier (102) operating on the difference between the input signal and a feedback DAC (114). The DAC output is operatively coupled to the amplifier virtual ground through, for example, a set of precision capacitors. The input circuit is also coupled to the amplifier input through a continuous-time element such as a set of precision capacitors, approximately equal in capacitance to those coupled to the DAC (114). The amplifier (102) may have a moderate gain such as (16) with a high-pass characteristic beyond, for example 1 KHz. The DAC is controlled by the latched output of a digital signal processing block, which uses digital outputs from the flash converter (104) and the last latched output to predict the next value of the input signal. Converter control loop stability is afforded by providing a lowpass character to the prediction circuit. The converter produces a first digital result by adding the digital value produced by the flash (104), properly scaled, to the current value of the digital latch (112) driving the DAC (114). The output of the converter is a second digital result produced by passing the first digital result through a simple lowpass digital filter (116). The second digital result may be sub-sampled at any arbitrary phase of the input sampling clock to permit optimum-phase data recovery.

Description: RECURSIVE MULTI-BIT ADC WITH PREDICTOR BACKGROUND OF THE INVENTION Field of the Invention This invention relates in general to analog-to-digital converters (ADCs) and more particularly to high-speed, high-resolution, monolithic integrated circuit ADCs. Description of the Related Art Prior art analog to digital converters (ADCs) requiring high conversion rates and low latency typically use a string of stacked comparators connected to progressive voltages in a resistive ladder in a configuration well known as a basic flash converter. Significant amounts of research funds in telecommunications technology are continually being invested in order to increase speed and resolution within a limited number of prior art ADC circuit architectures. Basic flash converters operate at very high speeds, in large part because they produce a complete data conversion for each clock cycle. An n-bit flash converter, however, typically requires 2 π comparators, which are connected to progressively increasing voltage steps on a resistive ladder. The outputs of the comparators may be routed to a circuit widely known as a "thermometer" binary digital coding circuit to produce an n-bit binary output. As its name implies, the main advantage of the flash converter is its speed. Furthermore, these devices require no input sample-and- hold. On the other hand, flash converters suffer from several well-known limitations, which become more pronounced at resolutions beyond eight bits. As is pointed out above, the number of comparators increases exponentially with resolution. Consequently, 256 comparators are needed even for modest 8-bit resolution. Further limitations include differential phase errors from clock skew, very large circuit area, and significant charge blowback. Recent work directed toward overcoming the limitations described above has focused on multiple-stage flash converters (also known as "pipelined" flash converters) that use fewer bits per stage and thus reduce blowback, clock skew and circuit area. In these multiple-stage devices, each stage includes an input sample- and-hold circuit. Each stage in the pipeline then uses its own, separate flash converter to assess the magnitude of the input, and it then and feeds the digital result to a digital-to-analog converter (DAC) in a feedback loop. The difference between the DAC result and the input sample-and-hold output is amplified and fed to the sample-and-hold input of the next stage. Once the next stage has sampled the amplifier output signal, the previous stage is free to receive the next input signal. As an example of one problem associated with the pipelined arrangement, assume the amplifier in the pipelined flash converter has a moderate gain of 16. The input signal to the first stage must in any event be sampled and held while the first flash converter estimates its amplitude and while the estimated amplitude output of the feedback DAC is subtracted from the input signal. Note that if the input signal were not sampled and held, it could change in amplitude by an amount sufficient to render the first flash digital amplitude estimate invalid or to drive the first stage amplifier into saturation before the output error signal is even sampled by the second stage. Thus, it is possible to increase the accuracy of a second flash converter in a pipelined stage by preceding it with an amplifier, but this in turn generally necessitates a sample-and-hold circuit. Pipelined flash converters are capable of high speeds, such as 100 MHz. Their resolution, however, has typically been limited to around ten bits because of differential linearity errors caused when the first error signal is handed off to the second stage in the pipeline. Another problem has been the lack of high integral linearity in the first stage DAC, which controls the overall linearity of the converter. These converters are therefore usually used in medium-resolution applications, from ten to twelve bits, where the inherent latency of three to five successive conversions does affect the quality of the output data. In order to overcome the differential linearity limitations in basic and pipelined flash converters, much work in increasing converter resolution has focused on using sample-and-hold circuits and recursive techniques that employ just one feedback DAC with high differential and integral linearity. In these devices, the input is always sampled and held, after which the error signal between the DAC and the input signal is used on multiple cycles to drive the DAC toward the input signal. The final result is the DAC digital input word. In early recursive converters, a single high-gain comparator processed the error signal. In these devices, the comparator output was then used to perform a binary search for the unknown value of the input signal using a technique widely known as successive approximation. A successive approximation register (SAR) in a feedback path received the comparator output and controlled the feedback DAC to perform a binary search for the held input voltage. Such converters required a relatively long time to convert the unknown input signal to a digital value because several clock cycles were required for each data conversion. With a good feedback DAC, such converters were capable of 16-bit resolution, but at least one clock cycle was required for each bit of resolution. Furthermore, an input sample-and-hold circuit was required to preserve the input amplitude during the time required for the conversion. Additionally, droop in the sample-and-hold over the time required for the conversion led to differential linearity inaccuracies in the converter unless redundant successive approximation steps near the end of the conversion were provided. A significant improvement in the basic successive approximation converter was made when the single comparator was replaced by a moderate-resolution flash converter and the high-gain amplifier was replaced with a moderate but controlled- gain amplifier. The signal corresponding to the error between the input signal and the feedback DAC could then be converted (after amplification) by the multi-bit flash converter and processed in the feedback path by an adder, whose output was latched and controlled the DAC. The output of the latch also formed the second input of the adder. Although these improved successive approximation devices could produce more than one bit on each clock cycle, they still required an input sample-and-hold circuit. Another drawback stemmed from the amplifier: In order to provide high accuracy in these converters, the gain of the amplifier had to be increased on progressive steps. The settling time of the amplifier on high-gain steps was long, however, and significantly slowed the converter. Further work on multi-bit recursive converters has focused on improving the speed and accuracy of the DAC in the feedback path. The four or five clock cycles required for the analog-to-digital conversion, however, still limits the overall speed of the converter. When converting analog signals, it is desirable to remove input signal energy in the frequency range near the sampling frequency of the converter. If the converter sampling frequency can be greatly increased, for example, by sampling the input every clock cycle, then the order and cost of the anti-aliasing filter required in the input signal amplifier chain can be greatly reduced. Accordingly, converters known as "sigma-delta" converters later became popular, since they were able to sample the input on each clock cycle and thus greatly ease anti-aliasing. This proved especially beneficial in filter design for audio applications. Early sigma-delta converters used simple, one-bit DACs, which provided inherently high integral linearity without requiring precision in the feedback DAC layout design. In the sigma-delta converter, an error signal produced by the difference between the input voltage and the one-bit DAC is fed to one or more discrete-time switched-capacitor integrating amplifiers before application to the one- bit comparator. The number of integrating amplifiers generally increases with the order of the loop. The one-bit sigma-delta is at present the common choice for audio applications. One drawback of the sigma-delta converter is that it requires a very high over-sampling ratio (ratio of sampling rate to Nyquist sampling rate) in order to achieve high resolution. Other drawbacks relate to the order of the converter loop. It is well known, for example, that the order of the signal processing feedback loop may be increased to increase the input signal bandwidth for the same converter resolution. The number of integrating amplifiers provided must be increased, however, in proportion to the order of the required loop filter. Other well-known limitations of the sigma-delta converter include spurious tones and an output noise spectral density that increases with frequency at a rate that is proportional to the order of the converter loop. In addition, the one-bit comparator output in such a converter must be processed over several clock cycles by a digital decimation filter at the output. It is well known that this decimation filter can become quite complex because it must have an order at least one higher than the basic loop in order to remove the large amount of converter quantization noise in the frequency region above the input signal Nyquist bandwidth. The reason for this increase in quantization noise is the inherent reduction of gain through an integrating amplifier with increasing frequency. Furthermore, sigma-delta converters sample the input on each clock cycle, and they do not produce a full conversion for every sample. First, the input sampling circuit is generally part of the input circuits for the discrete-time integrators used in these devices. Second, if an input sampling circuit were not used, inaccuracies in the integrator output would arise as a result of any time jitter or glitch energy in the DAC output every time it switched. Much early work on sigma-delta converters concentrated on increasing speed beyond the audio signal frequency range by increasing the order of the converter. Because increasing the order of a sigma-delta converter increases the slope of the output noise spectrum, a higher order decimation filter was typically required to recover the output data. Additionally, many techniques were developed to cope with increased stability problems and tones in higher order loops. Despite increasing the order of the loop, it is still difficult to construct a one-bit sigma-delta converter that can process signal frequencies with bandwidths an order of magnitude higher than the audio range. Nonetheless, processing such signal frequencies is required to fulfill many of the tasks required for telecommunications data transmission over, for example, copper wire subscriber loops. In an effort to increase the sigma-delta ADC sampling frequency for high-resolution telecommunications applications, designers have replaced the single comparator in the sigma-delta converter with a multi-bit flash converter at the output of the integrating amplifier chain in order to obtain more bits per conversion step. The multi-bit flash output is, in these designs, usually connected to a multi-bit feedback DAC. Such multi-bit flash signal processing reduces the quantization noise of the converter, but it loses the ease of DAC construction offered by the one-bit converter. Additionally, the quantization noise of the converter still increases with frequency in proportion to the order of the converter. One attempt to ease DAC layout requirements in multi-bit sigma-delta converters has concentrated on the implementation of several different multiple-unit capacitive charge redistribution and switching techniques. In these devices, the feedback DAC is usually implemented by switching identical precision capacitors to either a positive or negative reference voltage rather than by switching just one capacitor to a voltage output DAC. Several techniques exist to alternate the precision capacitors in such a way as to reduce detrimental integral linearity effects due to inaccuracies in any one capacitor. In some of these devices, for example, the feedback capacitors are alternated in a pseudo-random fashion to provide acceptable integral linearity. The multi-bit flash comparator in such a sigma-delta converter has certain speed advantages because the quantization noise form the flash is reduced in proportion to the number of comparators used. The signal bandwidth may be then also be increased considerably over the one-bit case, while still retaining a high signal-to-quantization-noise ratio. Because multiple capacitors are used in these systems, however, large switching current pulses are required in order to charge the input capacitor used for input voltage-to-charge conversion. Furthermore, the multi- bit sigma-delta converter still requires an input sampler and a complex output decimation filter with an order at least one higher than the basic loop. In many telecommunications applications, such as copper wire-based digital subscriber loops, the noise performance of the ADC is most important at the highest frequencies. In these applications, however, the line attenuates the transmitted signals at high frequencies far greater than at low frequencies. Moreover, the overall resolution of the ADC is not nearly as important as the ADC equivalent resolution or quantization noise spectral density at the highest signal frequencies, which is the very the frequency region where sigma-delta converters have the worst noise performance. What is needed is therefore a high-resolution ADC system that retains such advantages of the basic flash converter as sampling the input at clock rates as high as the clock rate itself and completing a conversion on each clock cycle. This is especially true in the area of high-speed data transmission over channels such as copper wires (including twister pairs). At the same time, the system should avoid the conventional need for a pre-amplification sample-and-hold circuit, since it would then also be able to avoid the transient currents required in existing devices in order to drive the input sampling circuit. The ADC should be able to produce a first digital result with a quantizing noise that does not increase significantly with frequency. This in turn would eliminate the requirement for a high-order decimation filter and make possible the use of a second- or third-order lowpass digital filter to produce a second digital result with increased resolution. This invention provides such as ADC system. SUMMARY OF THE INVENTION An analog-to-digital converter (ADC) according to the invention includes a fast internal analog to digital converter (ADC) such as a flash converter. The output of a continuous-time amplifier with a gain A and a high-pass transfer characteristic is coupled to the input of the fast internal ADC. A signal prediction circuit, which predicts a next value of the system input signal, is coupled in a feedback path. Its input is the output of the fast ADC, preferably scaled down in a scaling circuit by a factor equal to the gain A of the amplifier. The prediction circuit preferably has a low-pass characteristic. The output of the prediction circuit, which is preferably latched, is then applied as the input to a feedback digital-to-analog converter (DAC). An input differencing circuit then outputs an analog error signal that is the difference between the system input signal and the output from the DAC. The analog system input signal does not have to be sampled and held before being differenced with the feedback signal from the DAC. The digital system output signal is formed as the sum of the predicted next input signal value and the scaled output of the fast ADC. This summed signal is preferably low-pass filtered. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified block diagram of the flash analog-to-digital converter (ADC) according to the invention. Figure 2 illustrates a preferred differencing and error amplification portion of the circuitry of the ADC according to the invention. Figure 3 illustrates the general structure of a simplified feedback digital-to- analog converter (DAC) used in the invention, which utilizes capacitive charge redistribution techniques. Figure 4 is a block diagram that shows the structure of a prediction circuit used in the invention. DETAILED DESCRIPTION Figure 1 is a simplified block diagram of the analog-to-digital converter (ADC) according to the invention. As the figure shows, the analog input signal Vin, that is, the analog signal one wishes to convert into a digital approximation, forms one input to an analog input differencing circuit (subtractor) 100, the output of which forms an input signal to an amplifier 102. Note that no sample-and-hold circuit is required for the input signal Vin. Various other components may be needed to operate the ADC according to the invention. For example, a master clock must be provided to generate the clock pulses that synchronize several of the invention's components. As another example, voltage supplies are needed to provide not only reference voltages (described below), but also driving current and voltage for certain elements such as an operational amplifier. Because components and circuits such as these are so well understood, they are neither specifically discussed nor illustrated. The amplifier 102, for reasons given below, preferably has a moderate gain A of, for example, 16, as well as a high-pass characteristic. The amplifier's output, which is labelled as the analog signal S1 , is connected to the input of a conventional nine-bit flash converter 104. The converter 104 thus constitutes an internal ADC, within the ADC formed by the entire invention. Using a flash converter as a reduced-bit-length "core" converter provides speed without requiring the unacceptably large number of comparators one would need for, for example, a full 16-bit flash converter. The output of the flash converter 104 is shown as being ten- bit resolution. As is well understood in the art of ADC design, the "extra" bit corresponds to the result of the excess signal energy that falls between the lowest- order channel boundaries. In the following description of the invention, various gains and resolutions (in number of bits or specified by a number of components) are given. Unless stated otherwise, however, these numbers are only examples. They have been chosen in part because they are the values used in a functioning prototype of the invention, and in part because they provide both computational efficiency and stability given, for example, the physical limitations of existing amplifiers. Certain alternative choices are explained below. Others will be obvious to those skilled in the art of analog-to-digital converters. The ten-bit output of the flash converter 104 is scaled down by a factor equal to the gain of the amplifier 102 before being applied as input signals to both a digital summer 106 and a digital signal prediction circuit 108. In the case where the gain of the amplifier 102 is a power n of two, that is, 2**n, then the scaling down of the output signal of the flash converter 104 can be carried out quickly and easily using an n-bit digital shifter 110, as is well known in the field of digital design. In Figure 1 , the output of the divider/shifter 110 is labeled S3. If the gain of the amplifier 102 is chosen to be something other than a power of 2, then a suitable conventional digital dividing circuit will need to be used in place of the shifter 110. The 14-bit output (labeled S5) of the digital signal prediction circuit 108 is first latched by a conventional latching circuit 112, whose latched output value forms not only the input signal to a feedback digital-to-analog converter (DAC) 114 but also the second input signal to the digital summer 106. As the figure shows, the input signal to the filter/predictor 108 has ten bits, whereas its output has 14 bits. The additional four bits are generated by the scaling inherent in the filter, whose output signal should range over the full scale as the system input signal. The prediction circuit 108 thus needs to compensate for the four-bit downshifting of the shifter circuit 110 and increase the significance of its result by four bits. The output of the latch 112 is preferably updated on the positive-going edge of the ADC master clock so that, on every positive clock edge, the DAC 114 input is updated and held. The analog output of the DAC 114 forms the second input to the input differencing circuit 100. Finally, the 14-bit output (labeled S4) of the summer 106 is preferable passed through a digital low-pass filter 116, whose 16-bit output is also the final, digital converted output signal OUT of the entire ADC according to the invention. The lowpass filter 116 is preferably included in order to restrict the bandwidth of the output signal to be approximately the same as the bandwidth of the system input signal Vin. This, in turn, reduces the quantization noise in the frequency region above the passband of the input signal. It is well known that, at increased sampling frequencies, and with band limiting, one can achieve more bits of resolution by the use of a lowpass filter. An increase from 14 to 16 bits is merely one example of the results of a filter choice that proved effective in simulations of the invention. Any conventional design techniques may be used to select a proper digital lowpass filter 116. In other words, the output of the flash converter 104, after scaling to compensate for the gain of the preceding amplifier 102, is fed back to the amplifier 102 input along a negative path that also includes a circuit that predicts the next value of the input signal. Also, although the structure and function of the prediction circuit 108 is described in greater detail below, one can observe already at this point that if the latched value of the prediction circuit's output is as close to the digital equivalent of the analog input signal Vin as the given resolution allows, then the scaled output of the flash converter 104 will be negligible, so that the latched value will pass through the summer 106 to be the system's output signal OUT. Figure 2 shows in greater detail the preferred embodiment of the input portion of the invention, which includes the differencing circuit 100, the amplifier 102, and the DAC 114. Figure 2 also illustrates the preferred method by which the input voltage summation is carried out using capacitive charge redistribution techniques. Note that although this input portion resembles the input circuit of a conventional multi-bit, sigma-delta, oversampled ADC, it does not require the input signal (voltage) to be switched, or sampled and held. As Figure 2 shows, the input voltage signal Vin is applied to a capacitive bank Cin, which is constructed in a known manner using, for example, 64 unit capacitors. The unit of capacitance may be chosen in any conventional manner. The input is thus operatively coupled in a continuous-time manner to the virtual ground of an operational amplifier 202. The operational amplifier 202 forms the primary gain component of the amplifier 102, and has a feedback capacitor Cfb and a feedback resistor Rfb coupled in parallel in its feedback path. The capacitance of the feedback capacitor Cfb is preferably equal to four units. The DAC 114 is preferably a capacitor charge redistribution DAC, whose output is coupled to the same (subtractive) input of the operational amplifier 202 as the output of the capacitor bank Cin and the feedback path. The operational amplifier 202 thus has the preferred voltage gain of 16 with respect to the difference between the input voltage Vin and the DAC 114 output voltage since the ratio of Cfb to Cin is 64/4=16. The operational amplifier's 200 output constitutes the signal S1. The DAC 114 operates to produce an output charge with full amplitude equal to a positive reference voltage Vref times the capacitance, which is equal to the 64 unit capacitors of the capacitor bank 100 (Figure 2). This charge, as well as the charge resulting from the input voltage being applied to the 64 unit input capacitors, are summed at the summing junction of the operational amplifier and converted to an output voltage with an effective gain of 16 by the feedback capacitor Cfb. Signal polarities are chosen around the ADC control loop as shown so that, for positive input voltages and input charge, the DAC will tend to produce negative input charge. When the DAC code closely represents the ADC input voltage Vin, then the charge difference between the input and DAC charge is small and the output voltage from the operational amplifier remains small and thus well short of its saturation voltage. Figure 3 illustrates in simplified form the general structure of the preferred configuration of the capacitor charge redistribution DAC 114 used in the invention. This configuration is preferred because it is fast. Any other conventional DAC may be used instead, however, as long as its output charge is proportional to its digital input signal and it is fast enough to provide a stable output signal in less than one clock cycle. Assume now for the sake of simplicity that the input signal has only six bits. In the example, of Figure 3, the least significant three bits (LSB) of the input signal DAC_in drive a series of 2 3 = 8 functionally single-pole, single throw solid-state switches LSB_SW, each of which is connected to a corresponding branch of a resistive ladder (the series of resistors labeled R) that linearly divides a voltage range from +Vref to -Vref. The three most significant bits (MSB) of DACjn drive a series of 2 3 = 8 functionally single-pole, triple-throw solid-state switches MSB_SW, which couple eight corresponding unit capacitors 1C to either the corresponding bit, to +Vref, or to -Vref. The combined charges on the capacitors 1C form the DAC's 114 output signal DAC_out, which is coupled to the operational amplifier 102 (Figure 2). The switches MSB_SW and LSB_SW are preferably operated progressively using the well-known decoding technique known as "thermometer decoding." Because this technique, as well as the construction and operation of switches such as MSB_SW and LSB_SW, are well known in the art of analog and digital conversion, they are not described further here. In the illustrated embodiment of the invention, the input to DAC 114 has 14 bits. In the preferred embodiment of the invention, these are partitioned into 6 MSBs and 8 LSBs. There will therefore be 2 6 = 64 unit capacitors (and corresponding MSB_SW switches) and a ladder of 2 8 = 256 resistors R (and corresponding LSB_SW switches). Conventional thermometer decoding is then used to control the switches in the well-understood manner. Figure 3 thus illustrates the simplified 3:3 MSB:LSB cases merely to reduce the complexity of the figure while clarifying the structure. Figure 4 illustrates one embodiment of the digital signal prediction circuit 108 that was tested successfully in a simulation of the invention. In this figure, standard symbols Σ and z 1 are used to designate, respectively, summers and first-order backward shift operators. Thus, z "1 S 5 (t) = S 5 (t-1). In other words, each z "1 block latches and ouputs the value of its input from one time unit (in this case, clock cycle) earlier. Both the function and construction of summers and z ~1 blocks are well understood in the art of digital design and are therefore not described further. The block labeled 400 functions as an operator to increase by one bit the significance of its input signal. This can be done simply by proper routing of the wiring for the input bits, or by a single left-shift operation, which has the effect of multiplying the input value by two. Using conventional techniques, the circuit shown in Figure 4 can be shown to produce the following results: 54 = S3/(1-z "1 ) 2 55 = S3 [(2-z "1 )/(1-z- 1 ) 2 ] In other words, the prediction circuit is a second-order digital lowpass filter. It is well known that the error at the output of the summing junction of a closed-loop linear system is reduced by a factor equal to the inverse of the effective loop gain for any given frequency. Furthermore, in order to prevent saturation of the gain element, its input must be bounded to fall within limits (+/-) of no more than the inverse of its gain. It can be shown through conventional calculations using linear filter, as well as simulations, that the filter illustrated in Figure 4 meets these requirements for the chosen amplifier gain of 16. As such, the output of the filter will fall within 1/16 of the full-scale value R of the input signal for any frequencies in the chosen bandwidth. Consequently, the filter 108 acts to "predict" the next value of the input signal in the sense that the output of the filter will fall within the allowable 1/16 range to prevent amplifier saturation. Known design methods may be used, however, to replace the illustrated filter 108 with any other that meets the same requirements. The lowpass characteristic of the prediction circuit also ensures ADC control loop stability without requiring a lowpass function in the gain amplifier 102 that drives the flash converter 104. The output of the prediction circuit block is held in the a latch 112 and is updated only once per clock cycle because data patterns change with time in the digital signal prediction block. The flash converter 104 and the digital signal prediction block 108 operate at a higher frequency than that of the input signal Vin in order to avoid the introduction of any unwanted lags, and to provide adequate settling time for the various components. In a system involving conversion of signals for transmission over a twisted pair of copper conductors, for example, the bandwidth of the input signal was approximately 1 MHz, whereas the bandwidth of the flash converter and the prediction circuit were chosen to be roughly the same as that of the transmission channel itself, that is, roughly 35 MHz. Both the prediction circuit 108 and the filter 116 have low-pass characteristics. Because the result of the flash converter 104 is produced prior to the lowpass characteristic in the loop, however, the quantization noise spectral density produced by the flash converter is relatively constant with frequency. The first digital result does not therefore have a quantization noise spectral density proportional in frequency to the order of the digital filter in the digital signal prediction circuitry. As is mentioned above, one use of the invention that has proven particularly advantageous is for conversion of signals for transmission over twisted-pair conductors. At present, the bandwidth of such a transmission channel is roughly 35 MHz. Tests and experience have shown that the gain of the amplifier 102 (Figure 1) should not be more than about 25 in order to stay well within the stability range of the circuit. A gain of 16 was chosen because 16 is the largest power of two less than this maximum advisable gain. Having a power of two as the gain not only simplifies other circuitry, such as the divider 110, but it is also computationally efficient, since multiplication and division correspond to simple 4-bit left and right shifts of digital words. One other advantage of a gain of 16 is that it has proven efficient even from the standpoint of working within the physical limits of the components used to implement the amplifier. An increase of gain for its own sake, for example, means only that more energy goes into charging the various capacitors in the system. Nonetheless, the moderate gain of 16 is only one possible choice. For use in systems with greater or less bandwidth, conventional experiments, calculations and test results may be used to select a different gain. Necessary corresponding changes to other components of the system will then also be obvious. Certain differences and advantages of the invention as compared with the prior art should now be clear. Because the next value of the input signal is predicted with an accuracy higher than, for example, 1/16 of full scale, the amplifier 102 will never saturate even with no sample-and-hold circuit at the input of the converter. In addition, the output of the amplifier 102 with a gain of 16 is not saturated at the time of the next predicted sample. As a result, the ADC according to the invention is able to produce a full conversion on each clock cycle. Furthermore, unlike the prior art converters described above, the DAC 114 is not connected directly, or through any simple adder or SAR, to the flash comparator output. Instead, the DAC is driven by the highly accuracy digital signal processing circuit 108, which provides an estimate of the next, not the previous, value of the input signal. Providing to the DAC a highly accuracy estimate of the next value of the input signal allows the use of a moderate gain (for example, the illustrated gain of 16), amplifier to be used after the input differencing circuit 100. Such a gain of 16 is increases the effective resolution of the flash converter by 4 bits from 9 to 13 bits (16 = 2 4 ). If the ADC is clocked at a frequency that is 32 times the signal bandwidth, or 16 times the normal Nyquist sampling frequency, then another 2-bit increase in resolution results. The converter is then capable of achieving quantizing noise levels equivalent to 15 bits of resolution while still maintaining sample clock rates as high as, for example, 30 MHz or higher. If the converter output is obtained from the sum of the digital code driving the DAC and the next flash converter digital output (down-shifted by 4 bits, for example in circuit 110), then a complete data conversion is produced for each clock cycle. Because the use of integrating amplifiers is avoided, the output digital filter required to provide the 2-bit increase in resolution obtained by oversampling may be of low order and minimum complexity. The use of integrating amplifiers is avoided by guaranteeing system stability with a lowpass digital filter characteristic in the digital prediction circuit 108. One other advantageous feature of the invention is its use of continuous-time elements such as the precision capacitors in the bank 100 (Figure 2), which are approximately equal in capacitance to those coupled to the DAC 114. (There are preferably 64 unit capacitors in each.) The error amplifier 102 may also be made entirely of continuous-time components with a high-pass characteristic beyond, for example 1 KHz.

Claims: CLAIMS 1. An analog to digital converter for converting an analog system input signal (Vin) to a digital system output signal (OUT), comprising: a fast internal analog to digital converter (ADC - 104); an amplifier (102) with a gain A; a signal prediction circuit (108) coupled in a feedback path; a digital to analog converter (DAC -- 114) included in the feedback path; an input differencing circuit (100); in which: an output of the DAC (114) and the analog system input signal (Vin) are connected as inputs to the differencing circuit (110); an output of the differencing circuit (110) is connected as an input to the amplifier (102); an output of the amplifier (102) is connected as an input to the fast ADC (104); a scaled output of the fast ADC (104) is connected as an input of the signal prediction circuit (108); an output of the signal prediction circuit (108) is connected as an input of the DAC (114); the digital system output signal (OUT) is formed as a filtered sum of the output of the signal prediction circuit (108) and a scaled output of the fast ADC (104); whereby the output of the signal prediction circuit (108) lies continuously within a factor of 1/A times a full-scale range value of the analog input signal (Vin). 2. An analog to digital converter as in claim 1 , further comprising a scaling circuit (110) that reduces the output of the fast ADC (104) by a factor of the amplifier gain A and that is connected immediately after the ADC (104). 3. An analog to digital converter as in claim 1 , in which the amplifier (102) is a continuous-time analog amplifier. 4. An analog to digital converter as in claim 1 , in which the amplifier (102) has a high-pass transfer characteristic. 5. An analog to digital converter as in claim 1 , in which the signal prediction circuit (108) has a low-pass transfer characteristic. 6. An analog to digital converter as in claim 1 , in which the analog system input signal (Vin) is connected to the differencing circuit (100) as an unsampled and unheld input signal. 7. An analog to digital converter as in claim 1 , further including a digital latch circuit (112) connected in the feedback path between the signal prediction circuit (108) and the DAC (114). 8. An analog to digital converter as in claim 1 , further including a digital low-pass filter (116) having as its input the sum of the output of the signal prediction circuit (108) and a scaled output of the fast ADC (104) and having as its output the digital system output signal (OUT). 9. An analog to digital converter according to claim 1 , further comprising: a scaling circuit (110) that reduces the output of the fast ADC (104) by a factor equal to the amplifier (102) gain A and that is connected immediately after the ADC (104); a digital latch circuit (112) connected in the feedback path between the signal prediction circuit (108) and the DAC (114); a digital low-pass filter (116) that has, as its input, the sum of the output of the signal prediction circuit (108) and a scaled output of the fast ADC (104) and that has, as its output, the digital system output signal (OUT); in which: the amplifier (102) is a continuous-time amplifier with a gain A with a high- pass transfer characteristic; the analog system input signal (Vin) is connected to the differencing circuit (100) as an unheld input signal; the output of the signal prediction circuit (108) is connected to the the DAC via the digital latch circuit (112); and the signal prediction circuit (108) has a low-pass transfer characteristic. 10. A method for analog-to-digital conversion of an analog system input signal (Vin) to a digital system output signal (OUT) comprising the following steps: forming an error signal by differencing the analog system input signal and a feedback signal; amplifying the error signal by a factor A; converting the amplified error signal to digital form in a fast analog-to-digital converter ADC (104); predicting a next input signal value as a predetermined function of the converted and amplified error signal, the predicted next input signal value also forming the feedback signal; and forming the digital system output signal as the sum of the feedback signal and the converted and amplified error signal scaled down by the factor A. 11. A method as in claim 10, in which the step of amplifying the error signal comprises amplifying the error signal time-continuously. 12. A method as in claim 10, in which the step of amplifying the error signal also comprises amplifying the error signal with a high-pass characteristic. 13. A method as in claim 10, in which the step of predicting the next input signal value also comprises simultaneously low-pass filtering the amplified and converted error signal. 14. A method as in claim 10, in which the step of forming an error signal further comprises differencing the analog system input signal in an unsampled and unheld form.

Inventors: LEWYN LANNY, LEWYN, LANNY
Applicants: LEWYN CONSULTING INC, LEWYN CONSULTING, INC.
Application year: 1999
Main classification: H03M3
IPC codes: H03M1, H03M3