Abstract:
An apparatus performs adaptive analog-to-digital conversion. The apparatus according to one embodiment comprises a frequency modulator unit for changing an input analog signal into a modulated analog signal with a frequency spectrum in a bandwidth of interest, a parallel delta sigma conversion unit operatively connected to the frequency modulator unit, the parallel delta sigma conversion unit converting the modulated analog signal into a digital signal, and a controller operatively connected to the frequency modulator unit and the parallel delta sigma conversion unit, the controller adjusting at least one parameter relating to a frequency characteristic of the frequency modulator unit and/or the parallel delta sigma conversion unit.

Description:
PRIORITY CLAIMED 
   This application is a Non-Provisional application including the subject matter and claiming the priority date under 35 U.S.C. §119(e) of Provisional Application Ser. No. 60/607,577, filed Sep. 8, 2004, the contents of which are meant to be incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to analog-to-digital converters and control structures and methods for an analog-to-digital converter. 
   2. Description of the Related Art 
   Analog-to-digital (ADC) converters are essential components in today&#39;s electronic circuits and systems. ADC converters transform analog signals to digital signals. Conventional delta sigma (ΔΣ) analog-to-digital converters offer high resolution and linearity, high integration, little differential non-linearity, and low cost. Their performance is not limited by mismatched components within the converter, and has low noise sensitivity. Most of the circuitry in delta sigma ADCs is digital; hence the performance of delta sigma ADCs does not drift with time and temperature. 
   Two basic principles govern the operation of conventional delta sigma ADCs: oversampling and noise shaping. The sampling frequency in a delta sigma ADC is typically chosen to be much larger than the input signal bandwidth. Oversampling spreads the quantization noise power over a bandwidth equal to the sampling frequency. A delta sigma ADC usually contains a delta sigma modulator, a lowpass filter, and a decimator filter. The delta sigma modulator applies a lowpass filter to the input analog signal and a high pass filter to the noise, hence placing most quantization noise energy above the input signal bandwidth. The lowpass filter follows the delta sigma modulator, attenuating out-of-band quantization noise. The decimator filter downsamples the sampled output digital signal to the Nyquist rate. 
   Since delta sigma ADCs typically operate at an oversampled rate much larger than the maximum input signal bandwidth, their circuitry is complex and their speed is low. Because of speed limitations, delta sigma ADCs perform best in high-resolution, very-low frequency applications. 
   In order to successfully extend the use of delta sigma ADCs to higher frequency applications, a parallel delta sigma ADC architecture has been proposed. U.S. Pat. No. 5,196,852 by Ian Galton and “ A Nyquist - Rate Delta - Sigma A/D Converter” IEEE Journal of Solid - State Circuits , Vol. 33, No. 1, pp. 45–52 describe parallel delta sigma ADC systems. The system described in U.S. Pat. No. 5,196,852 achieves an effective oversampling ratio of N*M, where N is the oversampling ratio of each delta sigma ADC and M is the number of parallel delta sigma ADC channels. The system described in “ A Nyquist - Rate Delta - Sigma A/D Converter ” achieves an effective oversampling ratio of M without oversampling in the individual delta sigma ADCs, where M is the number of parallel delta sigma ADC channels. However, with the circuits described in above works, the parallel delta sigma ADCs do not self-adapt. Hence, only a limited predetermined range of incoming signal frequencies can be processed. 
   A disclosed embodiment of the application addresses these and other issues by utilizing a parallel, adaptive delta sigma analog-to-digital converter. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention are directed to an apparatus for adaptive analog-to-digital conversion. According to a first aspect of the present invention, an apparatus for adaptive analog-to-digital conversion comprises: a frequency modulator unit for changing an input analog signal into a modulated analog signal with a frequency spectrum in a bandwidth of interest; a parallel delta sigma conversion unit operatively connected to the frequency modulator unit, the parallel delta sigma conversion unit converting the modulated analog signal into a digital signal; and a controller operatively connected to the frequency modulator unit and the parallel delta sigma conversion unit, the controller adjusting at least one parameter relating to a frequency characteristic of the frequency modulator unit and/or the parallel delta sigma conversion unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 2A  illustrates a frequency modulator unit included in a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 2B  illustrates aspects of the operation of a local oscillator and a mixer included in a frequency modulator unit according to an embodiment of the present invention; 
       FIG. 2C  illustrates aspects of the operation of a tunable bandpass filter included in a frequency modulator unit according to an embodiment of the present invention; 
       FIG. 3  illustrates a delta sigma conversion unit included in a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 4A  illustrates a code generator that may be included in a delta sigma conversion unit of a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 4B  illustrates a code generator that may be included in a delta sigma conversion unit of a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 4C  illustrates a code generator that may be included in a delta sigma conversion unit of a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 5  illustrates a control unit that may be included in a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 6  illustrates aspects of the operation of a control unit that may be included in a parallel, adaptive delta sigma ADC according to an embodiment of the present invention; 
       FIG. 7  is a flow diagram illustrating operations performed by a frequency modulator unit according to an embodiment of the present invention; 
       FIG. 8  is a flow diagram illustrating operations performed by a delta sigma conversion unit according to an embodiment of the present invention; 
       FIG. 9A  is a flow diagram illustrating aspects of an exemplary normal mode operation of a control unit included in a parallel, adaptive ADC according to an embodiment of the present invention; 
       FIG. 9B  is a flow diagram illustrating aspects of an exemplary self calibration mode operation of a control unit included in a parallel, adaptive ADC according to an embodiment of the present invention; 
       FIG. 10  illustrates an exemplary Hadamard code generator included in a delta sigma conversion unit according to an embodiment of the present invention; 
       FIG. 11  illustrates a continuous time integrator circuit that may be included in a delta sigma ADC according to an embodiment of the present invention; 
       FIG. 12  illustrates a polarity reversal circuit that may be included in input and output multipliers of a delta sigma conversion unit according to an embodiment of the present invention; 
       FIG. 13A  illustrates an exemplary set of Hadamard codes that may be produced by a Hadamard code generator, according to an embodiment of the present invention; 
       FIG. 13B  illustrates an exemplary set of Hadamard input multiplier sequences that may be used by input multipliers in a delta sigma conversion unit, according to an embodiment of the present invention; 
       FIG. 13C  illustrates an exemplary set of Hadamard output multiplier sequences that may be used by output multipliers in a delta sigma conversion unit, according to an embodiment of the present invention; and 
       FIG. 14A  and  FIG. 14B  illustrate aspects of the operation for obtaining the output of a signal passed through Hadamard input multipliers and Hadamard output multipliers in a delta sigma conversion unit, according to a particular example of an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures. 
     FIG. 1  is a block diagram of a parallel, adaptive delta sigma ADC according to an embodiment of the present invention. The parallel, adaptive delta sigma ADC  100  illustrated in  FIG. 1  includes the following components: a frequency modulator unit  20 ; a delta sigma conversion unit  50 ; and a control unit  70  operatively connected as shown. An input analog signal  1  enters the parallel, adaptive delta sigma ADC  100  through frequency modulator unit  20 . Parallel, adaptive delta sigma ADC  100  converts input analog signal  1  to output digital signal  90 . Control unit  70  controls the operation of frequency modulator  20  and delta sigma conversion unit  50 . Delta sigma conversion unit  50  may also communicate with control unit  70  before outputting digital signal  90 . The parallel, adaptive delta sigma ADC  100  may be built for automatic, manual, or selectable automatic/manual control of output digital signal  90 . Operation of the parallel, adaptive delta sigma ADC  100  in  FIG. 1  will become apparent from the following discussion. 
     FIG. 2A  illustrates a frequency modulator unit  201  that may be included in a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Frequency modulator unit  201  contains a tunable bandpass filter  140  operatively connected as shown. Frequency modulator unit  201  may also contain a local oscillator (LO)  110  and a mixer  120  operatively connected as shown. The LO  110  and mixer  120  are needed when input analog signal  1  lies outside the bandwidth accessible to delta sigma conversion unit  50 . For some embodiments, frequency modulator unit  201  may contain a chain of local oscillators  110 . Control unit  70  controls tunable bandpass filter  140 . Control unit  70  may also control local oscillator  110 . 
   Local oscillator  110  is a semiconductor device or an electronic circuit that generates one or more signals of constant frequency. The frequencies generated by local oscillator  110  are determined by parameters of electronic components inside local oscillator  110  as is known in the art. Mixer  120  is a semiconductor device or electronic circuit that multiplies two signals of different frequencies to obtain a signal of intermediate frequency. A frequency υ LO  generated by local oscillator  110  is mixed by mixer  120  with all frequencies contained in input analog signal  1 , producing mixed analog signal  3 . Each frequency υ in  present in input analog signal  1  is shifted to two frequencies υ LO −υ in  and υ LO +υ in  mixed analog signal  3 . 
   Local oscillator  110  is preferably tunable. This may be accomplished with, for example, a single tunable local oscillator  110  or a chain of tunable local oscillators  110  as is known in the art. 
   Tunable bandpass filter  140  is a semiconductor device or an electronic circuit that isolates and extracts independent communication channels from the frequency wideband of mixed analog signal  3 , and cuts off frequencies in the frequency wideband of mixed analog signal  3  that are either too high or too low. Tunable bandpass filter  140  may be a transmultiplexer; a spectral subband coder that divides an analog signal into frequency segments, or spectral terms, computed for non-overlapped successive blocks of input data; a collection of single-sideband narrowband filters which perform a complex heterodyne, hence basebanding a selected center frequency; a channelizer with suitable RF switches; an antialias filter; a band pass filter; a lowpass filter; or a bandpass and lowpass filter. The center frequency and/or bandwidth of tunable bandpass filter  140  can be changed manually or automatically, so that the passband of tunable bandpass filter  140  matches the bandwidth of interest of delta sigma conversion unit  50 , hence eliminating aliasing. 
     FIG. 2B  illustrates aspects of the operation of a local oscillator  110  and a mixer  120  that may be included in a frequency modulator unit  201  according to an embodiment of the present invention. In case input analog signal  1  occupies a different frequency band than the frequency band parallel, adaptive delta sigma ADC  100  is designed for, the local oscillator  110  may be tuned such that the input signal is downshifted onto a desired bandwidth range. 
   In other words, the particular design values and construction of the parallel, adaptive delta sigma ADC  100  result in a device having a particular bandwidth range (bandwidth of interest). The local oscillator  10  may be used to shift the input signal onto the bandwidth range of the ADC  100 . For this purpose, local oscillator  10  may be tuned by control unit  70  to generate a signal of frequency υ LO  which is mixed with all frequencies of input analog signal  1  in mixer  120 , producing mixed analog signal  3 . Each frequency υ in  present in input analog signal  1  is shifted to two frequencies υ LO −υ in  and υ LO +υ in  in mixed analog signal  3  as illustrated in  FIG. 2B . υ LO  is chosen so that either υ LO −υ in  or υ LO +υ in  is located within the frequency band for which the parallel, adaptive delta sigma ADC  100  is designed. 
     FIG. 2C  illustrates aspects of the operation of a tunable bandpass filter  140  included in a frequency modulator unit  201  according to an embodiment of the present invention. Tunable bandpass filter  140  divides the frequency wideband of mixed analog signal  3  into frequency segments  5 A,  5 B, . . .  5 Y,  5 Z. The number of frequency segments and the width of frequency segments may be tuned by control unit  70 . All frequency segments are then mapped to the lowest frequency segment  5 Z, avoiding aliasing of frequency segments. 
     FIG. 3  illustrates a delta sigma conversion unit  501  that may be included in a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Delta sigma conversion unit  501  includes a code generator  150 , N number of delta sigma (ΔΣ) channels  155   1 ,  155   2 , . . .  155   N  connected in parallel, and an adder  260  operatively connected as shown. Each ΔΣ channel  155   i  includes an input multiplier  160   i , a lowpass ΔΣ ADC  180   i , an adaptable digital correction filter  200   i , a programmable decimation filter  220   i , and an output multiplier  240   i , where subscript “i” has values from 1 to N. 
   A lowpass ΔΣ ADC  180   i  is a delta sigma analog-to-digital converter and may be implemented using many possible converter types including low-order single-bit single-loop converter, high-order single-bit single-loop converter, single-bit multiloop (cascaded or MASH) converter and multi-bit (single-loop or multiloop) converter. All lowpass ΔΣ ADCs  180   i  (for i from 1 to N) are preferably substantially identical. In one embodiment of the current invention, all lowpass ΔΣ ADC  180   i  have the same order, number of bits, and signal delay. 
   Adaptable digital correction filters  200   i , programmable decimation filters  220   i , and output multipliers  240   i  where subscript “i” has values from 1 to N, are conventional digital electronic devices. They can be implemented using a custom ASIC, an off the shelf FIR filter, a field programmable gate array, or a sufficiently fast microprocessor. The filter coefficients for the adaptable digital correction filters  200   i  and the programmable decimation filters  220   i , may be stored in a register on the chip that holds the parallel, adaptive delta sigma ADC  100 , or in a separate memory off the chip, connected to a data bus. Adaptable digital correction filters  200   i , programmable decimation filters  220   i , and output multipliers  240   i , where subscript “i” has values from 1 to N, may also be combined into a single filter. 
   As will be further described below, adaptable digital correction filters  200   i  and programmable decimation filters  220   i  have programmable filter parameters such as length and bandwidth. Adaptable digital correction filters  200   i  correct inaccuracies introduced by lowpass ΔΣ ADCs  180   i  where subscript “i” has values from 1 to N, and may be implemented using known techniques. 
   Programmable decimation filters  220   i  are used to down-sample signals and to eliminate out-of-band noise, and may be implemented using known techniques. Code generator  150  generates a set of codes. Input multipliers  160   i  and output multipliers  240   i  are standard multiplier circuits whose multiplication values are supplied by codes from the set of codes provided by code generator  150 . Adder  260  may be a simple adder circuit that adds its inputs bit-by-bit. Control unit  70  may control code generator  150 , adaptable digital correction filters  200   i , and programmable decimation filters  220   i . 
   A power splitter  159  simultaneously inputs one of the frequency segments produced by tunable bandpass filter  140  (signal  5 A) to all ΔΣ channels  155   1 ,  155   2 , . . .  155   N . The set of codes generated by code generator  150  and applied by multipliers  160   i  decomposes signal  5 A into orthonormal components. Along with other types of codes, a set of Hadamard codes or a set of Gold codes can be generated by code generator  150  for orthonormal decomposition of signal  5 A. Each input multiplier  160   i  uses one and only one code from the set of codes generated by code generator  150 , to create a multiplication value for its frequency decomposing weighting function. Frequency decomposing weighting functions are orthonormal and channel specific. Inside channel i, signal  5 A is frequency-decomposed by input multiplier  160   i , passed through lowpass ΔΣ ADC  180   i , corrected by adaptable digital correction filter  200   i  for inaccuracies gained from lowpass ΔΣ ADC  180   i , down-sampled by programmable decimation filter  220   i  which also eliminates quantization noise, and multiplied by output multiplier  240   i  which applies a time-shifted version of the frequency decomposing weighting function of input multiplier  160   i , that undoes the frequency decomposing action of input multiplier  160   i . 
   The outputs from all ΔΣ channels  155  are summed by adder  260  to obtain a digital signal  7 . 
   If signal  5 A has a frequency spectrum of X GHz, each lowpass ΔΣ ADC  180   i  may be clocked at 2× GHz, which is the Nyquist rate of signal  5 A frequency spectrum. Each lowpass ΔΣ ADC  180   i  is designed to have a band-pass response of X/N, hence the widest bandwidth signal the N-channel parallel, adaptive delta sigma ADC  100  can digitize is X GHz (N*X/N). The oversampling rate (OSR) of delta sigma conversion unit  501  for the entire frequency band of signal  5 A is 1, since each lowpass ΔΣ ADC  180   i  is clocked at the Nyquist rate of signal  5 A frequency spectrum. However, within each ΔΣ channel  155   i  the OSR ratio is N, calculated as the ratio of each lowpass ΔΣ ADC  180   i  sampling frequency (2×) to the Nyquist frequency of the band-pass response of the channel (2*X/N). Therefore the parallel network of individual ΔΣ channels  155   i  has N times larger OSR than a single lowpass ΔΣ ADC  180   i  would exhibit. 
     FIG. 4A  illustrates a code generator  150 A that may be included in a delta sigma conversion unit  501  of a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Code generator  150 A is a code calculator that generates a set of codes in real-time, without any information except the desired length of the set of codes which is supplied by control unit  70 . Each code C i  in the set of codes is sent to a pair of input multipliers  160   i  and output multipliers  240   i . Input multipliers  160   i  and output multipliers  240   i  generate multiplication values from the code C i  sent to them. 
   To account for processing times of the ΔΣ ADC  180   i , adaptable digital correction filter  200   i , and programmable decimation filter  220   i , the codes supplied to output multipliers  240   i  are preferably time shifted or delayed relative to the input multipliers  160   i . This may be accomplished by, for example, a delay element (not shown), by the code calculator generator  150 A, or by the control unit  70 . 
     FIG. 4B  illustrates a code generator  150 B that may be included in a delta sigma conversion unit  501  of a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Code generator  150 B includes a memory  146  with registers or memory locations  148   1 ,  148   2 , . . .  148   N  that contain a set of codes. Code generator  150 B generates a set of output codes C 1 , C 2  . . . C N  by direct read from memory registers or memory locations  148   1 ,  148   2 , . . .  148   N . The code C i  in each register  148   i  is directly read into input multiplier  160   i  and output multiplier  240   i . Input multipliers  160   i  and output multipliers  240   i  generate multiplication values from code C i . 
     FIG. 4C  illustrates a code generator  150 C that may be included in a delta sigma conversion unit  501  of a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Code generator  150 C includes a memory  147  with M sets of registers or memory locations. Each register set “j”, with “j” from 1 to M, contains N individual registers or memory locations  148   1j ,  148   2j , . . .  148   Nj . Each register  148   ij  stores an internal code H ij . There are N×M internal codes H ij  as “i” runs from 1 to N and “j” runs from 1 to M. A switchable lookup table register  143  decides output codes C 1 , C 2  . . . C N  by direct read from among all internal codes H ij . Output codes C 1 , C 2  . . . . C N  are sent to all N ΔΣ channels  155   1 ,  155   2 , . . .  155   N  to input multiplier  160   i  and output multiplier  240   i  where subscript “i” has values from 1 to N. Input multipliers  160   i  and output multipliers  240   i  generate multiplication values from the code sent to them. Lookup table register  143  may store one lookup table or multiple lookup tables. An example of a lookup table is: 
             [           H   11           H   21         ⋯       ⋯         H   N1             ⋯       ⋯       ⋯       ⋯       ⋯           ⋯       ⋯       ⋯       ⋯       ⋯             H     1   ⁢   M             H     2   ⁢   M           ⋯       ⋯         H   NM           ]     .         
This table would be read by direct memory read, switching between rows to select one row for output codes C 1 , C 2  . . . C N .
 
An example of a multiple lookup table is:
 
   
     
       
         
             
           
             [ 
             
               
                 
                   
                     H 
                     11 
                   
                 
                 
                   
                     H 
                     11 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     11 
                   
                 
                 
                   
                     H 
                     21 
                   
                 
                 
                   
                     H 
                     21 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     21 
                   
                 
                 
                   … 
                 
                 
                   … 
                 
                 
                   
                     H 
                     N1 
                   
                 
                 
                   
                     H 
                     N1 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     N1 
                   
                 
               
               
                 
                   
                     H 
                     12 
                   
                 
                 
                   
                     H 
                     12 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     12 
                   
                 
                 
                   
                     H 
                     22 
                   
                 
                 
                   
                     H 
                     22 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     22 
                   
                 
                 
                   … 
                 
                 
                   … 
                 
                 
                   
                     H 
                     N2 
                   
                 
                 
                   
                     H 
                     N2 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     N2 
                   
                 
               
               
                 
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                     H 
                     
                       1 
                       ⁢ 
                       M 
                     
                   
                 
                 
                   
                     H 
                     
                       1 
                       ⁢ 
                       M 
                     
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     
                       1 
                       ⁢ 
                       M 
                     
                   
                 
                 
                   
                     H 
                     
                       2 
                       ⁢ 
                       M 
                     
                   
                 
                 
                   
                     H 
                     
                       2 
                       ⁢ 
                       M 
                     
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     
                       2 
                       ⁢ 
                       M 
                     
                   
                 
                 
                   … 
                 
                 
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                     H 
                     NM 
                   
                 
                 
                   
                     H 
                     NM 
                   
                 
                 
                   … 
                 
                 
                   
                     H 
                     NM 
                   
                 
               
             
             ] 
           
         
       
     
   
     FIG. 5  illustrates a control unit  700  that may be included in a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Control unit  700  may include the following operational components: a mode control unit  305 ; a bandwidth control unit  300 ; a self calibration unit  320 ; and a DAC  340 . Bandwidth control unit  300  controls code generator  150 , programmable decimation filters  220   1 , . . .  220   N , tunable bandpass filter  140 , and tunable local oscillator  110 . Self calibration unit  320  controls adaptable digital correction filters  200   1 , . . .  200   N  and DAC  340 , and communicates with bandwidth control unit  300 . 
   The parallel, adaptive delta sigma ADC  100  has two modes of operation: a normal operation mode and a self-calibration mode. Mode control unit  305  determines the operation mode of the parallel, adaptive delta sigma ADC  100 . 
   The parallel, adaptive delta sigma ADC  100  operates in normal operation mode most of the time. In normal operation mode, bandwidth control unit  300  determines the bandwidth of interest using internal algorithms or an external control signal manually or automatically commanded. Bandwidth control unit  300  calculates or looks up the proper code for code generator  150 , and calculates filter coefficients for programmable decimation filters  220   i , where “i” runs from 1 to N. Bandwidth control unit  300  also sets the center frequency and the bandwidth for tunable bandpass filter  140  and tunable local oscillator  110 . After sending commands to code generator  150 , programmable decimation filters  220   i , tunable bandpass filter  140 , and tunable local oscillator  110 , bandwidth control unit  300  remains idle until mode control unit  305  sends a command to change bandwidths again. 
   In self-calibration mode, self calibration unit  320  takes over bandwidth control unit  300 . Self calibration unit  320  sets filter coefficients for adaptable digital correction filters  200   i  to zero, and then instructs bandwidth control unit  300  to set a predetermined calibration bandwidth. Self calibration unit  320  then creates a calibration signal using the built-in DAC  340 . The DAC  340  feeds the calibration signal to tunable bandpass filter  140  as an input. The calibration signal is processed by tunable bandpass filter  140  in the same manner as in normal mode operation. The digital output of the delta sigma conversion unit  501  is fed back to self calibration unit  320 . Self calibration unit  320  calculates a final set of adaptable digital correction filter  200   i  coefficients for use when control unit  700  and parallel, adaptive delta sigma ADC  100  return to normal mode operation. 
   Mode control unit  305  may be a part of control unit  700 . In another embodiment of the invention, mode control unit  305  may be external to the parallel, adaptive delta sigma ADC  100 . The DAC  340  may be any conventional DAC. The DAC  340  function may also be generated by an alternate analog frequency synthesis technique. Mode control unit  305 , bandwidth control unit  300 , and self calibration unit  320  may be implemented using a single field programmable gate array; an ASIC; a microcontroller; a standard microprocessor; or a memory or set of memories. 
     FIG. 6  illustrates aspects of the operation of a control unit  700  that may be included in a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. Control unit  700  selects ( 902 ) the operation mode of the parallel, adaptive delta sigma ADC  100 . 
   If the selected operation mode is the normal operation mode ( 904 ), control unit  700  determines ( 905 ) the frequency or frequencies of interest for the parallel, adaptive delta sigma ADC  100 . Next, control unit  700  controls tunable local oscillator  110  by selecting ( 906 ) the frequency of tunable local oscillator  110 ; controls the tunable bandpass filter  140  by selecting the number of desired frequency segments ( 907 ), and/or the bandwidth of frequency segments ( 908 ), and/or the filter bandwidth ( 909 ); determines the length of code ( 910 ) and the type of code ( 911 ) for code generator  150 ; and controls programmable decimation filters  220   1  . . .  220   N  by selecting the filters&#39; function ( 912 ), cutoff frequency ( 913 ), and length ( 914 ). In one embodiment of the invention, code calculator generator  150 A directly calculates a code according to a selected code length ( 910 ). In other embodiments of the invention, code generators  150 B and/or  150 C choose an appropriate lookup table and/or lookup table read rate according to the selected code length ( 910 ) and the type of code ( 911 ). 
   If the selected operation mode is the self-calibration mode ( 903 ), control unit  700  determines a calibration signal ( 915 ) for an appropriate calibration bandwidth, and sends it to DAC  340 . Control unit  700  also controls the adaptable digital correction filters  200   1 , . . .  200   N  by calculating ( 916 ) proper filter correction coefficients. 
     FIG. 7  is a flow diagram illustrating operations performed by a frequency modulator unit  201  according to an embodiment of the present invention. An analog signal  1  ( 481 ) is input into frequency modulator unit  201 . A test is performed ( 482 ) to determine whether input analog signal  1  lies outside the bandwidth accessible by delta sigma conversion unit  501 . If input analog signal  1  lies outside the bandwidth accessible by delta sigma conversion unit  501 , input analog signal  1  frequency is shifted to a frequency accessible to delta sigma conversion unit  501 , using ( 483 ) tunable local oscillator  110  whose frequency is controlled ( 906 ) by control unit  700 . Input analog signal  1  is then filtered ( 484 ) by tunable bandpass filter  140  that is controlled ( 1100 ) by control unit  700 . The output of frequency modulator unit  201  is analog input signal  5 A ( 615 ). 
     FIG. 8  is a flow diagram illustrating operations performed by a delta sigma conversion unit  501  according to an embodiment of the present invention. Delta sigma conversion unit  501  is designed to convert a bandwidth or set of separate bandwidths present in input analog signal  1  into a digital signal. Analog signal  5 A ( 615 ) output from frequency modulator unit  201  has a continuous low frequency band or an arbitrary set of bandwidths. 
   Power splitter  159  splits ( 620 ) analog signal  5 A into N identical signal channels. Each signal channel is passed to a ΔΣ channel  155   i , with subscript “i” having values from 1 to N. In each ΔΣ channel  155   i , input multiplier  160   i  multiplies ( 830 ) the signal channel by a code from a code set ( 820 ) created by code generator  150 . Power splitter  159  and input multipliers  160   i  break the band or bands of interest of analog signal  5 A into many continuous low frequency bands, by decomposing input analog signal  5 A into several subbands in which frequencies of interest are translated bit-by-bit to low frequency in each of the ΔΣ channels  155   i . 
   Control unit  700  sets ( 750   a ) the type and length of the code set that may be generated by real time calculation ( 824 ) in a code calculator generator  150 A, by direct read ( 822 ) from a memory  150 B containing a lookup table, or by direct read ( 826 ) from a memory with switchable output  150 C containing a group of code sets located in a memory lookup table. The generated code set is sent ( 820 ) to input multipliers  160   i  and output multipliers  240   i . 
   The signal channel in each ΔΣ channel  155   i  is then converted ( 832 ) to a digital signal channel by lowpass ΔΣ ADC  180   i . Errors introduced by lowpass ΔΣ ADCs  180   i  are compensated ( 834 ) by the adaptable digital correction filters  200   i  whose filter correction coefficients are set ( 750   b ) by control unit  700 . Quantization noise is removed ( 836 ) by programmable decimation filters  220   i  whose decimation filter coefficients are set ( 750   c ) by control unit  700 . 
   The digital signal channel in each ΔΣ channel  155   i  is demodulated ( 838 ) by output multiplier  240   i  and is output from the ΔΣ channel  155   i . The output digital signal channels from all parallel ΔΣ channels  155   i  for “i” from 1 to N are added ( 840 ) together by adder  260 . The output ( 842 ) of the adder is digital signal  7 . 
   For analog input signal  5 A to be accurately digitally reconstructed from the outputs of programmable decimation filters  220   i , the coefficients of programmable decimation filters  220   i  are adapted ( 750   c ) by control unit  700  to the specific code ( 820 ) that was generated by code generator  150 . Hence, changing the code ( 820 ) generated by code generator  150  and the coefficients of programmable decimation filters  220   i  with “i” from 1 to N, changes the band or bands in analog input signal  5 A that can be processed by delta sigma conversion unit  501 . Therefore, by changing the code generated by code generator  150  and the coefficients for programmable decimation filters  220   i , the parallel, adaptive delta sigma ADC  100  can change bandwidth and dynamic range. One set of codes and programmable decimation filters coefficients may create a wideband low dynamic range modulator, while another set of codes and programmable decimation filters coefficients may create a narrow band, high dynamic range modulator. The parallel ΔΣ channels  155   i  may also be split into separate channel groups to create a modulator with separate pass-bands. In this case, separate bands of different bandwidth as well as continuous bands from analog signal  5 A can be processed by the parallel, adaptive delta sigma ADC  100 . 
     FIG. 9A  is a flow diagram illustrating aspects of an exemplary normal operation mode of a control unit  700  included in a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. In a normal operation mode, bandwidth control unit  300  included in control unit  700  sets ( 1200 ) the tunable local oscillator  110 , the tunable bandpass filter  140 , the code generator  150 , and the programmable decimation filters  220   i  to proper settings for a frequency band or bands of interest. The frequency band or bands of interest can be selected by bandwidth control unit  300 , or by an external command from an external signal. When signals of interest and consequently bands of interest are unknown, such as in passive listening of signals or in electronic signal warfare, bandwidth control unit  300  selects the frequency band or bands of interest using a self adaptive algorithm. When the input signals are known, such as in a RADAR signal emission, an external signal may direct the bandwidth control unit  300  to a specific frequency band with a desired dynamic range. 
   An exemplary self adaptive algorithm is presented in  FIG. 9A . In one logic flow, the bandwidth control unit  300  sets ( 1202 ) all N parallel ΔΣ channels  155   i  of delta sigma conversion unit  501  in use to form a wideband low dynamic range modulator to search for signals. Once a signal is detected ( 1204 ), bandwidth control unit  300  switches ( 1206 ) to a narrow band, high dynamic range configuration, to listen to the identified signal. This logic flow works well for large complex signals. A second logic flow may search for both large and small signals using ( 1208 ) half of the N parallel ΔΣ channels  155   i  to form a wideband low dynamic range modulator to search for large signals, and half ( 1210 ) of the N parallel ΔΣ channels  155   i  to form a narrow band, high dynamic range modulator to search for small signals. By slowly sweeping the center frequency of the high dynamic range band, bandwidth control unit  300  can locate ( 1212 ) small persistent signals. Once one or more signals of interest have been identified ( 1214 ), the bandwidth control unit  300  can allocate one or more parallel ΔΣ channels  155   i  of the delta sigma conversion unit  501  to listen ( 1216 ) to the signals of interest, while using the remaining parallel ΔΣ channels  155   i  to search ( 1218 ) for additional signals in other bands. 
     FIG. 9B  is a flow diagram illustrating aspects of an exemplary self-calibration operation mode of a control unit  700  included in a parallel, adaptive delta sigma ADC  100  according to an embodiment of the present invention. In self-calibration mode, the small errors introduced by delta sigma modulators  180   i  are quantified. Error quantification is important because it allows for error correction and compensation by adaptable digital correction filters  200   i . One exemplary implementation of error quantification and correction is shown in  FIG. 9B , where a known signal is input into the delta sigma conversion unit  501 , and the output of the delta sigma conversion unit  501  is examined to determine introduced errors. 
   When control unit  700  decides that it is time for a self calibration cycle ( 1230 ) based on some external input or some internal routine, it activates ( 1232 ) the self calibration unit  320 , which instructs the bandwidth control unit  300  to switch ( 1234 ) to a predetermined calibration bandwidth. The bandwidth control unit  300  sets ( 1236 ) the code generator  150 , tunable bandpass filter  140 , and decimation filters  220   i , to the proper settings for the desired bandwidth. The self calibration unit  320  then activates the DAC  340  to produce ( 1238 ) a known calibration signal which is injected ( 1242 ) into the delta sigma conversion unit  501 . The coefficients on the digital correction filters  200   i  are all set to zero ( 1240 ) so that no digital correction is performed. The uncorrected digital output signal  7  ( 1244 ) is fed back ( 1246 ) to the self-calibration unit and analyzed to determine what errors ( 1248 ) are introduced by the low pass ΔΣ ADCs  180   i . The self-calibration unit uses this signal to calculate ( 1250 ) a new set of coefficients ( 1254 ) for the adaptable digital correction filters  200   i  with “i” from 1 to N. 
   Optionally, the calibration signal could be reinjected into delta sigma conversion unit  501  ( 1242 ) using the most recent set of adaptable digital correction filters  200   i  coefficients. The output is re-measured to determine if errors are within tolerance ( 1249 ). Such errors are primarily nonlinearities and variations in gain from one ΔΣ channels  155   i  to another. If the errors are within tolerance, the self calibration is completed ( 1252 ) and the latest found set of coefficients for adaptable digital correction filters  200   i  with “i” from 1 to N, are stored to be used ( 1254 ) until the next self calibration cycle. Other variations on the self-calibration routine include calibrating each channel one at a time, and calibrating at various frequencies to eliminate frequency dependent errors. 
     FIG. 10  illustrates an example of code generator  150  (Hadamard code generator  1500 ) that may be included in a delta sigma conversion unit  501  according to an embodiment of the present invention. The orthogonal set of codes generated by a Hadamard code generator  1500  is used in a preferred embodiment of the current invention. A Hadamard code generator  1500  generates a Hadamard code from a conventional Hadamard matrix. The elements of a Hadamard matrix are 1s and −1s. The size of the Hadamard matrix used by Hadamard code generator  1500  is N×N, where N is the number of ΔΣ channels  155  in delta sigma conversion unit  50 . In order for the Hadamard matrix to exist, N can be a non-negative power of 2, or another number for which Hadamard matrices are known to exist. Such numbers may include all multiples of 4 smaller than 428. For N=1 the Hadamard matrix is H 1 =[1]. For N≧2, Hadamard matrix of order N, H N , is defined recursively as follows: 
             H   N     =       [           H     N   /   2             H     N   /   2                 H     N   /   2             -     H     N   /   2               ]     .           
The N-by-N Hadamard matrix has the property that H N H N   T =NI N  where I N  is the N-by-N identity matrix. The rows and columns of the Hadamard matrix are mutually orthogonal. The Hadamard codes  151  generated by Hadamard code generator  1500  are the individual rows of the N×N Hadamard matrix. The orthogonal set of codes  151  is sent to input multipliers  160  and to output multipliers  240 .
 
     FIG. 11  illustrates a continuous time integrator circuit  400  that may be included in a ΔΣ ADC  180   i  from a delta sigma conversion unit  501 , according to an embodiment of the present invention. An exemplary embodiment of this invention uses a continuous time 4 th  order cascade of resonator filters that exhibits high stability. A building block of the filter is a continuous time integrator  400  illustrated in  FIG. 8 . A transconductor stage charging a capacitor  440  implements the integrating function. A metal film resistor  420  is used in place of a transconductance element to improve and maintain the high degree of linearity required for high dynamic range performance of ΔΣ ADCs  180 . The schematic in  FIG. 11  is a single ended design. A fully differential design such as that shown in  FIG. 12  can also be used. 
     FIG. 12  illustrates a polarity reversal circuit  500  that may be included in input multipliers  160   i  and output multipliers  240   i  of a delta sigma conversion unit  501  according to an embodiment of the present invention. The polarity reversing circuit  500  reverses the differential input lines  520  and  521  to ΔΣ ADCs  180  at the appropriate times when the +/−1 Hadamard input multiplier sequences  152  and Hadamard output multiplier sequences  153  are applied to signal  5 A. An exemplary implementation of polarity reversal circuit  500  uses current steering bipolar switches  540  in emitter-follower unity gain buffers connected at the input of continuous time integrator circuit  400  of the first resonator in the continuous time 4 th  order cascade of resonators filter in ΔΣ ADCs  180 . Bipolar transistors perform well as high-speed current switches and the negative feedback of the emitter-follower buffer configuration maintains circuit linearity. The input lines  560  and  561  labeled H+ and H− in the circuit diagram of  FIG. 9  correspond to logical levels of 1 and −1 in a differential Hadamard input multiplier sequence  152  or Hadamard output multiplier sequence  153 . Polarity reversal circuit  500  is used to switch the inputs to the integrator  400 , between input value and inverted input value. Polarity reversal circuit  500  combines input multiplier  160   i  with the input stage of its corresponding lowpass ΔΣ ADC  180   i . As a result, input multipliers  160   i  do not add additional nonlinearities into the delta sigma conversion unit  501 . 
     FIGS. 13A ,  13 B,  13 C,  14 A and  14 B illustrate a particular, non-limiting example of the invention. In this example, it is assumed that the bandwidth control unit  300  of control unit  700  has decided or has been instructed to form a single broadband modulator using four channels, that is N=4. The four channels cover the entire Nyquist band of the delta sigma conversion unit  501 . The signals at various points in the delta sigma conversion unit  501  are calculated to demonstrate the basic principle of delta sigma analog-to-digital conversion. 
     FIG. 13A  illustrates an exemplary set of Hadamard codes  151  that may be produced by a Hadamard code generator  1500  for number N of ΔΣ channels  155   i  in delta sigma conversion unit  501  with N=4, according to an embodiment of the present invention. Since H 1 =[1], it follows that 
             H   2     =       [           H   1           H   1               H   1           -     H   1             ]     =     [         1       1           1         -   1           ]             
and
 
             H   4     =       [           H   2           H   2               H   2           -     H   2             ]     =       [         1       1       1       1           1         -   1         1         -   1             1       1         -   1           -   1             1         -   1           -   1         1         ]     .             
The Hadamard codes  151  produced by Hadamard code generator  1500  for N=4 are the rows of Hadamard matrix H 4 . Hence the Hadamard codes  151  for N=4 are:
 C1=[1 1 1 1]   C 2=[1 −1 1 −1]   C 3=[1 1 −1 −1]   C 4=[1 −1 −1 1]. 
Hadamard input multiplier sequences  152  are generated from Hadamard codes C 1 , C 2 , C 3  and C 4  for input multipliers  160 . Similarly, Hadamard output multiplier sequences  153  are generated from Hadamard codes C 1 , C 2 , C 3  and C 4  for output multipliers  240 .
 
     FIG. 13B  illustrates an exemplary set of Hadamard input multiplier sequences  152  that may be used by input multipliers  160  in a delta sigma conversion unit  501  for number N of ΔΣ channels  155   i  in delta sigma conversion unit  501  with N=4, according to an embodiment of the present invention. Hadamard input multiplier sequences  152  z i (n) are obtained from Hadamard codes  151  by the following formula:
   z   i ( n )= C   i   [n  mod  N]   
where A mod B is the modulus function that returns the integer remainder value when A is divided by B, and i is the index for each ΔΣ channel  155  in delta sigma conversion unit  50 , i running from 1 to N. Other formulas for obtaining Hadamard input multiplier sequences  152  z i (n) are also possible and various lookup tables may also be used to supply the Hadamard code sequences as described in relation to  FIGS. 4B and 4C . The first element in matrices C i  has index 0, therefore C i  elements have indices from 0 to N−1. When N=4 and signal samples n run from 0 to 15, Hadamard input multiplier sequences  152  for the second ΔΣ channel  155 , i=2, are:
   z   2 (0)= C   2 [0 mod 4 ]=C   2 [0]=1  z   2 (4)= z   2 (8)= z   2 (12)= z   2 (0)=1   z   2 (1)= C   2 [1 mod 4 ]=C   2 [1]=−1  z   2 (5)= z   2 (9)= z   2 (13)= z   2 (1)=−1   z   2 (2)= C   2 [2 mod 4 ]=C   2 [2]=1  z   2 (6)= z   2 (10)= z   2 (14)= z   2 (2)=1   z   2 (3)= C   2 [3 mod 4 ]=C   2 [3]=−1  z   2 (7)= z   2 (11)= z   2 (15)= z   2 (3)=−1 
Hadamard input multiplier sequences  152  for all N ΔΣ channels  155  when N=4 are shown in  FIG. 13B . In each ΔΣ channel  155  i, sample n of signal  5 A is multiplied by z i (n).
 
     FIG. 13C  illustrates a set of exemplary Hadamard output multiplier sequences  153  that may be used by output multipliers  240   i  in a delta sigma conversion unit  501 , for number N of ΔΣ channels  155   i  in delta sigma conversion unit  50  with N=4, according to an embodiment of the present invention. The purpose of Hadamard output multiplier sequences  153  is to undo the frequency decomposing action of Hadamard input multiplier sequences  152 . Within each ΔΣ channel  155 , Hadamard output multiplier sequence  153  t i (n) is a delayed version of Hadamard input multiplier sequence  152  z i (n) to account for processing and signal delays between input multiplier  160  and output multiplier  240 . For the specific case of Hadamard input multiplier sequences  152  in  FIG. 13B , a set of Hadamard output multiplier sequences  153  t i (n) that may be used by output multipliers  240   i  can be obtained from Hadamard codes  151  by the following formula:
   t   i ( n )= C   i [( n+ 1)mod  N )] 
When N=4 and signal samples n run from 0 to 15, Hadamard output multiplier sequences  153  for the second ΔΣ channel  155 , i=2, are:
   t   2 (0)= C   2 [1 mod 4 ]=C   2 [1]=−1  t   2 (4)= t   2 (8)= t   2 (12)= t   2 (0)=−1   t   2 (1)= C   2 [2 mod 4 ]=C   2 [2]=1  t   2 (5)= t   2 (9)= t   2 (13)= t   2 (1)=1   t   2 (2)= C   2 [3 mod 4 ]=C   2 [3]=−1  t   2 (6)= t   2 (10)= t   2 (14)= t   2 (2)=−1   t   2 (3)= C   2 [4 mod 4 ]=C   2 [0]=1  t   2 (7)= t   2 (11)= t   2 (15)= t   2 (3)=1 
Hadamard output multiplier sequences  153  for all ΔΣ channels  155  when N=4 are shown in  FIG. 13C . In each ΔΣ channel  155  i, sample n of signal going into output multipliers  240  is multiplied by t i (n).
 
     FIG. 14A  illustrates aspects of the operation for obtaining the output of a signal passed through Hadamard input multipliers  160   i  and Hadamard output multipliers  240   i  in a delta sigma conversion unit  501 , according to a particular example of an embodiment of the present invention. For simplicity, the example uses a chain of Hadamard input multipliers  160  followed by Hadamard output multipliers  240 , without the lowpass ΔΣ ADCs  180  in the middle. The length of identical H(z) digital correction filters  200  is chosen to be 6. The 6 samples from a signal  5 A enter Hadamard input multipliers  160  S 0 =[x[0], x[1], x[2], x[3], x[4], x[5]]. Inside input multipliers  160 , signal S 0  is multiplied by corresponding Hadamard input multipliers sequences  152  labeled as C 1  set, C 2  set, C 3  set, C 4  set in  FIG. 13B . Signals S 1 , S 2 , S 3 , S 4  for channels  1 , 2 , 3  and  4  respectively, are output from input multipliers  160 :
   S 1 =[x[ 0 ]*z   1 (0), x[ 1 ]*z   1 (1), x[ 2 ]*z   1 (2), x[ 3 ]*z   1 (3), x[ 4 ]*z   1 (4), x[ 5 ]*z   1 (5)]=[ x[ 0 ],x[ 1 ],x[ 2 ],x[ 3 ],x[ 4 ],x[ 5]];   S 2 =[x[ 0 ]*z   2 (0), x[ 1 ]*z   2 (1), x[ 2 ]*z   2 (2), x[ 3 ]*z   2 (3), x[ 4 ]*z   2 (4), x[ 5 ]*z   2 (5)]=[ x[ 0 ],−x[ 1 ],x[ 2 ],−x[ 3 ],x[ 4 ],−x[ 5]]; 
and so on.
 
Signals S 1 , S 2 , S 3  and S 4  are sent to digital correction filters  200  or order  6 . Each order  6  digital correction filter  200  is represented by filter functions [h(0),h(1),h(2),h(3),h(4),h(5)]. Signals G 1 , G 2 , G 3 , and G 4  for channels  1 , 2 , 3  and  4  respectively, are output from digital correction filters  200 :
   G 1 =h (0) x[ 0 ]+h (1) x[ 1 ]+h (2) x[ 2 ]+h (3) x[ 3 ]+h (4) x[ 4 ]+h (5) x[ 5];   G 2 =h (0) x[ 0 ]−h (1) x[ 1 ]+h (2) x[ 2 ]−h (3) x[ 3 ]+h (4) x[ 4 ]−h (5) x[ 5]; 
and so on.
 
Signals G 1 , G 2 , G 3  and G 4  are then input into Hadamard output multipliers  240  where they are each multiplied by the Hadamard output multiplier sequences  153  corresponding to the order of the last sample input into the system, x[5]. The Hadamard output multiplier sequences  153  corresponding to the order of the last sample input into the system are labeled t 1 (5) ,t 2 (5),t 3 (5),t 4 (5) in  FIG. 5C . The output obtained from network adder  260  is sum G 1   t   1 (5)+G 2   t   2 (5)+G 3   t   3 (5)+G 4   t   4  (5). As seen in  FIG. 14A , the output is 4h(2)x[2].
 
     FIG. 14B  illustrates aspects of the operation for obtaining the output of a signal passed through Hadamard input multipliers  160   i  and Hadamard output multipliers  240  in a delta sigma conversion unit  501 , according to a particular example of an embodiment of the present invention. The algorithm presented in  FIG. 14A  is repeated in  FIG. 14B  for the next time instant when the input is S 0 =[x[1], x[2], x[3], x[4], x[5], x[6]]. Within each channel, signal S 0  is multiplied by corresponding Hadamard input multipliers sequences  152  labeled as D 1  set, D 2  set, D 3  set, D 4  set in  FIG. 13B , sent to digital correction filters  200 , multiplied by the Hadamard output multiplier sequences  153  t 1 (6),t 2 (6),t 3 (6),t 4 (6) from  FIG. 5C  corresponding to the order of the last sample input into the system, x[6], and added back in network adder  260 . The result is 4h(2)x[3]. Therefore the output signal of network adder  260  is simply a delayed and scaled version of the input signal. 
   The parallel, adaptive delta sigma ADC  100  is compatible with high speed, BiCMOS mixed signal processes, SiGe processes, and monolithic integration, as well as ASIC implementation. The filtering operations inside parallel, adaptive ADC  100  are linear, and therefore can be interchanged and/or combined. 
   Although detailed embodiments and implementations of the present invention have been described above, it should be apparent that various modifications are possible without departing from the spirit and scope of the present invention.