Abstract:
An analog-to-digital converter (ADC) having a two-channel or multi-channel structure processes background timing calibration. Signals from the ADC are directly compared for the calibration. Additional signal or interruption of circuit is not required. A dynamic calibration is processed. A timing-skew error is kept in a low level and a process mismatch is not a concern. Moreover, sampling frequency and input signal frequency are improved. A high sampling frequency and a high speed of signal inputting are achieved; and chip area can be greatly shrunk because the extra calibration circuits are simple digital circuits.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an analog-to-digital converter; more particularly, relates to dynamically calibrating timing of narrow-band signals and wide-band signals in a successive circulation by using a two-channel or multi-channel time-interleaved analog-to-digital converter having background timing calibration without interruption of circuit.  
       DESCRIPTION OF THE RELATED ARTS  
       [0002]     General timing-skew calibrations for a time-interleaved analog-to-digital converter includes the following method  
         [0003]     a. Foreground calibration method: Before running the circuit , a standard input signal is generated as a calibration authority for a calibration of timing-skew error of each various phase. But, because timing skew will slightly changes with the temperature and the outside environment on running the circuit, a main disadvantage of this method is that a dynamic calibration of timing-skew error is not available and the converter becomes inferior after a period of operation.  
         [0004]     b. Correction of cross-correlation function test: In a multi-channel AD C, timing-skew information are obtained by comparing cross-correlation functions of each two channels. When there is a timing skew in a phase, the cross-correlation functions of two channels are different in the multi-channel ADC and so a timing-skew error can be known. But this method can only be applied in a narrow-bandwidth operation. When the bandwidth of the input signal is wider than a half of the sampling frequency of a single channel, signal aliasing appears and fails this method.  
         [0005]     In U.S. Pat. No. 5,294,926, “Timing and amplitude error estimation for time-interleaved analog-to-digital converters”, a pre-calibration is processed. When a reference sine-wave signal is inputted into a time-interleaved analog-to-digital converter, a timing position of zero crossing is found by digital interpolation. Time periods of each two adjacent channels are compared and the differences are expressed in digital signals. When digital signals are different, a timing skew is determined and a timing-skew calibration is done according to the information. This method separates the signals and the sine waves and, so, is a foreground calibration method.  
         [0006]     Although the above methods processes timing-skew calibrations, the foreground calibration method is not able to process a calibration dynamically and the correction of cross-correlation function test can be applied to narrowband signals only. Hence, the prior arts do not fulfill users&#39; requests on actual use.  
       SUMMARY OF THE INVENTION  
       [0007]     The main purpose of the present invention is to dynamically calibrate timing of both narrow-band signals and wide-band signals in a successive circulation without interruption of circuit while maintaining smallest timing-skew error.  
         [0008]     To achieve the above purpose, the present invention is a time-interleaved analog-to-digital converter having a timing calibration, which is a two-channel or multi-channel time-interleaved analog-to-digital converter. The two-channel background calibrated time-interleaved analog-to-digital converter comprises a multi-phase clock generator, a clock random chopper, a first programmable delay unit, a second programmable delay unit, a first ADC, a second ADC, a data recovery chopper, a CP and a random sequence generator. The multi-channel time-interleaved analog-to-digital converter comprises a multi-phase clock generator, a multi-channel clock random chopper, a multi-channel programmable delay unit, a multi-channel analog-to-digital converter, a multi-channel data recovery chopper, a multi-channel CP and a random sequence generator. Accordingly, a novel time-interleaved analog-to-digital converter having a timing calibration is obtained.  
     
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0009]     The present invention will be better understood from the following detailed descriptions of the preferred embodiments according to the present invention, taken in conjunction with the accompanying drawings, in which  
         [0010]      FIG. 1  is a structural view showing the first preferred embodiment according to the present invention; and  
         [0011]     FIG. 2  is a structural view showing the second preferred embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     The following descriptions of the preferred embodiments are provided to understand the features and the structures of the present invention.  
         [0013]     Please refer to  FIG. 1 , which is a structural view showing a first preferred embodiment according to the present invention. As shown in the figure, the present invention is a time-interleaved analog-to-digital converter (ADC) having a timing calibration. The first embodiment is a two-channel background calibrated time-interleaved ADC  1 , comprising a multi-phase clock generator  11 , a clock random chopper  12 , a first programmable delay unit  131 , a second programmable delay unit  132 , a first ADC  141 , a second ADC  142 , a data recovery chopper  15 , a calibration processor  16  (CP) and a random sequence generator  17 .  
         [0014]     Therein, the CP  16  comprises a zero-crossing detector  161 , a first accumulator  162 , a bilateral peak detector (BPD)  163  and a second accumulator  164 . The first accumulator  162  can be a flexible-symbol accumulator; or, the first accumulator  162  and the second accumulator  164  can be replaced with a counter.  
         [0015]     When using the first preferred embodiment, the multi-phase clock generator  11  provides a first phase and a second phase to the clock random chopper  12  for a timing change. The random sequence generator  17  outputs a random signal to the clock random chopper  12  to decide the state when the first phase and the second phase transfer to the first programmable delay unit  131  and the second programmable delay unit  132 . When the random signal is 1, the state is a positive state and the first phase and the second phase are transferred to the first programmable delay unit  131  and the second programmable delay unit  132  respectively. Or, when the random signal is −1, the state is a negative state and the first phase and the second phase are transferred to the second programmable delay unit  132  and the first programmable delay unit  131  respectively. The occurrence of the positive state and negative state are 50% and 50%; and, the multi-phase clock generator  11  can be a phase-locked loop.  
         [0016]     The CP  16  outputs a digital control signal to the second programmable delay unit  132  for a timing-skew calibration until timing differences are equal between the first programmable delay unit  131  and the second programmable delay unit  132 . The first programmable delay unit  131  and the second programmable delay unit  132  generate output signals to the first ADC  141  and the second ADC  142  respectively to obtain a first digital signal and a second digital signal. The first digital signal and the second digital signal are outputted to the data recovery chopper  15  to obtain the original signal without aliasing and are transferred to the CP  16 . Then signals are kept inputting with circular works.  
         [0017]     The zero-crossing detector  161  of the CP  16  receives the first digital signal and the second digital signal from the data recovery chopper  15  to find out whether a value smaller then 0 is obtained by multiplying the first digital signal by the second digital signal; and a number of level crossings to 0 level is counted. The number of level crossings has a one-to-one mapping to a cross-correlation coefficient of the first digital signal and the second digital signal to obtain timing-skew information. Such an effect is achieved by simple logic gates without a great amount of calculations by large circuits, such as multipliers. When a number of level crossings under positive state and a number of level crossings under negative state are the same, there is no timing skew. When the number of level crossings under positive state is bigger than the number of level crossings under negative state, a timing difference of the first programmable delay unit  131  and the second programmable delay unit  132  is big and there is a positive timing-skew error. When the number of level crossings under positive state is smaller than the number of level crossings under negative state, the timing difference of the first programmable delay unit  131  and the second programmable delay unit  132  is small and there is a negative timing-skew error. Hence, the polarity of skew errors is acknowledged by the zero-crossing detector  161 ; and, an output signal of the zero-crossing detector  161  is multiplied by the random signal outputted from the random sequence generator to obtain a value of a related variable.  
         [0018]     The first accumulator  162  accumulates values of the related variable to obtain the state of the timing skew. For example, when there is a positive timing-skew error, the related variable is deducted to a smaller value; on the contrary, when there is a negative timing-skew error, the related variable is added to a bigger value. Thus, the result after the accumulation of the first accumulator  162  is the value of the timing skew and shows the polarity of the skew error. The longer the duration and the bigger the number of the accumulation, the more authentic is the result. Hence, a negative feedback system is formed for processing timing-skew calibration to obtain a minimum timing skew.  
         [0019]     The accumulated result is then inputted into the BPD  163 . The BPD  163  is used to monitor the first accumulator with a decision threshold. When the accumulated result is bigger than the decision threshold, the BPD  163  outputs a signal with +1 value; when the accumulated result is smaller than the decision threshold, the BPD  163  outputs a signal with −1 value; and, the BPD  163  outputs a signal with 0 value when outside of the above situations. When the BPD  163  outputs a signal with a value other than 0, the first accumulator  162  is reset to zero for the next accumulation and thus the BPD  163  stays in a state of outputting a signal with a non-zero value only for a timing period to credibly obtain a polarity of a skew error.  
         [0020]     The second accumulator  164  accumulates output signals from the BPD  163 ; and the accumulated result is used to control the second programmable delay unit  132  to equalize timing differences (to a default value) for the second programmable delay unit  132  and the first programmable delay unit. When the accumulated result is added by 1, the timing skew is added with a fixed ultra-low level. Yet the timing skew is not added with a fixed value, but fluctuating around zero, which is regarded as a fluctuation noise. By precisely estimating a related parameter for the two-channel background calibrated time-interleaved ADC  1 , the fluctuation noise is restrained to prevent from affecting the whole performance.  
         [0021]     Please refer to  FIG. 2 , which is a structural view showing a second preferred embodiment. As shown in the figure, the second embodiment is a multi-channel time-interleaved ADC  2 , comprising a multi-phase clock generator  21 , a multi-channel clock random chopper  22 , a multi-channel programmable delay unit  23 , a multi-channel ADC  24 , a multi-channel data recovery chopper  25 , a multi-channel CP  26  and a random sequence generator  27 . Therein, the multi-channel CP  26  comprises a multi-channel zero-crossing detector  261  and a multi-channel co-relater together with accumulators  262 ; the multi-channel co-relater together with accumulators  262  comprises a co-relater and a plurality of accumulators; and, the accumulator is a flexible-symbol accumulator or a general accumulator. The multi-phase clock generator  21  generates a plurality of phases; and, the number of the phases, which has to be an even number, is obtained by dividing a sampling period of each ADC by a timing difference of each phase. From the number of the phases, a number of channels of the multi-channel time-interleaved ADC  2  is obtained, such as a four-channel time-interleaved ADC, a six-channel time-interleaved ADC, etc. Furthermore, the multi-channel time-interleaved ADC  2  can be used for narrow-band signals and wide-band signals, where the bandwidth of a signal has an upper limit bigger than a reciprocal of two times of the sampling period; and has a lower limit of a reciprocal of two times of the timing difference of phase. Consequently, signals in the bandwidth between the upper limit and the lower limit are calibrated by the multi-channel time-interleaved ADC  2 .  
         [0022]     When using the second preferred embodiment, the multi-phase clock generator  21  provides n phases (ψ i , i=0, 1, 2 . . . N−1, N is an even number) to the multi-channel clock random chopper  22 . The random sequence generator  27  outputs a first random signal and a second random signal. The first random signal determines a timing change and the second random signal determines a state from a positive state and a negative state. The multi-channel clock random chopper  22  generates output signals to the multi-channel ADC  24  to be transformed into multi-channels of signals; and the signals are recovered by the multi-channel data recovery chopper  25  to be transferred to the multi-channel CP  26 .  
         [0023]     When the first random signal is 1 and the second random signal is in a positive state, the n phases are grouped by two of a phase and an adjacent phase with a sequence of (ψ i , ψ i+1 ) into (ψ o , ψ 1 ), (ψ 1 , ψ 2 ), . . . , (ψ i , ψ i+1 ). The multi-channel co-relater together with accumulators  262  uses a reference phase as a base and a farer phase to the reference phase is used for a calibration to obtain a timing difference to a default value between the phase ψ i  and the adjacent phase ψ i+1 . On the contrary, when the second random signal is in a negative state, a group of a phase and an adjacent phase with a sequence of (ψ i+1 , ψ i ) is obtained and a fare r phase to the reference phase is used for a calibration to obtain a timing difference to a default value between the phase ψ i+1  and the adjacent phase ψ i .  
         [0024]     Or, when the first random signal is 0 and the second random signal is in a positive state, the n phases are grouped by two of a phase and an adjacent phase with a sequence of (ψ i−1 , ψ i ) into (ψ 1 , ψ 2 ), (ψ 2 , ψ 3 ), . . . , (ψ i−1 , ψ i ). The multi-channel co-relater together with accumulators  262  uses a reference phase as a base, and a farer phase to the reference phase is used for a calibration to obtain a timing difference to a default value between the phase ψ i−1  and the adjacent phase ψ i . On the contrary, when the second random signal is in a negative state, a group of a phase and an adjacent phase with a sequence of (ψ i , ψ i−1 ) is obtained and a farer phase to the reference phase is used for a calibration to obtain a timing difference to a default value between the phase ψ i  and the adjacent phase ψ i−1 .  
         [0025]     The multi-channel zero-crossing detector  261  receives the signals from the multi-channel data recovery chopper  25  for an accumulation and a histogram of zero crossing to obtain information of phase calibration. And the multi-channel co-relater together with accumulators  262  outputs a signal to the multi-channel programmable delay unit  23  for a calibration to optimize the multi-channel time-interleaved ADC  2 .  
         [0026]     A high-pass filter can be further applied at an output of the multi-channel ADC  24  to filter offset errors in the multi-channel ADC  24 . Because the multi-channel time-interleaved ADC  2  compares the polarities of skew errors only to obtain information of timing skew, the present invention is not affected by the gain errors in the ADC so that yield rate is improved with easy fabrication and a better calibration is obtained at the same time.  
         [0027]     To sum up, the present invention is a time-interleaved ADC having a timing calibration, where only simple logic circuits are used in the present invention for narrow-band signals and wide-band signals and offset errors in ADC are restrained so that a better calibration, a dynamic calibration, is obtained without interruption of circuit.  
         [0028]     The preferred embodiments herein disclosed are not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.