Patent Publication Number: US-7724173-B2

Title: Time-interleaved analog-to-digital-converter

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a time-interleaved analog-to-digital converter. It also relates to a method for controlling the time-interleaved analog-to-digital converter. 
   DESCRIPTION OF RELATED ART 
   A time-interleaved (TI) analog-to-digital converter (ADC) utilizes a plurality of sub ADCs with a common analog input. The plurality of sub ADCs operate at a common first clock frequency or sampling frequency. A multi-phase clock unit is used to generate individual clock signals for each sub ADC. The individual clock signals generated by the multi-phase clock unit are mutually displaced in time, so as to achieve effective sampling for the TI-ADC at a second clock frequency or sampling frequency which is higher than the first clock frequency or sampling frequency with a factor equal to the number of sub ADCs in the TI-ADC. 
   The sub ADCs in a TI-ADC are normally operated in an order corresponding to their physical placement on a chip. For example, the sub ADCs may be physically arranged in an array with one sub ADC in each row. The sub ADCs may be operated starting with the sub ADC in the first row followed by the second row (which is placed adjacent to the first row), followed by the third row (which is placed adjacent to the second row), etc., until the last row is reached, whereupon the first row is used again. 
   Offset errors in the sub ADCs contribute to distortion, such as spurious tones, in the output of the TI-ADC. To some extent, such offset errors may be compensated for by applying digital signal processing (DSP) either to individual digital outputs of the sub ADCs or to a combined output of the TI-ADC. Such compensation requires DSP circuitry, which adds an undesired portion to the circuit area and power consumption of the TI-ADC. Hence, such arrangements for reducing the influence of offset errors may be complex. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a TI-ADC with reduced susceptibility to offset errors. It is a further object of the present invention to provide a TI-ADC with reduced complexity. 
   According to a first aspect, a method is provided for operating a time-interleaved analog-to-digital converter for converting an analog input to a digital output using a time-interleaved analog-to-digital converter, wherein the time-interleaved analog-to-digital converter comprises an array of M sub ADCs, where M is an even integer, and each row of the array comprises one of the M sub ADCs. The method comprises the step of, for every sampling instant n, where n is an integer in a sequence of integers, converting the analog input by means of the sub ADC in row k(n) of the array, wherein 1≦k(n)≦M, a value between 1 and M is assigned to k(n) for the first sampling instant, and k(n+1) is selected such that 
   a) k(n+1)&gt;M/2 if k(n)≦M/2, otherwise k(n+1)≦M/2; 
   b) M/2−1≦|k(n+1)−k(n)|≦M/2+1; and 
   c) k(n+1)=k(m+1) if and only if n−m is an integer multiple of M. 
   The value of k(n+1) may be chosen according to
         if k(n)&lt;M/2 and k(n) is odd, then k(n+1)=k(n)+M/2+1;   if k(n)=M/2 and k(n) is odd, then k(n+1)=M;   if k(n)≦M/2 and k(n) is even, then k(n+1)=k(n)+M/2−1;   if k(n)=M/2+1, then k(n+1)=1;   if k(n)&gt;M/2+1 and k(n)−M/2 is odd, then k(n+1)=k(n)−M/2−1;   if k(n)=M and k(n)−M/2 is even, then k(n+1)=M/2; or   if M/2+1&lt;k(n)&lt;M and k(n)−M/2 is even, then k(n+1)=k(n)−M/2+1.       

   Alternatively, if M/2 is an odd number, k(n+1) may be chosen according to
         if k(n)=1 then k(n+1) is set to M/2+1;   if k(n)≦M/2, k(n)≠1, and k(n) is odd, then k(n+1) is set to k(n)+M/2−1;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2+1;   if k(n)=M then k(n+1) is set to M/2;   if M/2&lt;k(n)&lt;M and k(n) is odd, then k(n+1) is set to k(n)−M/2−1; or   if M/2&lt;k(n)&lt;M and k(n) is even, then k(n+1) is set to k(n)−M/2+1.       

   According to a second aspect, a computer program product comprises computer program code means for executing the method when said computer program code means are run by an electronic device having computer capabilities. 
   According to a third aspect, a computer readable medium has stored thereon a computer program product comprising computer program code means for executing the method when said computer program code means are run by an electronic device having computer capabilities. 
   According to a fourth aspect, a time-interleaved analog-to-digital converter comprising an array of M sub ADCs, where M is an even integer, wherein each row of the array comprises one of the M sub ADCs is provided. The time-interleaved analog-to-digital converter comprises a control unit arranged to, for every sampling instant n, where n is an integer in a sequence of integers, select the sub ADC in row k(n), 1≦k(n)≦M, of the array that is used for converting an analog input to a digital output. The control unit is adapted to assign a value between 1 and M to k(n) for the first sampling instant and to, for a given value of k(n), select k(n+1) such that 
   a) k(n+1)&gt;M/2 if k(n)≦M/2, otherwise k(n+1)≦M/2; 
   b) M/2−1≦|k(n+1)−k(n)|≦M/2+1; and 
   c) k(n+1)=k(m+1) if and only if n−m is an integer multiple of M. 
   The control unit may be arranged to select k(n+1) according to
         if k(n)&lt;M/2 and k(n) is odd, then k(n+1) is set to k(n)+M/2+1;   if k(n)=M/2 and k(n) is odd, then k(n+1) is set to M;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2−1;   if k(n)=M/2+1, then k(n+1) is set to 1;   if k(n)&gt;M/2+1 and k(n)−M/2 is odd, then k(n+1) is set to k(n)−M/2−1;   if k(n)=M and k(n)−M/2 is even, then k(n+1) is set to M/2; or   if M/2+1&lt;k(n)&lt;M and k(n)−M/2 is even, then k(n+1) is set to k(n)−M/2+1.       

   If M/2 is an odd number, the control unit may be arranged to select k(n+1) according to
         if k(n)=1 then k(n+1) is set to M/2+1;   if k(n)≦M/2, k(n)≠1, and k(n) is odd, then k(n+1) is set to k(n)+M/2−1;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2+1;   if k(n)=M then k(n+1) is set to M/2;   if M/2&lt;k(n)&lt;M and k(n) is odd, then k(n+1) is set to k(n)−M/2−1; or   if M/2&lt;k(n)&lt;M and k(n) is even, then k(n+1) is set to k(n)−M/2+1.       

   The control unit may comprise a multi-phase clock unit arranged to generate an individual clock signal at each of a plurality of clock terminals of the multi-phase clock unit. Each of said plurality of clock terminals may be connected to one of the sub ADCs. 
   The multi-phase clock unit may include a delay-locked loop and/or a plurality of delay elements connected in a ring. 
   According to a fifth aspect, an integrated circuit comprises the time-interleaved ADC. 
   According to a sixth aspect, an electronic apparatus comprises the time-interleaved ADC. The electronic apparatus may be but is not limited to a monitor, a projector, a television set, or a radio transceiver. 
   Further embodiments of the invention are defined in the dependent claims. 
   It is an advantage of some embodiments that a large fraction of the signal power or energy of an error in the output of the time-interleaved ADC caused by offset errors may appear at the Nyquist frequency of the time-interleaved ADC where it can be removed by filtering. As a consequence, the need for compensating for offset errors using complex digital signal processing may be reduced or eliminated, which is a further advantage. 
   It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps, or components, but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects, features and advantages of the invention will appear from the following detailed description of the invention, reference being made to the accompanying drawings, in which: 
       FIG. 1  is a block diagram of a TI-ADC according to an embodiment; 
       FIGS. 2   a  and  b  are flow charts for controlling the order of operation of sub ADCs in a TI-ADC according to embodiments; 
       FIG. 3   a  is a graph of an offset error sequence resulting from a straightforward selection of the order of operation of sub ADCs in a TI-ADC when offset errors of the sub ADC vary linearly over an array of sub ADCs; 
       FIG. 3   b  is a graph of the offset error sequence resulting from a selection of the order of operation of sub ADCs in a TI-ADC according to the flow chart in  FIG. 2   a  when offset errors of the sub ADC vary linearly over an array of sub ADCs; 
       FIGS. 3   c  and  d  are graphs of a first and a second term of the offset error sequence shown in  FIG. 3   b , respectively; 
       FIG. 4  is a block diagram of an embodiment of a control unit connected to an array of sub ADCs; 
       FIG. 5  shows example waveforms for a multi-phase clock unit; and 
       FIG. 6  is a block diagram of an embodiment of a multi-phase clock unit. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1  shows a block diagram of a time-interleaved (TI) analog-to-digital converter (ADC)  1  according to an embodiment. The TI-ADC  1  may comprise an array  20  of M sub ADCs ADC 1 , . . . , ADC M , where M may be an even integer. Each row of the array  20  may comprise exactly one of the sub ADCs ADC 1 , . . . , ADC M , as indicated in  FIG. 1 . The placement of sub ADCs in  FIG. 1  reflects the physical placement of sub ADCs on a chip or integrated circuit, i.e. ADC 1  may be placed in a first row of the array, ADC 2  may be placed in a second row adjacent to the first row, etc., and ADC M  may be placed in a last row of the array. Each of the sub ADCs ADC 1 , . . . , ADC M  operate at a common first clock frequency or sampling frequency. Sampling instants of the sub ADCs ADC 1 , . . . , ADC M  are mutually displaced in time, so as to achieve effective sampling of the TI-ADC  1  at a second clock frequency or sampling frequency, which is higher than the first clock frequency or sampling frequency with a factor equal to the number M of sub ADCs ADC 1 , . . . , ADC M . 
   The TI-ADC may comprise a control unit  10 . The control unit  10  may be adapted to control the order in which the sub ADCs ADC 1 , . . . , ADC M  are used. The control unit  10  may additionally be adapted to generate common clock signals and individual clock signals for the sub ADCs ADC 1 , . . . , ADC M . The control unit  10  may further be adapted to generate control signals for a selector unit  30 . Outputs of the sub ADCs ADC 1 , . . . , ADC M  may be operatively connected to the selector unit  30 . The selector unit  30  may be adapted to select one of the outputs of the sub ADCs ADC 1 , . . . , ADC M  and forward the selected output to an output  3  of the TI-ADC  1 . The selection may be based on the control signals supplied to the selector unit  30  by the control unit  10 . 
   An offset error of a sub ADC ADC 1 , . . . , ADC M  may be modeled with a constant added to the input of said sub ADC ADC 1 , . . . , ADC M . Offset errors may e.g. show an approximately linear variation over the array  20 , e.g. due to so called parameter gradients for circuit elements such as transistors and resistors. Operating or using the sub ADCs ADC 1 , . . . , ADC M  in a straightforward order such that if a sample is converted by ADC k , the following sample is converted by ADC k+1  for k=1, 2, . . . , M−1, and if a sample is converted by ADC M , the following sample is converted by ADC 1  in such a situation may result in a large amount of distortion in a signal band or Nyquist band of the TI-ADC. 
   According to an embodiment of the invention, the need for complex digital signal processing for compensating for offset errors may be reduced or eliminated using a method for controlling the TI-ADC  1  that provides a modified order of operating the sub ADCs ADC 1 , . . . , ADC M . In the following, ADC k(n) , which is placed in row k(n) of the array  20 , denotes the sub ADC that is used to convert the analog input at the sample instant n. The sequence k(n) determines the order in which the sub ADCs ADC 1 , . . . , ADC M  are used. For example, if k(n)=1, then ADC 1  is used to convert the analog input at sample instant n, if k(n)=2, then ADC 2  is used to convert the analog input at sample instant n, etc. An arbitrary or predefined value may be assigned to k(n) for the first or initial sample instant. Further, for every n, given a value of k(n), k(n+1) may be chosen such that 
   a) k(n+1)&gt;M/2 if k(n)≦M/2, otherwise k(n+1)≦M/2; 
   b) M/2−1≦|k(n+1)−k(n)|≦M/2+1; and 
   c) k(n+1)=k(m+1) if and only if m−n is an integer multiple of M. 
   The condition c) ensures that each sub ADC ADC 1 , . . . , ADC M  is used to convert every M:th sample of the analog input. Conditions a) and b) results in that the sequence k(n) has an oscillating or toggling component with approximate amplitude of M/4. For example, for a TI-ADC where the offset errors has an approximately linear variation over the array  20 , this results in that the distortion due to the offset errors has a spectral distribution such that a relatively large fraction of the total distortion energy is located at a frequency corresponding to half the sampling frequency, i.e. at the Nyquist frequency, of the TI-ADC  1 . Said fraction of the total distortion energy may be removed using a linear filter, e.g a low-pass filter. The complexity of said linear filter may be reduced using oversampling. Hence, the need for complex digital signal processing to compensate for the offset errors may be reduced. Consequently, the complexity of the TI-ADC may be reduced. 
   An embodiment of the method is illustrated with a flow chart in  FIG. 2   a . For every value of n, given a value of k(n), k(n+1) may be chosen according to
         if k(n)&lt;M/2 and k(n) is odd, then k(n+1) is set to k(n)+M/2+1 in step  108 ;   if k(n)=M/2 and k(n) is odd, then k(n+1) is set to M in step  109 ;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2−1 in step  110 ;   if k(n)=M/2+1, then k(n+1) is set to 1 in step  111 ;   if k(n)&gt;M/2+1 and k(n)−M/2 is odd, then k(n+1) is set to k(n)−M/2−1 in step  112 ;   if k(n)=M and k(n)−M/2 is even, then k(n+1) is set to M/2 in step  113 ; or   if M/2+1&lt;k(n)&lt;M and k(n)−M/2 is even, then k(n+1) is set to k(n)−M/2+1 in step  114 .       

   Examples of the order in which the sub ADCs ADC 1 , . . . , ADC M  may be operated according to the flow chart of  FIG. 2   a  are given below for a few different values of M. The sub ADCs ADC 1 , . . . , ADC M  are operated cyclically with a cycle that repeats itself every M:th sample. Only one cycle is shown in the examples. 
   M=2: 
   ADC 1 , ADC 2    
   M=4: 
   ADC 1 , ADC 4 , ADC 2 , ADC 3    
   M=6: 
   ADC 1 , ADC 5 , ADC 3 , ADC 6 , ADC 2 , ADC 4    
   M=12: 
   ADC 1 , ADC 8 , ADC 3 , ADC 10 , ADC 5 , ADC 12 , ADC 6 , ADC 11 , ADC 4 , ADC 9 , ADC 2 , ADC 7    
   M=14: 
   ADC 1 , ADC 9 , ADC 3 , ADC 11 , ADC 5 , ADC 13 , ADC 7 , ADC 14 , ADC 6 , ADC 12 , ADC 4 , ADC 10 , ADC 2 , ADC 8    
   Alternatively, if M/2 is an odd number, the order in which the sub ADCs ADC 1 , . . . , ADC M  are operated may be selected in an order given by the flow chart in  FIG. 2   b . Then, for every value of n, given a value of k(n), k(n+1) may be chosen according to
         if k(n)=1 then k(n+1) is set to M/2+1 in step  207 ;   if k(n)≦M/2, k(n)≠1, and k(n) is odd, then k(n+1) is set to k(n)+M/2−1 in step  208 ;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2+1 in step  209 ;   if k(n)=M then k(n+1) is set to M/2 in step  210 ;   if M/2&lt;k(n)&lt;M and k(n) is odd, then k(n+1) is set to k(n)−M/2−1 in step  211 ; or   if M/2&lt;k(n)&lt;M and k(n) is even, then k(n+1) is set to k(n)−M/2+1 in step  212 .       

   Using the embodiment according to the flow chart of  FIG. 2   b  results in a reverse order of operation compared with using the embodiment according to the flow chart of  FIG. 2   a . For example, for M=14, the sub ADCs will be operated in the following order (showing one cycle of 14 samples). 
   ADC 1 , ADC 8 , ADC 2 , ADC 10 , ADC 4 , ADC 12 , ADC 6 , ADC 14 , ADC 7 , ADC 13 , ADC 5 , ADC 11 , ADC 3 , ADC 9    
   An example showing the consequences of using the embodiment according to the flow chart of  FIG. 2   a  will be given in the following. In the example, the offset errors are modeled with a linear variation over the array  20 . The offset error o(k) associated with the sub ADC ADC k  is modeled with 
             o   ⁡     (   k   )       =           c   1     ⁡     (     k   -       M   +   1     2       )       +     c   2       =         o   1     ⁡     (   k   )       +     c   2               
where c 1  and c 2  are constants. The constant c 2  give rise to an overall DC offset of the TI-ADC  1 . Such a DC offset may be ignored in some applications. Alternatively, it may be compensated for using a simple subtraction with a constant. Therefore, only the first term o 1 (k) of o(k) will be considered in the following. For a given sequence k(n) defining the order in which the sub ADCs ADC 1 , . . . , ADC M  are used, a offset error sequence o 1 (k(n)) will result. The offset errors have the same effect as if the offset error sequence o 1 (k(n)) would be added to the samples of the analog input signal in an ADC without offset errors.
 
   As an example, the case M=14 will be considered. First, if the sub ADCs ADC 1 , . . . , ADC M  are operated in the straightforward order where the sequence k(n)= . . . , 1, 2, 3, . . . , 13, 14, 1, 2, . . . , the resulting offset error sequence o 1 (k(n)) will be as illustrated in  FIG. 3   a . Only one cycle of 14 samples of the repetitive offset error sequence o 1 (k(n)) is shown in  FIG. 3   a.    
   If the sequence k(n) is instead chosen according to the embodiment illustrated with the flow chart in  FIG. 2   a , the resulting offset error sequence o 1 (k(n)) will be as illustrated in  FIG. 3   b . For this choice of the sequence k(n), i.e. k(n)= . . . , 1, 8, 3, 10, 5, 12, 6, 11, 4, 9, 2, 7, 1, 8, 3, . . . , the offset error sequence o 1 (k(n)) may be decomposed into a sum of two sequences o 2 (n) and o 3 (n) illustrated in  FIG. 3   c  and  FIG. 3   d , respectively. The sequence o 2 (n) is toggling or oscillating with a frequency corresponding to half the sampling frequency of the TI-ADC. Hence, it may be filtered out using a linear filter. Only the sequence o 3 (n) contribute to distortion at frequencies below the Nyquist frequency of the TI-ADC. As in  FIG. 3   a ,  FIGS. 3   b - d  show one cycle of 14 samples. The ratio between the total signal energies over a cycle of 14 samples for the offset error sequence o 1 (k(n)) of  FIG. 3   a  and the sequence o 3 (n) of  FIG. 3   d  is 4.0625. Hence, using the method illustrated with the flow chart of  FIG. 2   a , the distortion energy at frequencies below the Nyquist frequency of the TI-ADC due to offset errors is approximately a factor four smaller than when the sub ADCs ADC 1 , . . . , ADC M  are used in the straightforward order determined by k(n)= . . . , 1, 2, 3, . . . , 13, 14, 1, 2, . . . for the considered example. 
   According to an embodiment, the control unit  10  may be arranged to, for every n, select the sub ADC ADC k(n)  in row k(n), 1≦k(n)≦M, of the array  20  that is used for converting the analog input at the sample instant n by means of assigning an arbitrary or predefined value to k(n) for the first sample instant and selecting k(n+1) such that 
   a) k(n+1)&gt;M/2 if k(n)≦M/2, otherwise k(n+1)≦M/2; 
   b) M/2−1≦|k(n+1)−k(n)|≦M/2+1; and 
   c) k(n+1)=k(m+1) if and only if n−m is an integer multiple of M. 
   For example, the control unit  10  may be arranged to select k(n+1) according to the flow chart of  FIG. 2   a , i.e.
         if k(n)&lt;M/2 and k(n) is odd, then k(n+1) is set to k(n)+M/2+1;   if k(n)=M/2 and k(n) is odd, then k(n+1) is set to M;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2−1;   if k(n)=M/2+1, then k(n+1) is set to 1;   if k(n)&gt;M/2+1 and k(n)−M/2 is odd, then k(n+1) is set to k(n)−M/2−1;   if k(n)=M and k(n)−M/2 is even, then k(n+1) is set to M/2; or   if M/2+1&lt;k(n)&lt;M and k(n)−M/2 is even, then k(n+1) is set to k(n)−M/2+1.       

   Alternatively, if M/2 is an odd number, the control unit  10  may be arranged to select k(n+1) according to the flow chart of  FIG. 2   b , i.e.
         if k(n)=1 then k(n+1) is set to M/2+1;   if k(n)≦M/2, k(n)≠1, and k(n) is odd, then k(n+1) is set to k(n)+M/2−1;   if k(n)≦M/2 and k(n) is even, then k(n+1) is set to k(n)+M/2+1;   if k(n)=M then k(n+1) is set to M/2;   if M/2&lt;k(n)&lt;M and k(n) is odd, then k(n+1) is set to k(n)−M/2−1; or   if M/2&lt;k(n)&lt;M and k(n) is even, then k(n+1) is set to k(n)−M/2+1.       

   The control unit  10  may include a finite state machine comprising a combinational logic block that determines k(n+1) based on k(n). Alternatively, the control unit may include a programmable logic unit, such as a central processing unit (CPU) or a field-programmable gate array (FPGA), programmed to compute k(n+1) based on k(n). 
   In an alternative embodiment, the control unit may include a multi-phase clock unit ( 11 ) having M clock terminals C 1 , . . . , C M  as illustrated with a block diagram in  FIG. 4  for M=14. The multi-phase clock unit ( 11 ) may be adapted to generate individual clock signals at each of the M clock terminals. Each of the individual clock signals may have the first clock frequency. The individual clock signals may be mutually displaced in time. Examples of waveforms for the individual clock signals are shown in  FIG. 5  for M=14. For each waveform in  FIG. 5 , one cycle of the individual clock signal is shown. Since k(n) is repetitive with a cycle of M samples, the above described functionality of the control unit  10  may be achieved by connecting each of the clock terminals C 1 , . . . , CM to an individual sub ADC ADC 1 , . . . , ADC M  such that the sub ADCs are operated in an order determined e.g. by the sequence k(n) given by the flow chart of  FIG. 2   a  or the sequence k(n) given by the flow chart in  FIG. 2   b . An example of such a connection in accordance with the flow chart in  FIG. 2   a  is given in  FIG. 4  for M=14, wherein C 1  is connected to ADC 1 , C 2  is connected to ADC 9 , C 3  is connected to ADC 3 , C 4  is connected to ADC 11 , C 5  is connected to ADC 5 , C 6  is connected to ADC 13 , C 7  is connected to ADC 7 , CB is connected to ADC 14 , C 9  is connected to ADC 6 , C 10  is connected to ADC 12 , C 11  is connected to ADC 4 , C 12  is connected to ADC 10 , C 13  is connected to ADC 2 , and C 14  is connected to ADC 8 . For example, each sub ADC ADC 1 , . . . , ADC M  may be adapted to sample the analog input signal at an edge, such as a rising edge or a falling edge, of the individual clock signal supplied to it. 
   The control unit  10  may also include additional circuitry (not shown in  FIG. 4 ), e.g. for generating various control signals for components of the TI-ADC  1 , such as the selector unit  30  ( FIG. 1 ). 
   The multi-phase clock unit  11  may, e.g., be implemented with a delay-locked loop (DLL). Alternatively, the multi-phase clock unit  11  may be implemented with a plurality of delay elements, such as D flip-flops D 1 , . . . , D M  connected in a ring, as shown in  FIG. 6 . The D flip-flops D 1 , . . . , D M  may be clocked with a clock signal clk having the second clock frequency. In a start-up phase, the output of one of the D flip-flops D 1 , . . . , D M  may be set to ‘1’, while the outputs of the other D flip-flops D 1 , . . . , D M  are reset to ‘0’. During operation of the multi-phase clock unit  11 , said ‘1’ will be transferred around the ring of D flip-flops so as to generate the individual clock signals at the outputs of the D flip-flops D 1 , . . . , D M . Alternatively, the output of two or more adjacent D flip-flops D 1 , . . . , D M  may be set to ‘1’ during the start-up phase, e.g. to obtain a different duty cycle of the individual clock signals. 
   The TI-ADC  1  may be comprised in an integrated circuit. The TI-ADC may further be comprised in an electronic apparatus, such as but not limited to a monitor, a projector, a television set, or a radio transceiver. 
   The invention may be embedded in a computer program product, which enables implementation of the method and functions described herein. The invention may be carried out when the computer program product is loaded an run in a system having computer capabilities. Computer program, software program, program product, or software, in the present context mean any expression, in any programming language, code or notation, of a set of instructions intended to cause a system having a processing capability to perform a particular function directly or after conversion to another language, code or notation. 
   The present invention has been described above with reference to specific embodiments. However, other embodiments than the above described are possible within the scope of the invention. Different method steps than those described above, performing the method by hardware or software, may be provided within the scope of the invention. The different features and steps of the invention may be combined in other combinations than those described. The scope of the invention is only limited by the appended patent claims.