Abstract:
A mixed signal test system for testing a semiconductor device having both an analog function and a digital function achieves improved resolution and low cost. The test system is formed of a functional test unit for testing a digital function of a device under test (DUT), an analog test unit (ATU) for testing an analog function of the DUT, and a synchronous control unit for synchronizing operations between the functional test unit and the analog test unit. The analog test unit includes a digitizer for converting an analog output of the DUT into a digital signal, and an acquisition memory for storing the digital signal from the digitizer in specified addresses. The wave form of the analog output is repeated by a plurality of cycles and a sampling clock for the digitizer is phase shifted by a predetermined amount for each cycle.

Description:
FIELD OF THE INVENTION 
   This invention relates to a semiconductor test system for testing semiconductor devices such as mixed signal ICs and LSIs, and more particularly, to a semiconductor test system having a digitizer for continuously performing AD conversion of an analog signal from a device under test where an equivalent sampling frequency in the AD conversion is substantially increased. 
   BACKGROUND OF THE INVENTION 
   In testing semiconductor devices such as ICs and LSIs by a semiconductor test system, such as an IC tester, a semiconductor IC device to be tested is provided with test signals produced by an IC tester at its appropriate tester pins (channels) at predetermined test timings. The IC tester receives output signals from the IC device under test generated in response to the test signals. The output signals are strobed by strobe signals with predetermined timings to be compared with expected data to determine whether or not the IC device properly performs the intended functions. This is a basic process for testing a logic device by a semiconductor test system. 
   A semiconductor device to be tested may also include analog functional blocks such as an AD converter and/or a DA converter as well as a digital functional block. Such a semiconductor device is sometimes called a mixed signal IC. An example of such a mixed signal IC is a semiconductor integrated circuit designed for modems, audio and/or video devices, and the like. 
   An example of semiconductor test system for testing such a mixed signal IC device (mixed signal test system) in the conventional technology is shown in  FIGS. 4-7 .  FIG. 4  shows a basic structure in the conventional mixed signal test system and  FIGS. 5-7  relate to a digitizer in the mixed signal test system. A device under test (DUT) is a mixed signal IC including an analog function and a digital function. When testing, the DUT is place on a test station to receive test signals from the mixed signal test system and produce response outputs. The mixed signal test system of  FIG. 4  includes a functional test unit (FTU) for testing a digital function of the DUT and an analog test unit (ATU) for testing an analog function of the DUT and a synchronous control unit  40  for synchronizing the functional test unit (FTU) and the analog test unit (ATU) with one another. 
   In  FIGS. 4 and 5 , the functional test unit (FTU) includes a timing generator TG, a pattern generator such as an algorithmic pattern generator (ALPG) or a sequential pattern generator (SQPG), and a format controller (FC). The functional test unit (FTU) has a large number of tester pins (channels), such as 256 pins, corresponding to terminal pins of the device to be tested (DUT) . At the output of the format controller FC, each tester pin provides a test pattern to the corresponding pin of the DUT. 
   The timing generator TG generates timing signals such as a rate clock to synchronize the timing of the functional test unit and provides the timing pulses to the pattern generator SQPG. The test pattern generator SQPG generates a test pattern based on a test program in response to the rate clock from the timing generator TG. The timing generator TG also generates timing data and wave form data to be used in the format controller FC to produce tester rates, delay timings and wave forms in the test pattern. The test pattern at the output of the format controller FC is provided to the DUT through a pin electronics PE. 
   The synchronous control unit  40 , although not shown, includes an event master and a digital/analog synchronous controller. In receiving signals generated by the pattern generator SQPG, the synchronous control unit  40  produces a start signal and a trigger signal to be provided to the analog test unit (ATU). The start signal and trigger signal are used to synchronize test patterns generated by the functional test unit FTU and test signals generated by the analog test unit and measurement timings in the analog test unit. A clock generator  48  receives clock signals such as the rate clock from the timing generator TG and a clock from a synthesized signal generator (SSG) in the analog test unit to produce appropriate clock signals to be used in the analog test unit (ATU). 
   In the example of  FIG. 4 , the analog test unit (ATU) includes a variety of functional blocks such as a digital arbitrary wave form generator (DAW) for generating digital wave form data, an acquisition memory (AQM) for storing digital codes of an output signal of the DUT, a synthesized signal generator (SSG) for generating signals of various frequencies, an arbitrary wave form generator (AWG) for generating signals with wave forms defined by the wave form data from DAW, a digitizer (DGT) for converting an analog signal into a digital signal, a time measurement unit (TMU) for measuring time intervals and frequencies of an incoming signal, a precision voltage generator (PVS) for generating a reference DC voltage, a precision voltage meter (PVM) for measuring a DC voltage, a digital signal processor (DSP) for digital processing on digital data and a controller (CPU) for an overall operational control of the analog test unit(ATU). 
   Plural sets of the above listed resources may be provided in the analog test unit for performing signal generation and signal measurements in response to the synchronous signal from the synchronous control unit  40 . The analog test unit and the terminal pins of the DUT are connected through the pin electronics (PE). 
     FIG. 5  schematically shows a structure in the digitizer (DGT) in the analog test unit (ATU). The digitizer DGT of  FIG. 5  includes a filter (FLT)  60  and an AD converter (ADC)  30 . Since a wide variety of output signals, such as high speed signals or high precision wave forms, will be produced by the DUT, the AD converter  30  may constitute a plurality of AD converters with different degrees of speed and resolution. For example, the AD converter may include a combination of a high speed AD converter with 12-bit resolution and 100 MHz sampling rate and a high precision AD converter with 26-bit resolution and 100 KHz sampling rate. 
   The filter  60  is an antialiasing filter which is typically a low pass filter to prevent aliasing effects involved in a sampling process. A plurality of such filters with different pass band frequencies may be selectively used depending on the sampling frequencies. Typically, as an antialiasing filter, the filter  30  removes frequency components higher than ½ of the sampling frequency f c  from the output signal of the DUT received through the pin electronics PE. The output of the filter  60  is provided to the AD converter  30 . 
   The AD converter  30  samples an input signal from the filter  60  at each edge of the sampling clock  40   clk  having a sampling frequency f c  and converts the sampled voltage to a digital signal, i.e., code data  30   s . The code data  30   s  is stored in the acquisition memory (AQM)  50  in response to a memory timing signal  47   s  from the synchronous control unit  40 . The stored data in the acquisition memory (AQM)  50  is used for signal analysis and evaluation such as by the digital signal processor (DSP)  64 . 
   Since high resolution data can be obtained by increasing the number of sampling points, generally, a digitizer uses a highest possible sampling frequency to achieve both high even higher than the highest sampling frequency of an AD converter, an example of circuit arrangement such as shown is  FIG. 6  is used in the conventional technology. In  FIG. 6 , two AD converters  31  and  32  are arranged so as to operate in an interleave fashion for increasing the overall sampling speed by two times of each AD converter. 
   Namely, the digitizer of  FIG. 6  includes a filter (FLT)  60 , a first AD converter  31 , a second AD converter  32 , and a multiplexer  35 . The filter  60  is designed to function as an antialiasing filter for an equivalent sampling frequency f ce  which is two times higher than a sampling frequency of each of the AD converters  31  and  32 . The synchronous control unit  40  provides sampling clocks  41   clk  and  42   clk  to the first and second AD converters  31  and  32 , respectively. The synchronous control unit  40  also provides a square wave clock  45   s  to the multiplexer  35 , and a memory timing signal  47   s  to the acquisition memory  50 . 
     FIGS. 7A-7C  are timing charts showing the timing relationship between the first and second AD converters  31  and  32  and an overall sampling rate at the output of the multiplexer  35 . As shown in  FIG. 7A , the first AD converter  31  samples an input signal S i  from the filter  60  by a first sampling clock  41   clk  from the synchronous control unit  40  which is the highest possible sampling frequency. As shown in  FIG. 7B , the second AD converter  32  samples the input signal S i  from the filter  60  by a second sampling clock  42   clk  from the synchronous control unit  40  which is the highest possible sampling frequency. 
   The multiplexer  35  receives the digitized codes from the first and second AD converters  31  and  32  and alternately selects the codes at the timing of each rising edge and falling edge of the square clock signal  45   s  having the same repetition rate as that of the sampling clocks  41   clk  and  42   clk . The clock signal  45   s  has a square shape so as to have the same time interval between any adjoining two edges. Thus, an output signal  35   s  of the multiplexer  35  has an equivalent sampling frequency f ce  which is two times higher than the clock rate of the first or second sampling clock. 
   In the example of  FIGS. 6 and 7 , although only two AD converters are shown just for an illustration purpose, three or more AD converters are used to establish three or more higher equivalent sampling rates. Namely, in the conventional technology, to increase the overall sampling rate, a plurality of AD converters are arranged to operate in parallel fashion while the outputs of the AD converters are combined to form a serial signal having a repetition rate which is the plurality of times higher than that of each AD converter. 
   In the conventional technology, however, to increase the overall sampling rate, the number of circuit components such as AD converters increases in proportion to the increase of the sampling rate. As a consequence, in the conventional technology involving the interleave method, there is a problem that the circuit size and cost of the digitizer increases with the increase of the sampling rate. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a digitizer which is capable of increasing an equivalent sampling rate without involving any substantial increase in the circuit components. 
   It is another object of the present invention to provide a digitizer which is capable of increasing an equivalent total sampling rate with using a single AD converter and without increasing a frequency of a sampling clock to an AD converter. 
   It is a further object of the present invention to provide a mixed signal semiconductor test system which is capable of converting an analog output signal of a device under test to a digital signal with high conversion speed and high resolution. 
   It is a further object of the present invention to provide a mixed signal semiconductor test system which is capable of converting an analog output signal of a device under test to a digital signal with high conversion speed and high resolution and storing the digital signal in a memory at a predetermined address sequence. 
   In the present invention, the mixed signal test system for testing a semiconductor device having both an analog function and a digital function is comprised of a functional test unit for testing a digital function of a device under test (DUT) by providing a logic test pattern to the DUT and evaluating a response output of the DUT, an analog test unit for testing an analog function of the DUT by providing a test signal to the DUT and evaluating an analog output of the DUT, and a synchronous control unit for synchronizing operations between the functional test unit and the analog test unit, wherein the analog test unit includes a digitizer for converting the analog output of the DUT whose wave form in a fixed time period T is repeated by a plurality of cycles into a digital signal wherein a sampling clock for sampling the analog output is phase shifted by a predetermined amount for each cycle, and an acquisition memory for storing the digital signal from the digitizer in specified addresses thereof. 
   In another aspect of the present invention, an address generator is provided to generate address data in a predetermined order to store the digital data from the digitizer in the continuous addresses of the acquisition memory in the order of sampling points on the analog output with a difference of the phase shift. 
   According to the present invention, an input analog signal which repeats the same wave form in the time period T by M cycles is sampled by the digitizer for the M cycles wherein a phase of the sampling clock is shifted by a predetermined amount ΔP (delta phase) for each cycle. As a result, the data obtained in the AD conversion process of the present invention shows resolution M times higher than that obtained in the normal AD conversion, i.e., an equivalent ;sampling frequency is increased by M times. Therefore, a digitizer of high resolution and high speed is achieved without using a plurality of AD converters or a higher frequency sampling clock. Accordingly, the mixed signal test system having a high performance digitizer is realized with low cost and small circuits size. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram showing an example of structure of the mixed signal test system of the present invention with an emphasis on a digitizer. 
       FIGS. 2A and 2B  are timing charts showing a wave form and timing relationship in the digitizer used in the mixed signal test system of FIG.  1 . 
       FIG. 3  is a schematic block diagram showing a basic configuration of a phase shifter incorporated in the digitizer of the present invention. 
       FIG. 4  is a schematic block diagram showing a basic structure of a mixed signal semiconductor test system in the conventional technology. 
       FIG. 5  is a schematic block diagram showing a basic structure of a digitizer in mixed signal test system of conventional technology. 
       FIG. 6  is a schematic block diagram showing a basic structure of a digitizer in the conventional technology for increasing an equivalent sampling rate by two times with use of two AD converters. 
       FIG. 7  is a timing chart showing the wave forms and timing relationships in the conventional digitizer of  FIG. 6  using the two AD converters. 
       FIG. 8  is a schematic circuit diagram showing an example of address generator for generating address data to store digitized codes in an acquisition memory with a predetermined address sequence. 
       FIG. 9  is a schematic circuit diagram showing another example of address generator for generating address data to store digitized codes in an acquisition memory with a predetermined address sequence. 
       FIGS. 10A-10E  are timing charts corresponding to the  FIGS. 2A-2B  for explaining the address sequence to be generated by the address generator of  FIGS. 8 and 9  for accessing the acquisition memory. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention is shown in  FIGS. 1-3 . An example of structure of the mixed signal semiconductor test system of the present invention is shown in a block diagram of  FIG. 1  in which an emphasis is placed on a digitizer DGT. In the present invention, to increase the sampling rate of the digitizer, a phase of the sampling clock signal is shifted by a predetermined degree for each cycle of an input analog signal. 
   This invention is based on the fact that, almost always, a wave form of a time period T shown in  FIGS. 2A and 2B  in an output analog signal of a device under test (DUT) is repeated by a plurality of times. This is because, in a mixed signal test system, clock and other signals can be freely supplied to the DUT from the test system so that the repetition rate of the output analog signal of the DUT can be controlled or predictable by the test system. Thus, in the present invention, a digitizer DGT in the mixed signal test system includes only one AD converter (ADC) where a sampling phase is shifted at each cycle of the input analog signal, thereby increasing an equivalent sampling frequency and sampling resolution in the AD conversion process. 
   The wave form of the input analog signal and a timing relationship in the digitizer used in the mixed signal test system of  FIG. 1  are shown in  FIGS. 2A and 2B . As noted above, it is assumed that the input signal S i  to be digitized has a wave form which repeats two cycles or more as a unit of a constant time period T. Since the mixed signal test system provides a start signal, a clock signal or other signals to the DUT, it is also assumed that the timing of the test system and one cycle period T of the analog signal can be matched with one another. 
   Even though such timings between the analog input signal and the test system are not synchronized, the present invention of shifting the phase of the sampling clock is still feasible. For example, if the test system can measure each time period (time length of one cycle) of the analog signal such as by a time measurement unit (TMU) of  FIG. 4 , an appropriate sampling clock for the input analog signal can be easily determined. Thus, the phase shift of the sampling clock can be conducted to increase the overall sampling rate in the digitizer. 
   In the example of  FIG. 1 , the digitizer DGT includes a filter  60 , an AD converter  30 , a phase shifter  20 , a controller  15 , and a synchronous control unit  40 . The filter  60  and the AD converter  30  are the same as that shown in the conventional technology of FIG.  5 . The synchronous control unit  40  provides a sampling clock  40   clk  and a constant period signal  40   s  to the phase shifter  20 . The constant period signal  40   s  is a pulse signal repeating with a time period T which is the same time length of one cycle of an input analog signal. 
   The controller  15  provides information regarding an amount of phase shift, or delta phase ΔP (FIG.  2 A and  10 B), to the phase shifter  20 . The delta phase ΔP is added to the phase of the sampling clock  40   clk  in the next cycle (period) T. In the example of  FIG. 3 , such information on the phase shift is M which is typically a number of cycles the same wave form of period T is repeated in the analog signal. Based on the number “M” from the controller  15 , the phase shift is performed by M times, thereby increasing an overall sampling frequency of the AD conversion by M times. 
   In receiving the sampling clock  40   clk  and the constant period signal  40   s  from the synchronous control unit  40  as well as the phase shift information “M” from the controller  15 , the phase shifter  20  produces a phase shifted sampling clock  20   clk  for the AD converter  30 . The phase shifter  20  produces the sampling clock  20   clk  by adding the delta phase shift ΔP at each period T to the sampling clock of the previous period. Namely, in the case where the input analog signal of a time period T is AD converted for M cycles, the delta phase ΔP is 360°/M. For example, if M is 8, 360°/8=45°, thus, in the first period T, the phase shift is zero, while in the second period T, the phase shift is 45°, and in the following periods, the respective phase shifts are 90°, 135°, 180°, . . . 360°. 
   In this manner, the phase, of the sampling clock is shifted by the unit of the delta phase ΔP at each period T of the analog signal. The output of the AD converter  30  is stored in the acquisition memory (AQM)  50  for the analysis in the later stages of the test system. The above noted operation in the digitizer DGT of the present invention is equivalent to have M AD converters in parallel and combine the digital outputs to form a serial form. Thus, the overall sampling rate of the digitizer DGT is increased by M times. 
   It should be noted that, for the present invention be effective, the analog output signal of the DUT with the constant time period T must repeat for M cycles. As can be seen in the foregoing, the smaller the delta phase ΔP, the higher the sampling rate and sampling resolution it becomes. However, for such a small delta phase ΔP, a sample and hold circuit (not shown) included in the AD converter  30  must be capable of high performance such as high voltage accuracy. 
   An example of basic configuration of the phase shifter  20  is shown in FIG.  3 . In this example, the phase shifter  20  is comprised of a frequency multiplier  22  and a frequency divider  24  connected in series. The frequency multiplier  22  multiplies the frequency of the sampling clock  40   clk  by M times and the frequency divider  24  divides the output frequency of the multiplier  22  by M to form the sampling clock  20   clk  for the AD converter  30 . The controller  15  provides such information “M” to the frequency multiplier  22  and the frequency divider  24 . 
   As shown in  FIG. 3 , the constant period signal  40   s  is provided to the frequency divider  24 . In this arrangement, every time when the constant period signal  40   s  is received, i.e., at every time period T, the frequency divider  24  skips its dividing operation. Namely, the dividing operation corresponding to one pulse in the output of the frequency multiplier  22  is disabled by an edge of the constant period signal  40   s . As a result of which, the sampling clock  20   clk  is phase shifted by ΔP at each time period T of the input analog signal. In the example of  FIG. 3 , if the synchronous control unit  40  is able to provide a sampling clock of M times higher frequency than the sampling clock  40   clk , the frequency multiplier  22  is unnecessary. Such a phase shifting operation can be also achieved by, for example, a phase lock loop (PLL) IC available in the market. 
   Timing charts of  FIG. 2A and 2B  show the case where the AD conversion is performed for two cycles of the input analog signal Si, i.e., M=2. In other words, the phase is shifted by 180° in the second cycle, i.e., the delta phase ΔP=180°. In  FIG. 2 , the first cycle is denoted by T 1  and the second cycle is denoted by T 2 , where the first cycle T 1  and second cycle T 2  have the same time period T. In the first cycle T 1 , the sampling clock  20   clk  does not involve any phase shift, and thus is the same as the original sampling clock  40   clk  of FIG.  2 B. In the second cycle T 1 , as shown in  FIG. 2A , the sampling clock  20   clk  is phase shifted by ΔP=180° relative to the original sampling clock  40   clk  of FIG.  2 B. 
   Since the frequency of the sampling clock  20   clk  is unchanged, the AD converter is able to convert the input analog signal to a digital signal. Further, the sampling points on the analog signal are shifted by a 50% duty cycle, i.e, 180°, of the original sampling clock  40   clk , the digital data obtained by the sum of the first cycle T 1  and the second cycle T 2  is equivalent to that would obtained by the sampling frequency of two times higher than the original sampling clock  40   clk . 
   Although the digitizer in the foregoing can be most advantageously used in the mixed signal test system, other applications are also feasible. For example, the digitizer of the present invention can be used as an AD converter for an input analog signal which repeats the same wave form at least two times. By shifting the phase of the sampling clock for each of M cycles of the input signal by the phase sifter  20 , an equivalent sampling rate and sampling resolution is increased by the factor of M. 
   The output of the digitizer DGT is stored in the acquisition memory  50  in the order of the AD conversion, i.e., from the digital data of the sampling points  1   1 ,  2   1 ,  3   1 , . . .  8   1 ,  1   2 ,  2   2 ,  3   2 , . . .  8   2  of FIG.  10 A. It is also possible to store the digitized data in the order different from the above, such as  1   1 ,  1   2 ,  2   1 ,  2   2 ,  3   1 ,  3   2 , . . .  8   1 ,  8   2 , i.e., the order of the delta phase ΔP of the sampling points of the analog signal S i  in FIG.  10 A. In such a case, the digital data is stored in the acquisition memory (AQM)  50  in a manner that would be acquired by an AD converter actually having a sampling speed of M times (two times in the example of  FIG. 10 ) higher than the AD converter  30 , thereby enabling to directly use the digital data in the acquisition memory (AQM)  50  for signal analysis, etc. 
     FIG. 8  shows an example of circuit diagram of an address generator  70  for generating the address sequence noted above for storing the output of the digitizer in the acquisition memory  50 . In the example of  FIG. 7 , the address generator  70  includes a period counter  72 , a first adder  74 , a gate circuit  76 , a second adder  78  and a register  79 . The constant period signal  40   s , having the same time period T of the input analog signal, from the synchronous control unit  40  is provided to the period counter  72  and the gate circuit  76 . The phase shifted sampling clock  20   clk  from the phase shifter  20  is provided to the register  79 . The first adder  74  is provided with data “M” which indicates a number of cycles of the input analog signal for which the AD conversion noted above is performed. 
   The period counter  72  is reset to “0” at the start of operation and is incremented by one in receiving the constant period signal  40   s . The period counter  72  provides an output signal  72   s  to an input of the second adder  78  whose other input is provided with an output of the gate circuit  76 . The first adder is provided with the cycle number “M” as noted above at its one input and an output signal  79   s  of the register  79  at the other input. The first adder  74  thus provides the sum (accumulated data) of the two inputs to the gate circuit  76 . 
   The gate circuit  76  sets its output to low only when the constant period signal  40   s  is valid (such as high) while supplies the accumulated data from the first adder  74  to the second adder  78  when the constant period signal  40   s  is invalid (such as low). The second adder  78  provides the sum of the output signal  72   s  of the period counter  72  and the accumulated data  76   s  from the gate circuit  76  to the register  79 . In receiving the output data of the second adder  78 , the register  79  generates an address signal  79   s  by the timing of the sampling clock  20   clk . 
   By the arrangement described above, the address generator  70  generates address signal which accesses the acquisition memory  50  in the order of the delta phase ΔP relative to the input analog signal. Thus, the data stored in the acquisition memory  50  is in the order that would have been obtained directly by a digitizer operating by a sampling frequency of M times higher than the sampling clock  40   clk  or  20   clk . In the above example of  FIG. 8 , if the number “M” is a power of two, i.e., 2, 4, 8, 16, etc., the lower bits of the address signal  79   s  may be produced by a counter which increments by one at every pulse of the constant period signal  40   s  while the upper bits of the address signal  79   s  may be produced by a counter which increments by one at every sampling clock  20   clk . 
     FIG. 9  is a schematic circuit diagram showing another example of address generator for generating address data to store digital data from the digitizer in the acquisition memory with a predetermined address sequence. As in the above example, this circuit arrangement is effective when the data “M” is a power of two. In the example of  FIG. 9 , an address generator  70  includes a lower bit counter  82 , an upper bit counter  83 , and a flip-flop  89 . The lower bit counter  82  is provided with the constant period signal  40   s . The upper bit counter  83  and the flip-flop  89  are provided with the phase shifted sampling clock  20   clk . 
   The lower bit counter  82  increments by one at every constant period signal  40   s  to produce a lower bit signal  82   s . The upper bit counter  83  increments by one at every sampling clock  20   clk  to produce a higher bit address signal  79   H . The lower bit signal  82   s  is latched by the timing of the sampling clock  20   clk  by the flip-flop  89 , thereby producing a lower bit address signal  79   L . The lower bit address signal  79   L  and the higher bit address signal  79   H  are provided to the acquisition memory  50  to access the acquisition memory to store the data in the order of the phase shift ΔP in the sampling clock on the analog signal. 
   To summarize the address sequence generated by the address generator  70  in  FIGS. 8 and 9 , reference is made to the timing charts of  FIGS. 10A-10E . Like the example of  FIGS. 2A-2B , the AD conversion process in  FIG. 10  shows the situation where an input analog signal with a time period T is converted to a digital signal by sampling the analog signal for two cycles, T 1  and T 2 . In the first cycle T 1 , there is no phase shift involved while in the second cycle T 2 , the phase of the sampling clock  20   clk  is shifted by 180° from the first cycle. 
   To store the digitized data in the memory  50  in the order of  1   1 ,  1   2 ,  2   1 ,  2   2 ,  3   1 ,  3   2 , . . .  8   1 ,  8   2 , of the sampling points on the input signal S i , i.e., with the increment of the delta phase ΔP of the sampling points, the above noted address generator  70  generates the address under a formula AD=Q+(M×N). In this formula, AD is the address data generated by the address generator  70 , M is a number of cycles of the analog signal used for the AD conversion, Q is a current cycle where Q=0, 1, . . . M−1, and N is a position of the sampling pulse. 
   In the example of  FIG. 10 , since the number of cycle is two, the variables M, Q and N take such numbers as shown in  FIG. 10D  where the number of sampling points in one cycle is, for example, eight. Thus, the address data AD generated by the address generator  70  is 0, 2, 4, . . . 14, 1, 3, 5, . . . 15 as shown in FIG.  10 E. Therefore, in the address “0” of the memory  50 , the digital data of the sampling point  1   1  (first cycle T 1 ) is stored, in the address “1”, the data of the sampling point  1   2  (second cycle T 2 ) is stored. Further, in the address “2” of the memory  50 , the digital data of the sampling point  2   1  (first cycle T 1 ) is stored, and in the address “3”, the data of the sampling point  2   2  (second cycle T 2 ) is stored, and so on. As a consequence, the digital data is stored in the acquisition memory  50  as if the data were acquired by an AD converter actually having a sampling speed two times higher than the AD converter  30 . 
   As described in the foregoing, according to the present invention, an input analog signal which repeats the same wave form in the time period T by M cycles is AD-converted for the M cycles wherein a phase of the sampling clock is shifted by a predetermined amount ΔP for each cycle. As a result, the data obtained in the AD conversion process of the present invention shows resolution M times higher than that obtained in the normal AD conversion. In other words, an equivalent sampling frequency is increased by M times. Therefore, a digitizer of high resolution and high speed is achieved without using a plurality of AD converters or a higher frequency sampling clock. Accordingly, the mixed signal test system having a high performance digitizer is realized with low cost and small circuit size. 
   Although only a preferred embodiment is specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing the spirit and intended scope of the invention.