Abstract:
There is provided a differential comparator circuit that is mounted on a test apparatus for testing a device under test outputting differential signals including noninverted signals and inverted signals. The differential comparator circuit includes: a difference signal subtracting circuit operable to compute and output a difference signal indicative of a difference between the noninverted signal and the inverted signal; a first threshold value subtracting circuit operable to compute and output a first threshold voltage indicative of a difference between a first comparative voltage generated based on ground potential of the device under test and a reference voltage generated based on the ground potential of the device under test; and a first comparing circuit operable to compare the difference signal and the first threshold voltage to output a comparison result.

Description:
CROSS REFERENCE TO THE RELATED APPLICATION  
       [0001]     This patent application claims priority from a Japanese Patent Application No. 2004-236808 filed on Aug. 16, 2004, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a differential comparator circuit, a test head, and a test apparatus. More particularly, the present invention relates to a differential comparator circuit that is mounted on a test apparatus for testing a device under test outputting differential signals.  
         [0004]     2. Description of Related Art  
         [0005]      FIG. 1  shows a configuration of a test apparatus  10  according to a conventional art. The test apparatus  10  includes a subtraction circuit  14 , an H side comparing circuit  16 , and an L side comparing circuit  18 . The subtraction circuit  14  amplifies differential signals output from a device under test (hereinafter, referred to as DUT)  12  with an amplification factor α, and takes and outputs a difference. Then, the H side comparing circuit  16  compares the signals output from the subtraction circuit  14  with an H side threshold value (VrefH) to output a comparison result. Moreover, the L side comparing circuit  18  compares the signals output from the subtraction circuit  14  with an L side threshold value (VrefL) to output a comparison result. In this manner, the test apparatus  10  measures a cross-point of the differential signals output from the DUT  12  by providing the subtraction circuit  14 . When the amplification factor α is constant in such a subtraction circuit  14 , it is possible to measure the cross-point of the differential signals output from the DUT  12 .  
         [0006]      FIG. 2  shows a first example of a configuration of an amplifying circuit included in the subtraction circuit  14  according to a conventional art. When a gain G is very large in the amplifying circuit shown in  FIG. 2 , the amplification factor α=−R OUT /R IN , and thus nonlinearity of the gain G can be ignored. However, it is difficult that such an amplifying circuit increases a gain to realize speedup.  
         [0007]      FIG. 3  shows a second example of a configuration of an amplifying circuit included in the subtraction circuit  14  according to a conventional art. Although the amplifying circuit shown in  FIG. 3  can realize speedup, a gain is small. Therefore, since nonlinearity of the gain becomes large, it is impossible to hold an amplification factor constant.  
       SUMMARY OF THE INVENTION  
       [0008]     Therefore, it is an object of the present invention to provide a test apparatus that can solve the foregoing problems. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.  
         [0009]     According to the first aspect of the present invention, there is provided a differential comparator circuit that is mounted on a test apparatus for testing a device under test outputting differential signals including noninverted signals and inverted signals. The differential comparator circuit includes: a difference signal subtracting circuit operable to compute and output a difference signal indicative of a difference between the noninverted signal and the inverted signal; a first threshold value subtracting circuit operable to compute and output a first threshold voltage indicative of a difference between a first comparative voltage generated based on ground potential of the device under test and a reference voltage generated based on the ground potential of the device under test; and a first comparing circuit operable to compare the difference signal and the first threshold voltage to output a comparison result.  
         [0010]     The difference signal subtracting circuit may amplify the difference between the noninverted signal and the inverted signal at a predetermined amplification factor to output the difference signal, and the first threshold value subtracting circuit may amplify the difference between the first comparative voltage and the reference voltage at the same predetermined amplification factor as that of the difference signal subtracting circuit in order to output the first threshold voltage.  
         [0011]     The differential comparator circuit may further include: a second threshold value subtracting circuit operable to compute and output a second threshold voltage indicative of a difference between a second comparative voltage generated based on the ground potential of the device under test and a reference voltage generated based on the ground potential of the device under test; and a second comparing circuit operable to compare the difference signal and the second threshold voltage and output a comparison result, in which the first comparing circuit may detect and output that the difference signal is larger than the first threshold voltage, and the second comparing circuit may detect and output that the difference signal is smaller than the second threshold voltage.  
         [0012]     The first comparing circuit may detect and output that the inverted signal is larger than the noninverted signal, and the second comparing circuit may detect and output that the inverted signal is smaller than the noninverted signal.  
         [0013]     The difference signal subtracting circuit may amplify the difference between the noninverted signal and the inverted signal at a predetermined amplification factor to output the difference signal, and the second threshold value subtracting circuit may amplify the difference between the second comparative voltage and the reference voltage at the same predetermined amplification factor as that of said difference signal subtracting circuit in order to output the second threshold voltage.  
         [0014]     According to the second aspect of the present invention, there is provided a test head of a test apparatus that tests a device under test outputting differential signals including noninverted signals and inverted signals. The test head includes a test module operable to measure the differential signals, the test module including: a difference signal subtracting circuit that computes and outputs a difference signal indicative of a difference between the noninverted signal and the inverted signal; a first threshold value subtracting circuit that computes and outputs a first threshold voltage indicative of a difference between a first comparative voltage generated based on ground potential of the device under test and a reference voltage generated based on the ground potential of the device under test; and a first comparing circuit that compares the difference signal and the first threshold voltage to output a comparison result.  
         [0015]     According to the third aspect of the present invention, there is provided a test apparatus that tests a device under test outputting differential signals including noninverted signals and inverted signals. The test apparatus includes a test module operable to measure the differential signals, the test module including: a difference signal subtracting circuit that computes and outputs a difference signal indicative of a difference between the noninverted signal and the inverted signal; a first threshold value subtracting circuit that computes and outputs a first threshold voltage indicative of a difference between a first comparative voltage generated based on ground potential of the device under test and a reference voltage generated based on the ground potential of the device under test; and a first comparing circuit that compares the difference signal and the first threshold voltage to output a comparison result.  
         [0016]     The summary of the invention does not necessarily describe all necessary features of the present invention. The present invention may also be a sub-combination of the features described above.  
         [0017]     According to the present invention, it is possible to provide a differential comparator circuit that can accurately measure a differential signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a view showing a configuration of a test apparatus according to a conventional art.  
         [0019]      FIG. 2  is a view showing a first example of a configuration of an amplifying circuit included in a subtraction circuit.  
         [0020]      FIG. 3  is a view showing a second example of a configuration of the amplifying circuit included in the subtraction circuit.  
         [0021]      FIG. 4  is a view exemplary showing a configuration of a test apparatus according to an embodiment of the present invention.  
         [0022]      FIG. 5  is a view showing an alternative example of a differential comparator circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The invention will now be described based on the preferred embodiments, which do not intend to limit the scope of the present invention, but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention.  
         [0024]      FIG. 4  is a view exemplary showing a configuration of a test apparatus  100  according to an embodiment of the present invention. The test apparatus  100  includes a performance board (PB)  102  on which a DUT  101  is mounted on and a test head  104  that has a plurality of test modules  106  therein. The plurality of test modules  106  mounted on the test head  104  includes a test module for an analog test, a test module for a digital test, etc. However, in the present embodiment, it will be described about a configuration and an operation of the test module  106  for testing the DUT  101  that outputs differential signals including noninverted signals and inverted signals.  
         [0025]     The test module  106  has a differential comparator circuit  118 , which includes a first threshold value subtracting circuit  108 , a difference signal subtracting circuit  110 , a second threshold value subtracting circuit  112 , an H side comparing circuit  114 , and an L side comparing circuit  116 , and DA converters  120 ,  122 ,  124 , and  126 . The first threshold value subtracting circuit  108 , the difference signal subtracting circuit  110 , and the second threshold value subtracting circuit  112  are a subtraction circuit that uses an operational amplifier having the same gain, and amplify signals input with the same amplification factor α.  
         [0026]     The difference signal subtracting circuit  110  inputs the noninverted signals output from the DUT  101  from its positive input terminal, and inputs the inverted signals output from the DUT  101  from its negative input terminal. Then, the difference signal subtracting circuit  110  computes and outputs a difference signal indicative of a difference between the noninverted signal and the inverted signal by amplifying the difference between the noninverted signal and the inverted signal at a predetermined amplification factor α, and supplies it to the H side comparing circuit  114  and the L side comparing circuit  116 .  
         [0027]     The DA converter  120  generates a first comparative voltage (VrefH) from a first comparative voltage data (D H ) to supply it to the first threshold value subtracting circuit  108  using ground potential of the DUT  101  supplied from the performance board (PB)  102  as a standard. Moreover, the DA converter  122  generates a reference voltage (Vref) from a reference voltage data (D R ) to supply it to the first threshold value subtracting circuit  108  using the ground potential of the DUT  101  supplied from the performance board (PB)  102  as a standard. Moreover, the DA converter  124  generates a second comparative voltage (VrefL) from a second comparative voltage data (DL) to supply it to the second threshold value subtracting circuit  112  using the ground potential of the DUT  101  supplied from the performance board (PB)  102  as a standard. Moreover, the DA converter  126  generates the reference voltage (Vref) from the reference voltage data (DR) to supply it to the second threshold value subtracting circuit  112  using the ground potential of the DUT  101  supplied from the performance board (PB)  102  as a standard.  
         [0028]     In addition, the differential comparator circuit  118  may not have the DA converters  122  and  126 . In this case, the ground potential of the DUT  101  supplied from the performance board (PB)  102  may be supplied to the first threshold value subtracting circuit  108  and the second threshold value subtracting circuit  112  as the reference voltage (Vref).  
         [0029]     The first threshold value subtracting circuit  108  inputs the first comparative voltage (VrefH) generated by the DA converter  120  based on the ground potential of the DUT  101  from the positive input terminal, and inputs the reference voltage (Vref) generated by the DA converter  122  based on the ground potential of the DUT  101  from the negative input terminal. Then, the first threshold value subtracting circuit  108  computes and outputs a first threshold voltage indicative of a difference between the first comparative voltage (VrefH) and the reference voltage (Vref) and supplies it to the H side comparing circuit  114  by amplifying the difference between the first comparative voltage (VrefH) and the reference voltage (Vref) at the same predetermined amplification factor α as that of the difference signal subtracting circuit  110 .  
         [0030]     The second threshold value subtracting circuit  112  inputs the second comparative voltage (VrefL) generated by the DA converter  124  based on the ground potential of the DUT  101  from the positive input terminal, and inputs the reference voltage (Vref) generated by the DA converter  126  based on the ground potential of the DUT  101  from the negative input terminal. Then, the second threshold value subtracting circuit  112  computes and outputs a second threshold voltage indicative of a difference between the second comparative voltage (VrefL) and the reference voltage (Vref) and supplies it to the L side comparing circuit  116  by amplifying the difference between the second comparative voltage (VrefL) and the reference voltage (Vref) at the same predetermined amplification factor α as that of the difference signal subtracting circuit  110 .  
         [0031]     The first comparative voltage (VrefH), the second comparative voltage (VrefL), and the reference voltage (Vref) are generated by DA converting circuits connected to the ground potential of the DUT  101 , and are supplied to the first threshold value subtracting circuit  108  and the second threshold value subtracting circuit  112 .  
         [0032]     The H side comparing circuit  114  inputs the difference signal supplied from the difference signal subtracting circuit  110  and the first threshold voltage supplied from the first threshold value subtracting circuit  108 . Then, the H side comparing circuit  114  compares the difference signal and the first threshold voltage, and detects that the difference signal is larger than the first threshold voltage to output its result as a comparison result. In other words, the H side comparing circuit  114  detects that the inverted signal included in the differential signal output from the DUT  101  is larger than the noninverted signal in order to output its result.  
         [0033]     The L side comparing circuit  116  inputs the difference signal supplied from the difference signal subtracting circuit  110  and the second threshold voltage supplied from the second threshold value subtracting circuit  112 . Then, the L side comparing circuit  116  compares the difference signal and the second threshold voltage, and detects that the difference signal is smaller than the second threshold voltage to output its result as a comparison result. In other words, the L side comparing circuit  116  detects that the inverted signal included in the differential signal output from the DUT  101  is smaller than the noninverted signal in order to output its result.  
         [0034]     As described above, by providing the first threshold value subtracting circuit  108  and the second threshold value subtracting circuit  112  having the same gain as that of the difference signal subtracting circuit  110 , it is possible to reduce a measurement error by nonlinearity of the gain of the difference signal subtracting circuit  110  and to accurately measure a cross-point of the differential signals. Therefore, according to the test apparatus  100  including the differential comparator circuit  118 , it is possible to test the DUT  101  that outputs the differential signals with high precision.  
         [0035]     Moreover, since a common noise from the ground potential of the DUT  101  is superimposed on all of the differential signal input into the difference signal subtracting circuit  110 , the first comparative voltage (VrefH) and the reference voltage (Vref) input into the first threshold value subtracting circuit  108 , and the second comparative voltage (VrefL) and the reference voltage (Vref) input into the second threshold value subtracting circuit  112 , the common noise is removed by the difference signal subtracting circuit  110 , the first threshold value subtracting circuit  108 , and the second threshold value subtracting circuit  112 , and thus the accurate comparison result can be output from the H side comparing circuit  114  and the L side comparing circuit  116 . Therefore, according to the test apparatus  100  including the differential comparator circuit  118 , it is possible to test the DUT  101  that outputs the differential signals with high precision.  
         [0036]      FIG. 5  shows an alternative example of a configuration of the differential comparator circuit  118  according to the present embodiment. The differential comparator circuit  11   a  according to this example has a first differential signal amplifying unit  200 , a second differential signal amplifying unit  270 , a first threshold value amplifying unit  201 , a second threshold value amplifying unit  202 , an H side comparing circuit  204 , an L side comparing circuit  206 , a resistor  208 , a resistor  210 , a resistor  248 , and a resistor  250 . The first differential signal amplifying unit  200  includes a transistor  212 , a resistor  214 , a transistor  216 , a resistor  218 , and a current source  220 . The second differential signal amplifying unit  270  includes a transistor  252 , a resistor  254 , a transistor  256 , a resistor  258 , and a current source  260 . The first threshold value amplifying unit  201  includes a transistor  222 , a resistor  224 , a transistor  226 , a resistor  228 , and a current source  230 . The second threshold value amplifying unit  202  includes a transistor  232 , a resistor  234 , a transistor  236 , a resistor  238 , and a current source  240 .  
         [0037]     A differential amplification circuit is made up of the first differential signal amplifying unit  200  and the resistors  208  and  210 , a differential amplification circuit is made up of the second differential signal amplifying unit  270  and the resistors  248  and  250 , a differential amplification circuit is made up of the first threshold value amplifying unit  201  and the resistors  208  and  210 , and a differential amplification circuit is made up of the second threshold value amplifying unit  202  and the resistors  208  and  210 . In addition, the first differential signal amplifying unit  200 , the second differential signal amplifying unit  270 , the first threshold value amplifying unit  201 , and the second threshold value amplifying unit  202  are the same configurations, and respectively include a transistor, a resistor, and a current source having the same characteristic. In other words, the first differential signal amplifying unit  200 , the second differential signal amplifying unit  270 , the first threshold value amplifying unit  201 , and the second threshold value amplifying unit  202  amplify the input signals at the same amplification factor to output the amplified signals.  
         [0038]     In the first differential signal amplifying unit  200 , the noninverted signal included in the differential signal output from the DUT  101  is applied to the transistor  212 , and the inverted signal included in the differential signal output from the DUT  101  is applied to the transistor  216 . Then, the first differential signal amplifying unit  200  amplifies the noninverted signal and the inverted signal, inputs the amplified noninverted signal into the positive input terminal of the L side comparing circuit  206 , and inputs the amplified inverted signal into the negative input terminal of the L side comparing circuit  206 .  
         [0039]     In the second differential signal amplifying unit  270 , the noninverted signal included in the differential signal output from the DUT  101  is applied to the transistor  252 , and the inverted signal included in the differential signal output from the DUT  101  is applied to the transistor  256 . Then, the second differential signal amplifying unit  270  amplifies the noninverted signal and the inverted signal, inputs the amplified noninverted signal into the positive input terminal of the H side comparing circuit  204 , and inputs the amplified inverted signal into the negative input terminal of the H side comparing circuit  204 .  
         [0040]     In the first threshold value amplifying unit  201 , the reference voltage (Vref) generated based on the ground potential of the DUT  101  is applied to the transistor  222 , and the first comparative voltage (VrefH) generated based on the ground potential of the DUT  101  is applied to the transistor  216 . Then, the first threshold value amplifying unit  201  amplifies the reference voltage (Vref) and the first comparative voltage (VrefH), inputs the amplified reference voltage (Vref) into the positive input terminal of the H side comparing circuit  204 , and inputs the amplified first comparative voltage (VrefH) into the negative input terminal of the H side comparing circuit  204 .  
         [0041]     In the second threshold value amplifying unit  202 , the reference voltage (Vref) generated based on the ground potential of the DUT  101  is applied to the transistor  232 , and the second comparative voltage (VrefL) generated based on the ground potential of the DUT  101  is applied to the transistor  236 . Then, the second threshold value amplifying unit  202  amplifies the reference voltage (Vref) and the second comparative voltage (VrefL), inputs the amplified reference voltage (Vref) into the positive input terminal of the L side comparing circuit  206 , and inputs the amplified second comparative voltage (VrefL) into the negative input terminal of the L side comparing circuit  206 .  
         [0042]     The H side comparing circuit  204  compares electric potential of the positive input terminal and electric potential of the negative input terminal to output its comparison result. In other words, the H side comparing circuit  204  detects that the sum of a voltage of the noninverted signal input from the second differential signal amplifying unit  270  and the reference voltage (Vref) input from the first threshold value amplifying unit  201  is larger than the sum of a voltage of the inverted signal input from the second differential signal amplifying unit  270  and the first comparative voltage (VrefH) input from the first threshold value amplifying unit  201 , in order to output it as a comparison result.  
         [0043]     The L side comparing circuit  206  compares electric potential of the positive input terminal and electric potential of the negative input terminal to output its comparison result. In other words, the L side comparing circuit  206  detects that the sum of a voltage of the noninverted signal input from the first differential signal amplifying unit  200  and the reference voltage (Vref) input from the second threshold value amplifying unit  202  is smaller than the sum of a voltage of the inverted signal input from the first differential signal amplifying unit  200  and the second comparative voltage (VrefL) input from the second threshold value amplifying unit  202 , in order to output it as a comparison result.  
         [0044]     As described above, since the first differential signal amplifying unit  200 , the second differential signal amplifying unit  270 , the first threshold value amplifying unit  201 , and the second threshold value amplifying unit  202  are made up of a differential amplification circuit, it is possible to speed up an operation of the differential comparator circuit  118 . Moreover, since the first differential signal amplifying unit  200 , the second differential signal amplifying unit  270 , the first threshold value amplifying unit  201 , and the second threshold value amplifying unit  202  are made up of a differential amplification circuit having the same gain, it is possible to reduce a measurement error by nonlinearity of a gain of the first differential signal amplifying unit  200  and the second differential signal amplifying unit  270  and to accurately measure a cross-point of the differential signals. Therefore, according to the test apparatus  100  including the differential comparator circuit  118 , it is possible to test the DUT  101  that outputs the differential signals with high precision.  
         [0045]     Although the present invention has been described by way of an exemplary embodiment, it should be understood that those skilled in the art might make many changes and substitutions without departing from the spirit and the scope of the present invention. It is obvious from the definition of the appended claims that embodiments with such modifications also belong to the scope of the present invention.  
         [0046]     As apparent from the above descriptions, according to the present invention, it is possible to provide the differential comparator circuit capable of measuring the differential signals precisely.