Measurement apparatus, test apparatus, and measurement method

A signal whose voltage level fluctuates with respect to a high voltage is measured with favorable accuracy. Provided is a measurement apparatus for measuring a fluctuation of a voltage level of an input signal with respect to a reference voltage, where the voltage level of the input signal fluctuates with respect to the reference voltage having a predetermined voltage level, the measurement apparatus including: a high-frequency passage section for allowing passage of a high-frequency component of a predetermined first band of the input signal; a reference voltage dividing section for dividing a voltage level of the reference voltage at a predetermined voltage dividing ratio; a signal voltage dividing section for dividing the voltage level of the input signal at the voltage dividing ratio; a low-frequency passage section for allowing passage of a low-frequency component of a second band of a differential signal that is according to a difference between a voltage level of the reference voltage outputted from the reference voltage dividing section and a voltage level of the input signal outputted from the signal voltage dividing section, the second band being lower than the first band; a composite section for combining a signal outputted from the high-frequency passage section and a signal outputted from the low-frequency passage section; and a measurement section for measuring a signal outputted from the composite section.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from a Japanese Patent Applications No. 2006-023273 filed on Jan. 31, 2006, the contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a measurement apparatus and a test method for measuring an input signal, and a test apparatus that is equipped with such a measurement apparatus. In particular, the present invention relates to a measurement apparatus that measures a fluctuation of a voltage level of an input signal with respect to a reference voltage, where the voltage level of the input signal fluctuates with respect to the reference voltage having a predetermined voltage level.

2. Related Art

In recent years, a demand is increasing for “HVIC” (High Voltage Integrated Circuit) that outputs a signal having a high voltage. For example, devices controlling motors of electric cars use such HVICs.

HVICs are devices that output a signal having predetermined amplitude with respect to about 0-2 kV for example. In testing such an HVIC, a test apparatus supplies, to an HVIC, a reference voltage having a high voltage of about 0-2 kV. In addition, the test apparatus detects a data pattern of an output signal, and judges quality of a device under test based on the data pattern.

SUMMARY

However, a voltage level of an output signal of an HVIC, fluctuates with respect to a high voltage of about 2 kV for example. In view of this, a measurement means of a high withstand voltage becomes necessary to measure such a signal. In addition, when an HVIC further outputs a signal whose reference is a ground potential, it is desirable that a test apparatus has a measurement means for high-voltage signals and a measurement means for low-voltage signals. However, from a cost point of view, it is not desirable to provide measurement sections respectively for high-voltage signals and low-voltage signals.

Therefore, an advantage of the present invention is to provide a measurement apparatus, a measurement method, and a test apparatus that are able to solve the foregoing problems. This advantage is achieved by combinations of features described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.

So as to solve the foregoing problems, according to the first aspect of the present invention, there is provided a measurement apparatus for measuring a fluctuation of a voltage level of an input signal with respect to a reference voltage, where the voltage level of the input signal fluctuates with respect to the reference voltage having a predetermined high voltage level, the measurement apparatus including: a high-frequency passage section for allowing passage of a high-frequency component of a predetermined first band of the input signal; a reference voltage dividing section for dividing a voltage level of the reference voltage at a predetermined voltage dividing ratio; a signal voltage dividing section for dividing the voltage level of the input signal at the voltage dividing ratio; a low-frequency passage section for allowing passage of a low-frequency component of a second band of a differential signal that is according to a difference between a voltage level of the reference voltage outputted from the reference voltage dividing section and a voltage level of the input signal outputted from the signal voltage dividing section, the second band being lower than the first band; a composite section for combining a signal outputted from the high-frequency passage section and a signal outputted from the low-frequency passage section; and a measurement section for measuring a signal outputted from the composite section.

The high-frequency passage section may output a differential waveform of the input signal, and the low-frequency passage section outputs an integral waveform obtained by integrating a difference between the voltage level of the reference voltage outputted from the reference voltage dividing section and the voltage level of the input signal outputted from the signal voltage dividing section.

The high-frequency passage section may include: a buffer that allows passage of the input signal; and a high-pass filter that allows passage of a frequency component of the first band of the input signal outputted from the buffer. The low-frequency passage section may include: a differential circuit that outputs the differential signal obtained from amplifying, at an amplification factor that is according to the voltage dividing ratio, the difference between the voltage level of the reference voltage outputted from the reference voltage dividing section and the voltage level of the input signal outputted from the signal voltage dividing section.

The low-frequency passage section may further include a low-pass filter that allows passage of a frequency component of the second band of the differential signal outputted from the differential circuit. The second band may be substantially adjacent to a low frequency side of the first band.

A cut-off frequency of the first band may be substantially equal to a cut-off frequency of the second band. A roll-off factor of the first band may be substantially equal to a roll-off factor of the second band. An amplitude gain for the high-frequency passage section may be substantially equal to an amplitude gain for the low-frequency passage section.

According to the second aspect of the present invention, there is provided a test apparatus for testing a device under test, the test apparatus including: a reference voltage application section for applying a reference voltage to the device under test, the reference voltage defining a reference of a voltage level of an output signal outputted from the device under test; a measurement apparatus for measuring the output signal; and a judgment section for judging quality of the device under test based on the output signal measured by the measurement apparatus, where the measurement apparatus includes: a high-frequency passage section for allowing passage of a high-frequency component of a predetermined first band of the output signal; a reference voltage dividing section for dividing a voltage level of the reference voltage at a predetermined voltage dividing ratio; a signal voltage dividing section for dividing the voltage level of the output signal at the voltage dividing ratio; a low-frequency passage section for allowing passage of a low-frequency component of a second band of a differential signal that is according to a difference between a voltage level of the reference voltage outputted from the reference voltage dividing section and a voltage level of the output signal outputted from the signal voltage dividing section, the second band being lower than the first band; a composite section for combining a signal outputted from the high-frequency passage section and a signal outputted from the low-frequency passage section; and a measurement section for measuring a signal outputted from the composite section.

The device under test may include: a low-voltage driving section for outputting a first output signal whose reference is a predetermined voltage level; and a high-voltage driving section for outputting a second output signal whose reference is the reference voltage having a higher voltage than a reference voltage level of the first output signal, and the measurement apparatus may measure the second output signal.

According to the third aspect of the present invention, there is provided a measurement method for measuring a fluctuation of a voltage level of an input signal with respect to a reference voltage, where the voltage level of the input signal fluctuates with respect to the reference voltage having a predetermined voltage level, the measurement method including: a high-frequency passage step for allowing passage of a high-frequency component of a predetermined first band of the input signal; a reference voltage dividing step for dividing a voltage level of the reference voltage at a predetermined voltage dividing ratio; a signal voltage dividing step for dividing the voltage level of the input signal at the voltage dividing ratio; a low-frequency passage step for allowing passage of a low-frequency component of a second band of a differential signal that is according to a difference between a voltage level of the reference voltage outputted in the reference voltage dividing step and a voltage level of the input signal outputted in the signal voltage dividing step, the second band being lower than the first band; a composite step for combining a signal generated in the high-frequency passage step and a signal generated in the low-frequency passage step; and a measurement step for measuring a signal generated in the composite step.

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.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1is a diagram showing one example of a configuration of a test apparatus100regarding an embodiment of the present invention. The test apparatus100tests a device under test200such as a semiconductor circuit, the device under test200operating by being provided with a reference voltage of a predetermined high voltage.

The device under test200is for example a device that outputs a first output signal having a waveform whose reference is a ground potential for example, and a second output signal having a waveform whose reference is a high voltage of about 0-2 kV for example. The device under test200includes a power source monitor section202, a high-voltage logic section204, a level shift circuit206, a high-voltage driving section208, a level shift circuit214, a low-voltage logic section216, a level shift circuit218, a low-voltage driving section220, and a plurality of input/output pins (226-238).

The input/output pins234and230are supplied with a voltage based on a first reference voltage. In the present example, the first reference voltage is the ground potential. The input/output pin234is supplied with the ground potential, and the input/output pin230is supplied with a voltage having a predetermined potential difference with respect to the ground potential.

The low-voltage driving section220has transistors222and224, and outputs a first output signal via the input/output pin238. The low-voltage driving section220is given a voltage to be supplied to the input/output pins234and230, and outputs the first output signal having amplitude that is according to a voltage level difference of the voltage. For example, the low-voltage driving section220is given a voltage to be supplied to the input/output pin230as a voltage level in the case where the first output signal indicates an H logic, and is given a voltage to be supplied to the input/output pin234as a voltage level in the case where the first output signal indicates an L logic.

A test pattern is inputted to the input/output pin232. The low-voltage logic section216controls, based on the test pattern, a gate voltage to be inputted to gate terminals of the transistors222and224. Accordingly, the low-voltage driving section220outputs a first output signal that is in accordance with the test pattern. The level shift circuit218shifts a gate voltage that the low-voltage logic section216outputs, to a voltage level that is in accordance with the characteristics of the transistors222and224.

The input/output pins226and228are given a voltage that is based on the second reference voltage that has a predetermined voltage level difference with respect to the first reference voltage and that has a voltage level that is higher than the first reference voltage. In the present example, the second reference voltage of about 0-2 kV is supplied to the input/output pin228. In addition, a voltage having a predetermined voltage level difference with respect to the second reference voltage is supplied to the input/output pin226.

The high-voltage driving section208has transistors210and212, and outputs a second output signal via the input/output pin236. The high-voltage driving section208is given a voltage to be supplied to the input/output pins226and228, and outputs the second output signal having amplitude that is according to a voltage level difference of the voltage.

The high-voltage logic section204controls, based on the test pattern inputted to the device under test200, a gate voltage to be inputted to gate terminals of the transistors210and212. Accordingly, the high-voltage driving section208outputs a second output signal that is in accordance with the test pattern.

The test pattern may be given to the high-voltage logic section204via the input/output pin232, the low-voltage logic section216, and the level shift circuit214. In this case, the level shift circuit214shifts a voltage level of the test pattern according to the voltage level difference between the first reference voltage and the second reference voltage. The test pattern may also be given to the high-voltage logic section204via a different path from with respect to the low-voltage logic section216. The level shift circuit206shifts the gate voltage outputted from the high-voltage logic section204, to a voltage level that is in accordance with the characteristics of the transistors210and212.

The test apparatus100includes a general-purpose power source10, a reference voltage application section12, a pattern generating section18, a measurement apparatus90, a high-voltage processing section23, and a judgment section24. The general-purpose power source10generates power supply for operating the test apparatus100. In the present example, the general-purpose power source10functions as a low-voltage application section for generating a voltage whose reference is the first reference voltage, and applying the generated voltage to the input/output pin230. Although the input/output pin234is connected to the ground potential in the present example, it is alternatively possible that the general-purpose power source10applies, to the input/output pin234, a first reference voltage having a predetermined voltage. According to such a configuration, a power supply voltage is able to be supplied to the low-voltage driving section220of the device under test200.

The reference voltage application section12generates a voltage that is based on the second reference voltage, and applies the generated voltage to the input/output pins226and228. For example, the reference voltage application section12applies a voltage of about 0-2 kV to the input/output pin228, and applies, to the input/output pin226, a voltage of about 15V whose reference is the voltage to be supplied to the input/output pin228.

The reference voltage application section12includes a high-voltage power source16and a floating power source14. The high-voltage power source16generates the second reference voltage, and applies the second reference voltage to the input/output pin228. The floating power source14generates a voltage whose reference is the second reference voltage outputted from the high-voltage power source16, and applies the generated voltage to the input/output pin226. According to such a configuration, a power supply voltage having a high voltage level is able to be supplied to the high-voltage driving section208of the device under test200.

A pin electronics section20includes a plurality of logic pins corresponding to the plurality of input/output pins (226-238) of the device under test200. Each logic pin is electrically connected to a corresponding one of the input/output pins (226-238). Conveyance of signals, power, or the like between the test apparatus100and the device under test200is performed via each logic pin of the pin electronics section20. It should be noted that althoughFIG. 1illustrates a plurality of pin electronics sections20separated from each other, each logic pin of the pin electronics sections20may be provided on a same substrate.

The pattern generating section18generates a test pattern and inputs the test pattern to the device under test200. For generating the test pattern, the pattern generating section18may base a test program given in advance.

The measurement apparatus90measures a second output signal that is outputted from the high-voltage driving section208. For example, the measurement apparatus90measures a fluctuation of a voltage level of the second output signal with respect to the second reference voltage. The measurement apparatus90includes a high-voltage processing section22and a pin electronics section20. Each of the high-voltage processing section22and the high-voltage processing section23is provided between the device under test200and a corresponding pin electronics section20. The high-voltage processing section22receives a second output signal that the high-voltage driving section208outputs, and processes the output signal according to the characteristics of the pin electronics section20. For example, the high-voltage processing section22shifts the signal level of the output signal, to a signal level that is in accordance with the withstand voltage of a corresponding logic pin of the pin electronics section20. Accordingly, even without a pin electronics section20of a high withstand voltage, the test apparatus100becomes able to detect data of an output signal having a high voltage. The configuration and the operation of the high-voltage processing section22are detailed later with reference to the drawings fromFIG. 2toFIG. 5.

The high-voltage processing section23receives a test pattern generated by the pattern generating section18, and processes the test pattern in accordance with the characteristics of the high-voltage logic section204. For example, the high-voltage processing section23may shift a voltage level of the test pattern according to the voltage level difference between the first reference voltage and the second reference voltage, just as the level shift circuit214.

The judgment section24judges the quality of the device under test200, based on an output signal outputted from the device under test200. For example, the judgment section24may judge the quality of the device under test200, based on a judgment as to whether the data pattern of the output signal outputted from the low-voltage driving section220and/or from the high-voltage driving section208matches the data pattern of a given expected-value signal. In this case, the pattern generating section18may generate the expected-value signal according to a test program.

The judgment section24may also judge whether the second reference voltage outputted from the high-voltage power source16lies within a predetermined expected voltage range. The judgment section24may judge quality of a device under test200based on an output signal, on condition that the voltage level of the second reference voltage lies within the expected voltage range. According to such a configuration, the test apparatus100is able to judge, with favorable accuracy, the quality of the device under test200that is driven with a high voltage.

The judgment section24may detect a voltage level of a second reference voltage in a conveyance path provided between the high-voltage power source16and the input/output pin228. It is desirable that the judgment section24detects a voltage level of a second reference voltage from an input terminal (input/output pin228) of the device under test200to which the second reference voltage is inputted.

The power source monitor section202monitors voltages to be supplied to the input/output pins226and228, and when the voltages do not lie within a predetermined voltage range, the power source monitor section202may stop an operation of the high-voltage logic section204. In addition, the power source monitor section202may monitor whether the difference between the voltages to be supplied to the input/output pins226and228lies within a predetermined voltage range.

FIG. 2is a diagram showing one example of a configuration of the high-voltage processing section22and the pin electronics section20. The high-voltage processing section22includes a high-frequency passage section60, a low-frequency passage section70, a signal voltage division section26, a reference voltage division section32, and a composite section80.

The high-frequency passage section60allows passage of a high frequency component of a predetermined first band of an output signal outputted from a high-voltage driving section208. The high-frequency passage section60may output a differential waveform of the output signal. In addition, the high-frequency passage section60may allow passage of an alternate-current component of the output signal. The high-frequency passage section60of the present example may have a buffer62, resistances64,66,78, and a high-pass filter68.

The buffer62receives an output signal from the high-voltage driving section208via the input/output pin236. The buffer62allows passage of the output signal after amplifying the amplitude of the output signal by means of a gain that is according to a resistance ratio between the resistances64and66.

The high-pass filter68allows passage of a frequency component of a first band of an output signal outputted from the buffer62. For example, the high-pass filter68may include a capacitor provided between the buffer62and the resistance78, and may include a transformer provided between the buffer62and the resistance78. The resistance78is provided between the high-pass filter68and the composite section80, and conveys a signal outputted from the high-pass filter68to the composite section80. It is preferable that the buffer62is a buffer that operates at a high speed. For example, a signal passage band in the buffer62is preferably a band that includes the above-mentioned first band.

The signal voltage division section26performs voltage division to a voltage level of an output signal from the high-voltage driving section208at a predetermined voltage dividing ratio and outputs the result. The signal voltage division section26in the present example has resistances28and30provided in series between the input/output pin236and the ground potential, and performs voltage division to the output signal according to a resistance ratio between the resistances28and30.

The reference voltage division section32performs voltage division to the voltage level of the second reference voltage at substantially the same voltage dividing ratio as that used by the signal voltage division section26, and outputs the result. The reference voltage division section32in the present example has resistances34and36provided in series between the high-voltage power source16and the ground potential, and performs voltage division to the second reference voltage according to a resistance ratio between the resistances34and36.

The low-frequency passage section70allows passage of a low frequency component of a second band of a differential signal that is according to a difference between a voltage level of the second reference voltage having undergone voltage division by the reference voltage division section32and a voltage level of an output signal having undergone voltage division by the signal voltage division section26. The second band that the low-frequency passage section70allows passage is lower than the first band that the high-frequency passage section60allows passage. The low-frequency passage section70may output an integral waveform resulting from integrating the differential signal. Moreover, the low-frequency passage section70may allow passage of a direct-current component of the differential signal.

The low-frequency passage section70in the present example has a differential circuit72and a low-pass filter74. The differential circuit72outputs a differential signal obtained by amplifying, at a predetermined amplification factor, a difference between the voltage level of the second reference voltage resulting from voltage division by the reference voltage division section32and the voltage level of the output signal resulting from voltage division by the signal voltage division section26.

Here, it is desirable that the amplification factor for the differential circuit72is determined based on the voltage dividing ratio for the signal voltage division section26and the reference voltage division section32. For example, the amplification factor for the differential circuit72may be a reciprocal number of the voltage dividing ratio for the signal voltage division section26and the reference voltage division section32. For example, when the signal voltage division section26outputs an output signal by dividing the voltage thereof to 1/200, the amplification factor of the differential circuit72may be 200.

According to such a configuration, an amplitude component of the signal having a high voltage is able to be detected by using a low withstand voltage circuit. Furthermore, the differential circuit72may be a so-called instrumentation amplifier. Instrumentation amplifiers have high input impedance, and so are suited for measuring minute signals.

The low-pass filter74allows passage of a frequency component of the aforementioned second band that belongs to a differential signal outputted by the differential circuit72. Moreover, when the signal passage band for the differential circuit72is the second band, the low-frequency passage section70does not have to be equipped with a low-pass filter74. The resistance76is provided between the low-pass filter74and the composite section80, and conveys, to the composite section80, a signal outputted from the low-pass filter74.

The composite section80combines a signal outputted from the high-frequency passage section60and a signal outputted from the low-frequency passage section70. For example, the composite section80may be a connection point that electrically connects an output end of the high-frequency passage section60to an output end of the low-frequency passage section70. Accordingly, a waveform of a signal outputted from the high-frequency passage section60and a waveform of a signal outputted from the low-frequency passage section70are added together.

Moreover, the pin electronics section20functions as a measurement section that measures a signal outputted from the composite section80. The pin electronics section20has a plurality of logic pins, where each logic pin has a comparator42and a driver44. The comparator42is given a predetermined threshold value, and compares a voltage level of a signal outputted from the composite section80to the threshold value. The comparator42outputs the comparison result. According to such a configuration, the comparator42converts an inputted signal into a binary signal.

A reference voltage of a signal outputted from the high-frequency passage section60will be the ground potential. In addition, a signal outputted from the low-frequency passage section70is based on a difference between an output signal and the second reference voltage. Consequently, a reference voltage of a signal outputted from the low-frequency passage section70will also be the ground potential. In addition, a signal outputted from the high-frequency passage section60is for example a differential waveform of an output signal, and a signal outputted from the low-frequency passage section70is for example an integral waveform of an output signal. Accordingly, by combining these signals, an amplitude component of an output signal is able to be extracted.

In addition, the high-frequency passage section60is able to be operated at a high speed, however cannot allow passage of a direct-current component. As opposed to this, the low-frequency passage section70cannot operate at a high speed, however is able to allow passage of a direct-current component.

In other words, if with one of either the high-frequency passage section60or the low-frequency passage section70, it is difficult to detect the amplitude waveform of an output signal. As opposed to this, the high-voltage processing section22in the present example combines the high-frequency passage section60and the low-frequency passage section70and combines the outputs from these sections, thereby enabling a low-frequency component and a high-frequency component of an output signal to be detected, and further enabling an amplitude waveform to be detected.

FIG. 3is a diagram showing one example of an operation of the high-voltage processing section22. As shown inFIG. 3, an output signal having a second reference voltage of 2 kV and amplitude of 15V is inputted to the high-voltage processing section22in the present example.

As described above, the high-frequency passage section60allows passage of a high-frequency component of the output signal, and outputs a differential waveform of the output signal. In addition, the low-frequency passage section70allows passage of a low-frequency component of the output signal, and outputs an integral waveform of the amplitude component of the output signal. During this operation, reference voltages of signals outputted from the high-frequency passage section60and the low-frequency passage section70will be the ground potential.

Then, the composite section80combines a signal outputted from the high-frequency passage section60and a signal outputted from the low-frequency passage section70. By performing such processing, an amplitude component of an output signal having a high voltage is able to be detected with favorable responsiveness.

FIG. 4is a diagram showing one example of a first band for the high-frequency passage section60and a second band for the low-frequency passage section70. It is preferable that the second band is substantially adjacent to a low frequency side of the first band. For example, the second band is a band in a range of the frequency zero to a predetermined cut-off frequency “fc”. In addition, the cut-off frequency of the low frequency side of the first band is substantially equal to the cut-off frequency of the high frequency side of the second band. According to this, it becomes possible to allow passage of the frequency component of an output signal without leakage.

Moreover, it is preferable that the roll-off factor of the low frequency side of the first band is substantially equal to the roll-off factor of the high frequency side of the second band. Moreover, it is preferable that the amplitude gain for the high-frequency passage section60is substantially equal to the amplitude gain for the low-frequency passage section70. Accordingly, the distortion of waveforms incident to combining waveforms is able to be lessened.

FIG. 5is a diagram showing another example of a configuration of the high-voltage processing section22and the pin electronics section20. The high-voltage processing section22in the present example is further equipped with a switch section40in addition to the configuration of the high-voltage processing section22having been described in relation toFIG. 2. The other constituting elements have substantially the same function and configuration as those of the constituting elements assigned the same reference numerals as inFIG. 2.

The switch section40switches between inputting, to the pin electronics section20, an output signal outputted from the composite section80, or a second reference voltage resulting from voltage division by the reference voltage application section12. Accordingly, it becomes possible to select a signal to be measured (i.e. either the output signal or the second reference voltage), and inputs the selected signal to the pin electronics section20.

The test apparatus100judges the quality of the device under test200, based on a judgment as to whether the data pattern of a signal outputted from the composite section80matches the data pattern of a given expected-value signal. However, when the second reference voltage does not lie within a predetermined reference voltage range, there is a possibility of misjudging the quality of the device under test200. To counter this problem, the high-voltage processing section22of the present example is further equipped with the switch section40, thereby enabling the comparator42to detect the voltage level of the second reference voltage. Furthermore, because the second reference voltage obtained by voltage division by the reference voltage dividing section32is inputted to the comparator42, the voltage level of the second reference voltage is able to be measured using the comparator42that detects the amplitude component of an output signal.

The pin electronics section20is further equipped with a control section46in addition to the configuration of the pin electronics section20having been described in relation toFIG. 2. The other constituting elements have substantially the same function and configuration as those of the constituting elements assigned the same reference numerals as inFIG. 2.

The control section46controls a threshold value to be given to the comparator42. When the comparator42measures the voltage level of the second reference voltage, the control section46sequentially changes the threshold value to be given to the comparator42. The comparator42also compares each threshold value with the voltage level of the second reference voltage. According to the result of comparison for each threshold value performed by the comparator42, the voltage level of the second reference voltage is able to be measured.

The measurement of the second reference voltage may be performed prior to testing the device under test200. For example, the test apparatus100may test the device under test200if the second reference voltage lies within a predetermined reference voltage range.

Furthermore, the measurement of the second reference voltage may be performed during testing of the device under test200. In such a case, the high-voltage processing section22may input the second reference voltage resulting from voltage division, to a logic pin different from the logic pin of the pin electronics section20connected to the composite section80. In this case, the high-voltage processing section22does not have to be equipped with a switch section40.

In addition, the switch section40may operate based on a test program given to the pattern generating section18. For example, the switch section40may detect, based on the test program, a timing when an output signal from the high-voltage driving section208is not compared to an expected-value signal, and cause the second reference voltage to be inputted to the comparator42at the detected timing. The switch section40may operate according to an expected-value signal that the pattern generating section18outputs based on the test program. For example, the switch section40may input the second reference voltage to the comparator42when the pattern generating section18does not output an expected-value signal. Alternatively, the switch section40may input the second reference voltage to the comparator42when the expected-value signal indicates a null value.

As apparent from the above description, the present invention enables to measure, with favorable accuracy, a signal whose voltage level fluctuates with respect to a high voltage, and to test, with favorable accuracy, a device under test that is operated with a high voltage as a reference.

In the above, the present invention has been described by way of an exemplary embodiment. However, it is needless to say that the technical scope of the present invention should not be limited by the above-described 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 appended claims that embodiments with such modifications also belong to the scope of the present invention.