Abstract:
In a method for fast, high-accuracy autoranging measurement, a current detection resistance is shared by a current measurement apparatus and a voltage source with current limiting function, a value of current output from the voltage source via the resistance being measured by the current measurement apparatus, and the current range of the current measurement apparatus and the compliance being automatically changed in accordance with the measured current value. The method includes a step of setting the compliance corresponding to the current output from the voltage source to a prescribed value ( 104 ), a step of measuring the value of the current output from the voltage source using the current measurement apparatus ( 106 ), a step of comparing the current measured by the trial measurement with the current range and judging whether or not the current range is optimum ( 108 ), and a step, in the case of a judgment that the current range is optimum, of outputting from the current measurement apparatus the value of current that was measured, and then increasing the current range and changing the compliance ( 112 ).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of measuring an output electric current of a voltage source unit (herein referred to as a source monitor unit or source measurement unit, and abbreviated as SMU) which includes an electric current measuring apparatus having a plurality of electric current ranges and an electric current controller by automatically selecting an appropriate electric current range, and more particularly to a method for changing the range while avoiding changes in the output voltage as much as possible and maintaining the speed and resolution of the measurement. 
   2. Related Art 
   In order to measure an electric current output from an SMU which is a voltage source having an electric current measurement unit or the like, autoranging has been proposed as a way of performing measurement and automatically selecting an appropriate current range. The SMU normally includes, in addition to the current measurement unit and voltage source, a current limit circuit which limits the output current so as not to exceed a limit value (hereinafter referred to as compliance), for the purpose of preventing destruction of the device under test (DUT) by excessive current. The current measurement unit and current control circuit require a resistance for the purpose of detecting the output current from the SMU, and this resistance is shared in the SMU for the benefit of cost and space reduction. Because this resistance is shared, the compliance is controlled by the current range. In the SMU, therefore, when the current range changes during autoranging measurement, an accompanying change of the compliance occurs. That is, at first the compliance is set by the user (hereinafter referred to as the true compliance), along with a correspondingly set current range. When the range setting is changed to a lower current range, and if the maximum current value measurable in the newly set current range (referred to as “upper limit value”) is lower than the true compliance, as a matter of convenience, the compliance is set equal to the upper limit of that current range. If the current range is changed to a value that is greater than the true compliance, the compliance is set equal to the true compliance. 
   By the above-described linking of the current range with the compliance, a voltage that the user does not expect may be applied to the DUT in a limited member of occasions. The hardware configuration of a conventional SMU in which a resistance is shared as the current measurement resistance and the current control resistance is described below. 
   Although the SMU usually has a current control mode and a voltage control mode, to simplify the following discussions, the operation of each part of the SMU will be described herein for the case of voltage control. 
     FIG. 7  of the accompanying drawings is a block diagram showing the configuration of a conventional SMU. The SMU generally includes: a voltage digital-analog converter (hereinafter referred to as voltage DAC)  701  for performing digital-to-analog conversion of a voltage setting value; a current digital-analog converter (hereafter referred to as current DAC)  702  for performing digital-to-analog conversion of a current limit value (compliance); an inverter  705  that inverts the output of the current DAC  702  so as to limit the negative current; a voltage error amplifier  703 ; a power amplifier  708 ; current error amplifiers  704  and  706 ; a range resistance  709 ; a current monitor amplifier  710 ; a voltage monitor amplifier  713 ; and a measurement analog-digital converter  711  for obtaining a digitized measurement value from the obtained output current or voltage value. 
   The voltage error amplifier  703  usually controls the power amplifier  708 . The voltage error amplifier  703  detects the amount of difference between the output voltage of the current DAC  701  and the output voltage fed back from a voltage follower  712  that functions to buffer the output voltage of the SMU, and controls the power amplifier  708  so that the set value of the voltage DAC  701  is the same as the output value of the SMU. The power amplifier  708  amplifies the voltage and current from the voltage error amplifier  703 , and provides the output power of the SMU. 
   The current error amplifiers  704  and  706  are usually in the standby condition, and function to limit the current under prescribed conditions. The output current of the power amplifier  708  is converted to a voltage by the range resistance  709 , and this voltage is normalized by the current monitor amplifier  710 . The output current therefrom is input to an analog-digital converter (hereinafter referred to as an ADC)  711  and digitized, so as to obtain an output current value corresponding to the output voltage of the SMU. 
   The voltage output from the current monitor amplifier  710  is fed back to the current error amplifiers  704  and  706 . The current error amplifiers  704  and  706  the respective perform control of the power amplifier  708  for each polarity, so that the current value does not exceed current value set at the DAC  702 . 
   During voltage output, the voltage error amplifier  703  usually controls the power amplifier  708 . If, for example, a sudden change occurs in the characteristics of the DUT, and the output value of the current monitor amplifier  710  exceeds the compliance set at the current DAC  702 , the current error amplifier  704  or  706  controls the power amplifier  708  in place of the current error amplifier  703 . With this, the voltage is controlled so that a current limited by the compliance set at the current DAC  702  is output. This condition is also referred to as the current loop condition. Under this condition, a signal indicating the current loop condition (the I−Loop status or I+Loop status) is sent to the controller from each of the current error amplifiers  704  and  706 . 
   The range resistance  709  is implemented by a plurality of resistances corresponding to a plurality of current ranges. The lower the current range, the larger the range resistance  709  is. These range resistances are switched in accordance with a range to be set based on a command from the controller (not shown in the drawing) using a switch. The range resistance  709  functions for both current measurement and current limitation. This is because the resistance is expensive due to the influence that the accuracy of the current range resistance has on current measurement accuracy and also because the number of elements or parts should be reduce for lower costs and space savings. Because the range resistance is shared between current measurement and current limitation, it is not possible for the compliance to exceed the upper limit value of the selected current range. 
   The optimum current range for high-resolution measurement is described below. First, we will discuss current ranges that can be measured. Because the full scale of input to the ADC  711  is constant, once the current range is established, it is not possible to measure a current that exceeds the current range. It is therefore necessary to make the current range larger than the current to be measured. Because the measurement resolution, on the other hand, is determined according to resolution relative to the full scale of input to the ADC  711 , the smaller the current range, the higher the resolution is. To measure with high resolution, therefore, it is preferable to measure with the current range that is greater than the current value to be measured and, at the same time, lowest. 
   The above describes conventional SMU hardware configuration and the conditions required for high resolution measurement. Next, we will consider a control method for autoranging measurement with high resolution. 
   Autoranging measurement refers to measurement during which automatic selection is made of the optimum range for each measurement point while achieving good resolution.  FIG. 8  of the accompanying drawings is a flowchart of conventional autoranging measurement. First, at step  800 , the measurement starts. At step  802 , the compliance is set to be the true compliance, and the current range is set to be a range that makes it possible to set the true compliance. At step  804 , an autoranging spot measurement is carried out, and the current range and the compliance are changed as necessary. At step  808 , it is determined whether or not the condition at that time satisfies the end condition set by the user. The end condition can be, for example, the number of measurements, the measurement time, or the final sweep value of the set voltage. The autoranging spot measurement (referred to as “spot measurement”) is repeated at steps  804  and  808  until the end condition is met. The term “spot measurement” as used herein refers to a measurement made by automatically setting the optimum current range and the output of the resulting measured value. Spot measurement is described later herein in detail. 
   When the current range is changed, a change in the compliance accompanies linked to the current range because of reasons attributed to the above-described hardware configuration, and the change is set at the current DAC  702 . The settings of the current range and the compliance after the change at step  804  are held until the next spot measurement. This is because it is common for two consecutive measurements to be mutually close or at least close enough to be of the same order, so that it is often possible to perform the consecutive measurements using the same current range. With this, it is possible to shorten the time spent performing range switching. 
     FIG. 9  of the accompanying drawings shows a flowchart illustrating step  804  in FIG.  8 . First, after setting the SMU output voltage to the user-specified setting at step  904 , at step  906  the current is provisionally measured. At this point, the compliance remains the same as the value previously set before step  900 . At step  908 , it is determined whether or not the current used in the range provisional measurement set at step  906  is the optimum current range. If the current range of the provisional measurement is not optimum, the range and compliance are changed at step  910 , and the process is returned to step  906 , at which a provisional measurement is again performed. The provisional measurement at step  906  and the changing of the compliance and current range at step  910  are repeated until the current range reaches the optimum range. When it is found that the optimum current range has been reached, the process proceeds to step  912 , at which the measured value is output as a the result of the measurement. The spot measurement then ends at step  914 . 
   With regard to the range change performed at step  910 , as will be described below, the compliance also changes when the current range is changed, and if the upper limit of the current range is smaller than the true compliance, the compliance is equal to the value of the current range upper limit. 
     FIG. 10  of the accompanying drawings is a flowchart showing the conventional current range switching as in the above-noted step  910 . At step  1002 , it is determined whether or not the current range is appropriate, based on the current range, the measured value, and the current loop condition signal from the hardware. At this point, if the current range is found too large compared with the measured value and if the measurement can be made with a lower range, the current range is lowered at step  1006 , and there is an accompanying change of the compliance to the upper limit value of the current range. On the other hand, if the current loop condition signal is detected from the hardware, that is, if the current range is excessively low, the current range is raised at step  1004 , and there is an accompanying change of the compliance to the upper limit value of the current range or to the true compliance, whichever is lower. 
   If the current range is changed and lowered, the compliance is not the true compliance, but the upper limit value of the current range after the change. At this point, if the current flowing in the DUT increases and exceeds the compliance, the hardware controls the output voltage of the SMU so that the current flow does not exceed the compliance. Therefore, the output voltage of the SMU may actually be different from the expected voltage value. 
   The above-noted operation is schematically illustrated in  FIG. 11  of the accompanying drawings. The SMU  1103  outputs a constant voltage to the DUT  1102  and performs autoranging current measurement. In this case, the direction of flow of current from the SMU  1103  into the DUT  1102  shall be taken as being positive. The other terminal of the DUT  1102  is grounded. The output voltage in this case, as shown by line  1116 , takes a step-like waveform. 
   First, as shown by the open square symbol at point  1104 , the compliance immediately after the start of the measurement is equal to the true compliance, and the current range is a range with which it is possible to measure a current corresponding to the true compliance. When the measurement starts, as shown by the arrows  1105 , the current range is changed to the optimum range, and accompanying the range change the compliance is also changed. When the output voltage from the SMU  1103  increases, the current flowing into the DUT  1102  also increases. As shown by symbol ▾ at point  1106 , when the increase in the output voltage from the SMU  1103  causes the current flowing in the DUT  1102  to attempt to exceed the compliance, the SMU  1103 , as shown by line  1107 , stops the increase of the output voltage, so that the current does not exceed the compliance, and outputs the current loop condition signal. When the controller detects this current loop condition signal, it increases the current range and the compliance, as indicated by symbol ▴ and symbol □ at point  1108 . As indicated by symbol ⋄ at point  1109 , the current limit of the SMU  1103  is released, and the output voltage returns to its original condition. During this process, by such voltage variations as the one illustrated by line  1107 , an unexpected voltage is applied across the terminals of the DUT  1102 . (For the purpose of comparison, see the expected output voltage indicated by line  1120  in  FIG. 11. ) 
   Two patent documents JP 58-148507A (Pages 3 and 4, and FIG. 7) and JP 08-262069A (Pages 6 and 7 and FIGS. 1 and 2) disclose an SMU which prevents an unexpected current from being caused to flow when the range is switched as in the SMU. More specifically, reference 1 (JP 58-148507A) discloses the stopping of the output from a controlling amplifier when a variable impedance means is changing. Patent reference 2 (JP 08-262069A) discloses a conventional SMU that is provided with an additional output-voltage hold loop. 
   In very low current ranges, in order to improve the stability of measurement accuracy, a certain length of integrating time is usually required. For this reason, there exists an autoranging method in which excessive range lowering is avoided. With such method, the current range is not set lower than a user-specified minimum range or a limit range, even if the current range required for measurement is found to be low. By doing this, it is possible to reduce the measurement time, while maintaining the resolution required by the user.  FIG. 12  is a flowchart showing another method of changing current ranges in such limit autoranging measurement as an alternative to the one shown in FIG.  10 . First, at step  1202 , it is determined whether or not the current range is large in comparison with the measured value. Next, at step  1204  it is determined whether or not the current range at that time is larger than the user-specified limit range. Only if the current range is larger than the limit range, the current range is reduced at step  1206 , and the compliance is changed to the upper limit value of that current range.  FIG. 13  shows the operation of the SMU at the time when the range is changed in this case. Similar to arrows  1105  in  FIG. 11 , changes are made of the range and the compliance as indicated by line  1301  in FIG.  13 . Compared to the case indicated by arrows  1105  in the above-noted  FIG. 11 , however, as shown by symbol □ at point  1302  the compliance does not fall below the user-specified limit range (in this case 1.00E-4 A). 
   However, in the above-noted case, it is difficult to maintain the optimum range at all times. Additionally, because there is no change to a range lower than the user-specified limit range, the dynamic range of the measurement is reduced. Therefore, if a range that is lower than the limit range is the optimum range, the resolution of measurements of minute currents is compromised. For example, in comparison with  FIG. 11 , in the case of  FIG. 13 , in a measurement as shown by symbol ▴ at point  1302 , because the current range does not fall below the limit range, the resolution indicated by symbol ◯ shown at point  1303  in  FIG. 13  is worse than that indicated by symbol ◯ at point  1110  in FIG.  11 . 
   Given the above, another method is proposed for the purpose of preventing the problems accompanying the limitation of the current range imposed by the compliance after the SMU output is set. In performing each autoranging spot measurement, the compliance is returned to the true compliance when the SMU output is set. In other words, the current range is returned to the value in effect immediately after the start.  FIG. 14  shows the operation of this method in contrast to that of FIG.  9 . First, at step  1400 , similar to step  904  in  FIG. 9 , the current range and compliance set prior to step  1400  are used. At step  1402 , the compliance is set to be the true compliance and the current range is set accordingly. At step  1404 , the output of the SMU is changed. Therefore, because the provisional measurement at step  1406  does not touch upon the compliance, it is possible to prevent the output voltage of the SMU from changing. An example of this is shown in FIG.  15 . As indicated by symbol ▴ at point  1502 , the current range is set at the optimum range for the measurement. As indicated by symbol □ at point  1503 , when the next spot measurement is started, the compliance is returned to the true compliance. The current range at that time is the value immediately after the start of the measurement, as indicated by symbol ▴ at point  1501 . As indicated by symbol ▴ at point  1504 , the current range is changed to the optimum range, and the measurement result is output. Therefore, the current range is not limited by the compliance and, as indicated by line  1505  in  FIG. 15 , the output voltage from the SMU does not exhibit unexpected variations. 
   According to the judgment made at step  1408  of  FIG. 14 , however, the current range may often not be optimum. For the reason, the probability may increase that the range is changed at step  1410 . Additionally, because initial value of the current range is suitable for the true compliance, the number of range changes executed before reaching the optimum range increases. As a result, the measurement time becomes long. For example, in each of the spot measurements performed at symbols ● at points  1522 ,  1524 ,  1525 ,  1526 , and  1527 , a range change is executed, with an attendant increase in the length of measurement time required. Additionally, although not shown in  FIG. 15 , when the number of provisional measurements increases, the range changes from the true compliance to the optimum range take also more time. 
   SUMMARY OF THE INVENTION 
   Given the above situation, demand exists for a method of performing high-accuracy, high-speed measurement of voltage-current characteristics using current autoranging, while suppressing unexpected output when the voltage and current settings are changed. 
   Accordingly, the present invention provides a method for performing high-accuracy, high-speed measurement of voltage-current characteristics using autoranging function. 
   Specifically, one aspect of the present invention is a method for measurement in which a resistance for current detection is shared by a current measurement device and a voltage source having current limiting function or compliance function, the value of current output from the voltage source is measured by the current measurement device using the resistance, and a compliance and the current range of the current measurement device are automatically changed in accordance with the measured current value, wherein the method comprising the steps of: 
   setting the current range and a true compliance to user-specified values, 
   setting the compliance for the current output from the voltage source to a prescribed value, 
   measuring the value of current output from the voltage source using the current measurement apparatus, 
   comparing the measured value of the current with the current range and determining whether or not the current range is optimum, 
   if the current range is found not to be optimum, changing the current range and performing another measurement of the current output from the voltage source with the newly changed current range, with the compliance being at a value not exceeding the user-specified value and at the upper limit value of the newly changed current range, and 
   if the current range is found optimum, having the current measurement apparatus output the resulting current measurement value, then raising the current range and making a corresponding change in the compliance. 
   In another preferred aspect of the present invention, if the current range is found to be optimum, the compliance is changed to either the user-specified value or the upper limit value of the raised current range, whichever is lower. 
   Another preferred aspect of the present invention includes, after the step of performing another measurement, if the current range is found not to be optimum, raising the current range and changing the compliance accordingly. 
   In yet another preferred aspect of the present invention, the lower limit of the current range is set beforehand with respect to the minimum output current value from the voltage source. 
   In another preferred aspect of the present invention, the step of determining whether or not the current range is optimum includes a step of calculating the ratio between the measured value of current output from the voltage source and the current range, and a step of determining whether or not the current range needs to be changed by comparing the calculated ratio with a prescribed ratio. 
   In another preferred aspect of the present invention, the step of determining whether or not the current range needs to be changed by comparing the calculated ratio with a prescribed ratio includes the calculation of the ratio between the measured value of current output from the voltage source and a current range that is lower than the presently set current range, and comparing this calculated ratio with a prescribed ratio. 
   In yet another preferred aspect of the present invention, the step of determining whether or not the current range needs to be changed sets the upper limit value of the maintained or changed current range as the true compliance. 
   In the foregoing aspects of the present invention, the compliance or the prescribed value or variation range of the current range can be set either by a user or by the measurement apparatus. The change of current ranges accompanying a variation in the output of the SMU can include up-ranging (raising of the current range) or down-ranging (lowering of the current range). Additionally, the timing of the changing of the current range can be established either immediately after the output of the measurement result or simultaneously with the SMU output setting or both. It is preferable that the determination of whether or not there is a need to change the current range be performed based on the whether, giving sufficient consideration to the resolution of the current range, the measurement of the current using this particular current range can be performed with sufficient accuracy (that is, whether the present current range is optimum). If the current can be measured with sufficient accuracy using a particular current range, the current range can be considered optimum or vice versa. 
   The above and other objects and features of the present invention will be more specifically described in the following detailed description, taken with the accompanying drawings. 

   
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
       FIG. 1  is a flowchart showing the change of the current range according to a first embodiment of the present invention. 
       FIG. 2  is a flowchart showing the change of the current range according to a second embodiment of the present invention. 
       FIG. 3  is a flowchart showing the change of the current range according to a third embodiment of the present invention. 
       FIG. 4  is a graph showing the SMU operation in the first embodiment of the present invention. 
       FIG. 5  is a graph showing the SMU operation in the second embodiment of the present invention. 
       FIG. 6  is a graph showing the SMU operation in the third embodiment of the present invention. 
       FIG. 7  is a block diagram showing an SMU of the past. 
       FIG. 8  is flowchart showing the measurement method of the past. 
       FIG. 9  is a flowchart showing a spot measurement method of the past. 
       FIG. 10  is a flowchart showing the change of the current range in autoranging of the past. 
       FIG. 11  is graph showing the change of the current range in autoranging of the past. 
       FIG. 12  is a flowchart showing the change of the current range in another example of autoranging in the past. 
       FIG. 13  is a graph showing the change of the current range with another example of autoranging in the past. 
       FIG. 14  is a flowchart showing the change of the current range in yet another example of autoranging in the past. 
       FIG. 15  is a graph showing the change of the current range in yet another example of autoranging in the past. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention are described below in detail, with reference to relevant accompanying drawings. The first embodiment of the present invention is described below with reference to FIG.  1  and FIG.  4 . 
     FIG. 1  is a flowchart showing a spot measurement applied to step  804  in  FIG. 8 , and  FIG. 4  is a graph showing the SMU according to in the first embodiment of the present invention. As shown in the lower right part of  FIG. 4 , the circuit diagram is the same as the one shown in FIG.  11 . 
   Turning first to the flowchart of  FIG. 1 , at step  100  the spot measurement starts. At step  104 , the output voltage of the SMU is changed so as to change the voltage applied to the DUT. Next, at step  106 , a provisional measurement is performed with the changed SMU output voltage. At step  108  a determination is made from the measured value obtained as to whether or not the current range used in the measurement has been an optimum range. If the current range is optimum, the process proceeds to step  110 , and the measurement result is output. At step  112 , the current range is raised with a prescribed width of variation (up-ranging) to accommodate a rise in the SMU output voltage, and either the upper limit value of the raised current range or the true compliance, whichever is lower, is set as the compliance. By doing this, the time required to change the current range can be shortened, compared with conventional methods. By setting the up-ranged current range as the compliance in this manner, it is possible to reduce the possibility that the voltage applied to the DUT from the SMU varies because of the compliance. 
   At step  108  if the current range is found not to be optimum; that is, if comparison of the current range with the measured current value shows that the current range is excessively high, so that it is not possible to carry out a measurement with sufficient accuracy, the process proceeds to step  116 . The current range and compliance are changed by a prescribed amount of variation, and the compliance is changed to the upper limit value of the current range, after which the process is returned to step  106 . In this case as well, in order to accommodate the next rise in the SMU output voltage after the output of the measurement result at step  110 , the current range is raised by a prescribed amount at step  112 , and the compliance is set to either the upper limit value of the changed current range or the true compliance, whichever is lower. Within step  116 , a conventional algorithm such as the one shown in  FIG. 10  or  FIG. 12  may be used. 
   Referring to the flowchart of  FIG. 4 , the operation of the SMU according to  FIG. 1  will be described. After the current is measured at the current range indicated by symbol ▴ at point  401  in  FIG. 4 , the current range is raised as indicated by symbol ▴ at point  402  in FIG.  4 . When this is done, with regard to what range the current range is to be raised, it is possible to use a prescribed value established by the measurement apparatus or a prescribed value established by the user. By making the range change in this manner, even if the current value after the increase in the SMU output voltage indicated by symbol ● at point  404  is larger than the current range at the immediately previous measurement, as indicated by symbol □ at point  403 , the current value is not limited to the compliance. For this reason, it is possible to reduce the possibility of the SMU outputting an expected voltage. Additionally, because it is possible to perform measurements at all measurement points in the optimum range, it is also possible to achieve the resolution indicated by  406 . Because the measurement result is obtained each time using the optimum range, it is possible to measure the current value with high resolution, and further reduce the possibility that a variation occurs in the output voltage from the SMU. Compared to the conventional methods, because the length of time during which the range (range-up) is raised is longer, it is less likely that the current value is limited by the compliance due to noise or the like. Additionally, compared with the arrow  1516  in  FIG. 15 , showing certain conventional art, because the current range after up-ranging is lower than the range immediately after starting, for example, as in the measurement shown by symbol ● at point  409 , the change in the current range may be as small as from 1.00E-02 A (10 mA) to 1.00E-04 A (100 μA). Because at arrow  1516  in the conventional art shown in  FIG. 15 , it is necessary to have a large variation from 1.00 A to 1.00E-04 A, with the present invention, the length of time required to change the current range may be reduced. 
   If the current value exceeds even the range value  402  indicated by ▴ after the up-ranging, however, even according to the above-described first embodiment it may not be possible to completely prevent a change in the SMU output voltage. Additionally, because in each of the spot measurements the probability of a range change to the optimum range is high, the measurement time may become overly long. Compared with the first embodiment, the second embodiment of the present invention provides a smaller probability of changing the range to the optimum range, and can therefore shorten the measurement time. Whereas in the first embodiment the range is raised every time a measurement result is output, according to the second embodiment according to the proportion of the measurement value relative to the current range is determined, and the range is raised only if this proportion exceeds a prescribed value. 
     FIG. 2  shows the flowchart of the spot measurement that can be applied to step  804  shown in FIG.  8 . Steps  200  to  210  are the same as steps  100  to  108  and  116  of the first embodiment. After outputting the measurement result at step  212 , at step  214  if the proportion of the measured value relative to the current range is larger than a prescribed value (indicated as ratio 1 in FIG.  2 ), at step  216  the range is raised before the next provisional measurement, with the compliance changed accordingly. That is, the compliance is set to either the upper limit value of the changed current range or the true compliance, whichever is smaller. In this case, the above-noted prescribed value (ratio 1) can be set by either the measurement apparatus or the user. Thus, it is possible to set an appropriate value according to the measurement made beforehand, thereby enabling a shortening of the measurement time. At step  214 , if the proportion of the measured value relative to the current range is less than the above-noted prescribed value (ratio 1), because the possibility of exceeding the compliance by the measured current in the next time is low, the current range is not changed. 
   The operation of the SMU in the second embodiment is described below, with reference made to the graph of FIG.  5 . After the measurement is started at the point indicated by symbol ▴ at point  505  in  FIG. 5 , because the proportion of the measurement value  501  relative to the range at the point indicated by symbol ● is greater then the prescribed value  503  at Δ (ratio 1), the current range at arrow  506  is raised by one step only. By doing this, it is possible to reduce the probability that the next measured value  509  indicated by ● is limited to the compliance, and the SMU outputs a voltage indicated by ⋄ at poin  514  that differs from an expected value. Additionally, because it is possible to perform measurements for all points with optimum ranges, it is possible to achieve the optimum resolution  504  indicated by ◯ at all times. 
   Here the proportion of the measured value  509  indicated by ● relative to the optimum range  508  indicated by ▴ is large, but the amount of change of the current is small enough to enable the next measurement to be performed also with the same 1.00E-04 A current range  511  indicated by ▴ as in the previous measurement, at arrow  507  it could happen that the range, once increased to 1.00E-03 A, is then lowered at arrow  522 . If the amount of change in the current is small in such cases as this one, because not only the number of times the range is changed at arrows  506  and  507  after a measurement, but also the number of changes made at arrow  522  and  513  to the optimum range increase, the number of range changes increases, the measurement time may become longer. In such cases, it is preferable to use the third embodiment of the present invention, which enables a reduction in the probability of an increase in the number of range changes. 
     FIG. 3  shows the flowchart of the current range changing according to the third embodiment. Step  300  of  FIG. 3  illustrates the part of the third embodiment corresponding to the range changing at step  210  in FIG.  2 . At step  302 , if the current range is found to be high and the range is lowered, another step is added to perform a judgment also as to the proportion of the measurement value relative to the next lower current range. At step  304 , a judgment is made as to the proportion of the measured value relative to the next lower current range. If the proportion exceeds a prescribed value (indicated as ratio 2 in the drawing), rather than changing the current range and the compliance, the process proceeds to step  314  via path  308 . For this reason, the frequency of range changing is reduced, so as to reduce the measurement time. The above-noted prescribed value (ratio 2) can be set by either the measurement apparatus or the user. At the above-noted step  304 , if the proportion is less than the prescribed value (ratio 2), at step  306  the current range is lowered, the compliance is set to the upper limit value of the current range, and at step  314  the current range change is ended. 
     FIG. 6  shows a graph illustrating the operation of the SMU in the third embodiment. At the measurement at point  603  as indicated by ●, the optimum range with respect to the current value is at point  606  indicated by ▴, but because the measured value  603  as indicated by ● is larger than the prescribed proportion (ratio 2)  605  indicated by ×, at point  602  with ▴ rather than perform a range change the measured value from the provisional measurement at step  206  is output as it is. Therefore, in addition to preventing a change in the current measurement range, which could lead to a change in the voltage, because the current range which has been raised at step  216  in the above-noted second embodiment has a reduced probability of being lowered at step  306 , it is possible to perform measurements with a higher speed than with the second embodiment. 
   As described in detail above, a method according to the present invention enables high-accuracy, high-speed autoranging measurement, with only a small probability of the application of an unexpected voltage. 
   The entire disclosure of Japanese Patent 
   Application 2003-3951, filed on Jan. 10, 2003, including the specification, claims, drawing, and summary thereof, is incorporated herein by reference in its entirety. 
   As the present invention can be embodied in several forms without departing from the spirit and essential characteristics thereof, the embodiments described herein are illustrative and not restrictive, the scope of the invention being defined by the appended claims rather than by the description preceding them. All changes falling within the bounds of the claims or the equivalence of the bounds thereof are therefore understood to be encompassed by the claims.