Abstract:
An improved crossover circuit for a V/I source includes a selector and a measurement circuit. The selector and measurement circuit both receive error signals indicative of differences between programmed and actual values of output voltage and current of the V/I source. In response to occurrences of predetermined events among the error signals, the measurement circuit activates the selector to pass one of the error signals to a control circuit for establishing a feedback loop. Different events cause different error signals to be selected, and hence cause different feedback loops of the V/I source to be activated. The improved crossover circuit provides increased control over the selection of feedback mode, and enhances the ability to individually optimize dynamic behavior of different feedback modes.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to automatic test equipment (ATE) for electronics, and, more particularly, to electronic sources that automatically switch between voltage control and current control depending upon programming and load conditions. 
     BACKGROUND OF THE INVENTION 
     ATE systems commonly include a variety of electronic sources for setting bias conditions and testing DC characteristics of devices. A type of electronic source known as a “V/I” source combines both voltage-forcing and current-forcing modes in a single instrument. The two modes generally share a common control circuit and output stage, but employ different feedback paths. The different feedback paths can be engaged programmatically, for example by explicitly setting a force-voltage or force-current mode, or can be engaged automatically, as described below. 
     Automatically controlled V/I sources accept programmed values for voltage and current, and switch between voltage-controlled and current-controlled modes automatically, as required, to ensure that neither the programmed voltage nor the programmed current is exceeded. For example, an automatically controlled V/I source programmed for 5 Volts and 5 mA would operate in voltage-controlled mode (at 5 Volts) when connected to grounded loads greater than 1 KΩ, but would automatically switch to current-controlled mode (at 5 mA) when driving grounded loads less than 1 KΩ. 
     V/I sources often include greater than two control modes. One type of V/I source provides one current-controlled mode and two voltage-controlled modes. This type of source functions as a current source with positive and negative voltage clamps. Another type of V/I source provides one voltage-controlled mode and two current-controlled modes. This type of source functions as a voltage source with positive and negative current limits. Some V/I sources include four feedback modes—two current-controlled modes and two voltage-controlled modes. Only three modes are allowed to be active at a time. The source can be used as either a current source with two clamps or a voltage source with two current limits, depending upon how the source is programmed. 
     FIG. 1 is a simplified illustration of a conventional V/I source  100 . The source  100  is configured as a voltage source with two current limits. A digital-to-analog converter (DAC)  110  establishes a desired output voltage for the V/I source  100 . A summing circuit  116  subtracts a voltage feedback signal from an output voltage of the DAC  110 , to produce an error voltage, V ERROR . A crossover circuit  122  selects one feedback path for passage to a control circuit  124 . When the V/I source  100  is operating in voltage-controlled mode, the crossover circuit  122  passes the V ERROR  to the control circuit  124 . The output of the control circuit  124  is fed to a gain stage  126  and to a shunt  128  before arriving at a device under test (DUT)  132 . When operating in voltage-controlled mode, the V/I source  100  forms a closed loop feedback system among the elements described above, and maintains an output voltage at the DUT  132  at the value prescribed by the DAC  110 . 
     The V/I source  100  also includes DACs  112  and  114  for establishing positive and negative current limits, respectively. A differential circuit  130  coupled to the shunt  128  produces a current feedback signal proportional to the voltage across the shunt. Summers  118  and  120  subtract the current feedback signal from the outputs of DACs  112  and  114 , to develop error signals I PosError  and I NegError , respectively. When the V/I source  100  operates in positive current-controlled mode, the crossover circuit  122  passes I PosError  to the control circuit  124 ; when it operates in negative current-controlled mode, the crossover circuit  122  passes I NegError . The circuit elements combine to form feedback systems that maintain the output current of the V/I source  100  at the value prescribed by DAC  112  or DAC  114 , depending upon which of the two current modes is operative. 
     The control circuit  124  typically includes an integrating circuit for establishing dominant frequency characteristics of the V/I source. The gain stage  126  may provide voltage gain, current gain, or both. The shunt  128  generally includes an array of different resistors that can be individually selected to accommodate different current ranges. 
     FIG. 2 illustrates a conventional crossover circuit  122  commonly used with the V/I source  100  of FIG.  1 . The crossover circuit  122  includes operational amplifiers (op amps)  214  and  224 , buffers  212  and  222 , diodes  216 ,  218 ,  226 , and  228 , and resistors  210 ,  220 , and  230 . The op amps  214  and  224  each have two distinct states of operation—an active state and an inactive state. 
     Taking the op amp  214  as an example, the op amp  214  assumes the active state whenever I PosError  is less than V ERROR . Under these conditions, diode  216  becomes reverse-biased and diode  218  conducts in the forward direction. A feedback loop is formed consisting of op amp  214 , diode  218 , buffer  222 , and resistor  220 . The feedback loop tends to drive the input of the buffer  212  to a level equal to I PosError . The buffer  212  then provides I PosError  to the input of the control circuit  124 . 
     Op amp  214  assumes the inactive state whenever I PosError  is greater than V ERROR . Diode  218  becomes reverse-biased and diode  216  conducts. I PosError  is thus cut off from the control circuit  124 , and feedback is closed locally around op amp  214  via diode  216 . 
     The negative polarity operates in an analogous manner. Op amp  224  assumes the active state whenever I NegError  is greater than V ERROR . A feedback loop is formed consisting of op amp  224 , diode  228 , buffer  222 , and resistor  230 . The loop tends to drive the input of the buffer  212  to I NegError , and establishes the negative current-controlled mode. When I NegError  is less than V ERROR , diode  228  becomes reverse-biased and diode  226  conducts, thus cutting off I NegError  from the control circuit  124  and locally closing feedback around the op amp  224 . 
     The crossover circuit  122  thus engages a current-controlled mode when either of the op amps  214  and  224  operates in its active state. When both op amps operate in their inactive states, the crossover circuit  122  engages voltage-controlled mode. Voltage-controlled mode is thus engaged whenever V ERROR  is greater than I NegError  and less than I PosError . In voltage-controlled mode, the crossover circuit  122  passes V ERROR  to the control circuit  124  via resistor  210 . 
     The crossover circuit  122  generally operates smoothly and accurately, making virtually seamless transitions between feedback modes. We have recognized, however, that the crossover circuit  122  can behave improperly during programming and output transients. For example, when programming a fast voltage step (via the DAC  110 ), the signal V ERROR  undergoes a voltage step from its steady-state value. The resulting step can momentarily cause V ERROR  to cross I PosError  or I NegError . These conditions cause the V/I source to inappropriately switch from voltage-controlled mode to one of its current-controlled modes. The mode change is “inappropriate” because it is not caused by excessive current flow; in fact, the output current may be zero. Rather, it is the natural consequence of applying a fast programming step. 
     FIG. 3 illustrates this condition. FIG. 3 is a V/I plot of the V/I source  100  operating with the crossover circuit of FIG.  2 . The curve  300  represents the output of the V/I source during its three distinct feedback modes: 
     1. The upper, horizontal portion of the curve  300  represents the output of the V/I source when the positive current loop is engaged and programmed to a value I ProgPos ; 
     2. The vertical portion represents the output when the voltage loop is engaged and programmed to a value V Prog ; and 
     3. The lower, horizontal portion represents the output when the negative current loop is engaged and programmed to a value I ProgNeg . 
     As long as the V/I source  100  operates in a settled state, the output of the V/I source will fall somewhere along the V/I curve  300 . This is not the case, however, during programming and output transients. As indicated above, the V/I source  100  assumes positive current-controlled mode whenever I ProgPos &gt;V ERROR  and negative current-controlled mode whenever I ProgNeg &lt;V ERROR . The shaded regions depicted in the left and right portions of FIG. 3 respectively illustrate these areas. During transients, the V/I source can momentarily operate in the shaded regions, inappropriately engaging the current limits even though the output current is within limits. The V/I source can also fail to engage its current limits, even though the programmed currents are exceeded. 
     Inappropriately switching to current-controlled mode dramatically slows the settling time of the V/I source. Once the V/I source changes to a current-controlled mode, it remains in that mode until the error signal of the active current-control loop and V ERROR  again cross. Depending upon the values of I ProgPos  and I ProgNeg , hundreds of microseconds may pass before the V/I source restores itself to voltage-controlled mode. This interval is exceedingly long compared with the normal settling time of the V/I source, i.e., when the current loops do not engage. In an automatic testing environment, delays in programming a V/I source translate directly to reduced testing throughput. A reduction in throughput detrimentally impacts testing efficiency. 
     What is needed is a crossover circuit for a V/I source that does not inappropriately switch feedback modes and consequently cause programming delays. 
     SUMMARY OF THE INVENTION 
     With the foregoing background in mind, it is an object of the invention to prevent an electronic source from assuming improper feedback modes during programming and output transitions. 
     To achieve the foregoing object, as well as other objectives and advantages, an improved crossover circuit is provided for use with an electronic source having a control circuit and providing at least one voltage-controlled mode and at least one current-controlled mode. The crossover circuit includes a selector and a measurement circuit. The selector and measurement circuits both receive a plurality of signals indicative of feedback voltage and feedback current of the electronic source. The measurement circuit monitors the plurality of signals and generates, in response to occurrences of predetermined events on the plurality of signals, at least one control signal. The selector operates in response to the at least one control signal, to select one of the plurality of signals for passage to the control circuit. A feedback loop is then formed for the electronic source employing the selected signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects, advantages, and novel features of the invention will become apparent from a consideration of the ensuing description and drawings, in which 
     FIG. 1 is a block diagram of a conventional electronic source that provides operation in one voltage-controlled mode and two current-controlled modes; 
     FIG. 2 is a simplified schematic of a conventional crossover circuit used in the electronic source of FIG. 1, which automatically makes transitions between the voltage-controlled mode and the two current-controlled modes; 
     FIG. 3 is a current vs. voltage graph of the electronic source of FIG. 1 using the crossover circuit of FIG. 2, depicting areas in which current limits may be active; 
     FIG. 4 is a simplified schematic of a crossover circuit constructed in accordance with the invention; 
     FIGS. 5A-5C are curves that illustrate the output voltage and feedback error signals of an electronic source of FIG. 1 as a function of programmed voltage, driving a resistive load with the electronic source equipped with the crossover circuit of FIG. 4; 
     FIG. 6 is a current vs. voltage graph of the electronic source of FIG. 1 using the improved crossover circuit of FIG. 4, depicting areas in which current limits of the improved circuit may be active; 
     FIG. 7 is a simplified schematic of a conventional integrator circuit, which could be used in connection with the control circuit of FIG. 1; 
     FIG. 8 is a block diagram of an alternative electronic source with which the crossover circuit of FIG. 4 can be used according to an alternative embodiment of the invention, which provides operation in two current-controlled modes and one voltage-controlled mode; and 
     FIG. 9 is a simplified schematic of the crossover circuit of FIG. 4 adapted for use with the alternative electronic source of FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Topology and Operation 
     FIG. 4 shows an illustrative embodiment of a crossover circuit  400  in accordance with the invention. The crossover circuit  400  can be used in place of the crossover circuit  122  of FIG.  2 . As shown in FIG. 4, the crossover circuit  400  includes a selecting circuit  410  and a measurement circuit  430 . The selecting circuit  410  and measurement circuit  430  both have inputs that receive the error signals V ERROR , I PosError , and I NegError . In response to occurrences of predetermined events among the error signals, the measurement circuit activates the selecting circuit  410  to pass one of the error signals to the control circuit  124 . 
     The measurement circuit  430  includes first and second comparators  432  and  434 . The outputs of the first and second comparators  432  and  434  are respectively coupled to SET and RESET inputs of a first latching circuit  440 . The first comparator  432  has a non-inverting input that receives I PosError . The second comparator  434  has a non-inverting input that receives V ERROR . The first and second comparators  432  and  434  each have an inverting input that receives a first reference voltage, V OS− . In the preferred embodiment, V OS−  is set to a small negative value, such as −7 mV. 
     The first latching circuit  440  “sets” when I PosError  drops below V OS−  and “resets” when V ERROR  drops below V OS− . In response to a “set” state of the first latching circuit  440 , the measurement circuit  430  asserts a control signal I PosCtl , which activates the selecting circuit  410  to pass I PosError  to the control circuit  124 . A feedback loop is then established wherein the output current of the V/I source  100  tends to match the value prescribed by the DAC  112 . In response to a “reset” state of the latching circuit  440 , the control signal I PosCtl  is de-asserted, and I PosError  is isolated from the control circuit  124 . I PosError  then has no affect on the output of the V/I source. 
     The measurement circuit  430  also includes third and fourth comparators  436  and  438 . These comparators have outputs that are respectively coupled to SET and RESET inputs of a second latching circuit  442 . The third comparator  436  has an inverting input that receives I NegError . The fourth comparator  438  has an inverting input that receives V ERROR . The third and fourth comparators  436  and  438  each have a non-inverting input that receives a second reference voltage, V OS+ . In the preferred embodiment, V OS+  is set to a small positive value, such as +7 mV. 
     The third latching circuit  442  “sets” when I NegError  exceeds V OS+ , and “resets” when V ERROR  exceeds V OS+ . When the latching circuit  442  is “set,” the measurement circuit  430  asserts a control signal I NegCtl , which causes the selecting circuit  410  to pass I NegError  to the control circuit  124 . When the latch  442  is “reset,” the measurement circuit  430  de-asserts I NegCtl  and consequently blocks I NegError  from the control circuit  124 . 
     With the first and second latching circuits  440  and  442  both “reset,” the measurement circuit  430  asserts a control signal V Ctl . Assertion of V Ctl , via a NOR gate  444 , activates the selecting circuit  410  to pass V ERROR  to the control circuit  124 . When V ERROR  is selected, the V/I source  100  tends to drive its output voltage to the value prescribed by the DAC  110 . 
     With the arrangement described above, one error signal is always selected, and only one error signal is ever selected at a time. I PosError  and I NegError  are never selected at the same time. The positive current DAC  112  should always produce a more positive output than the negative current DAC  114  (this rule is preferably enforced by design). Therefore, I PosError  should always be more positive than I NegError . Because I PosError  is selected only when it goes negative and I NegError  is selected only when it goes positive, the voltage difference between I PosError  and I NegError  ensures that conditions are never met to select both at the same time. In addition, owing to the operation of the NOR gate  444 , V ERROR  is selected only when neither current error is selected. Thus, there can never be an instance in which the V/I source operates with multiple feedback paths active, or when it operates “open loop.” 
     FIGS. 5A-C illustrate the behavior of the crossover circuit  400  as functions of programmed voltage, i.e. as functions of the output of the DAC  110 , with the V/I source driving a resistive load. As shown in FIG. 5A, the output voltage V Out  of the V/I source varies directly with programmed voltage, unless the V/I source is operating in positive or negative current limit. In current limit, output voltage remains constant, regardless of changes in programmed voltage. 
     As shown in FIG. 5B, V ERROR  remains at 0 Volts when the V/I source operates in voltage-controlled mode, because voltage feedback ensures that V Out =V PROG . When the V/I source enters current limit, however, V ERROR  deviates from 0 Volts, because V PROG  continues to change while V Out  remains constant. Eventually, V ERROR  levels off at a voltage determined by the output limits of the summer circuit  116 . 
     As shown in FIG. 5C, I PosError  operates at 0 Volts when the V/I source operates in positive current limit, because the output current of the V/I source equals the positive current limit from the DAC  112 . I PosError  is positive, however, when the V/I source operates in voltage-controlled, or negative current-controlled, mode. Analogously, I NegError  is maintained at 0 Volts when the V/I source operates in negative current limit, but is negative at all other times. Both I PosError  and I NegError  eventually level off as the summers  118  and  120  that produce them reach the limits of their respective output circuits. 
     A significant aspect of the crossover circuit  400  is that it compares error signals V ERROR , I PosError  and I NegError  with zero or near-zero voltages rather than with each other. The instant invention exploits this fact by responding to the zero-crossing, or near zero-crossing, of the error signals. 
     As the zero-crossings of the error signals indicate a need to change modes, the crossover circuit  400  could be configured with threshold voltages V OS+  and V OS−  both set to 0 Volts. We have discovered, however, that setting the thresholds to 0 Volts can cause the crossover circuit  400  to undesirably oscillate between modes. Providing small offsets effectively adds hysteresis to the crossover circuit  400 , and helps to ensure that noise signals do not cause the circuit to oscillate. 
     To illustrate the effect of hysterisis, consider a small negative offset voltage V OS−  on the comparators  432  and  434 . This offset voltage forces I PosError  to go slightly negative before the crossover circuit  400  switches to positive current control. Typically, this means that V Out  will attain a slightly greater value just before current limiting than just after. When the current limit engages, V Out  is pulled negative, causing V ERROR  to be pushed positive. The sudden upward movement of V ERROR  tends to push it out of the noise range of the crossover circuit and tends to prevent the V/I source from inappropriately re-entering voltage-controlled mode. By an analogous process, a small positive voltage V OS+  on the comparators  436  and  438  tends to prevent oscillations when switching into negative current limit. The offset voltages also inhibit oscillations when switching to voltage-controlled mode from either of the current-controlled modes. 
     As shown in FIGS. 5B-C, V ERROR  normally increases as the V/I source enters positive current limit and normally decreases as the V/I source enters negative current limit. These movements of V ERROR  generally ensure that the SET and RESET inputs of each of the latches  440  and  442  are not active at the same time. In rare instances, however, it is possible for both inputs to be active simultaneously. Under these conditions, the properties of the latches  440  and  442  determine the mode (SET or RESET) that dominates. Commercially available S/R latches generally specify which mode dominates in the event that both inputs are active at one time. For S-R latches constructed from cross-coupled NAND gates, the SET input dominates. Regardless of the type of S-R latch used, it is recommended that the crossover circuit be arranged so that the current limiting mode dominates under these conditions, to ensure that current limits are strictly enforced. 
     The crossover circuit  400  represents a significant advance over the prior circuit  122 . FIG. 6 is a V/I plot of the V/I source  100  operating with the crossover circuit  400 . The curve  600  represents the output of the V/I source during its three feedback modes. The shaded portions represent areas in which the current limits are engaged. In contrast with the V/I plot of FIG. 3, the V/I plot of FIG. 6 includes no areas in which the current limits are engaged when they should not be engaged. In addition, it includes no areas in which the current limits are not engaged when they should be engaged. Therefore, the crossover circuit  400  enables the V/I source  100  to always assume the appropriate feedback mode, even under transient conditions. The V/I source  100  can thus avoid the long programming delays incurred using the prior crossover circuit, and testing throughput is significantly increased. 
     We have discovered an additional and unintended benefit of the crossover circuit  400 —it readily allows the voltage and control loops to be individually stabilized. FIG. 7 shows a highly simplified schematic of the control circuit  124 , which includes a conventional integrator. The integrator consists of a operational amplifier  710  having its non-inverting input grounded, and its inverting input  714  receiving a selected error signal (one of V ERROR , I PosError  and I NegError ) via an input resistor  712 . A capacitor  716  connected between the inverting input  714  and the output of the operational amplifier operates in conjunction with the input resistor  712  to establish dominant frequency characteristics of the V/I source  100 . 
     When using the prior crossover circuit  122 , the same input resistor  712  of the control circuit  124  is used for voltage-controlled mode and both current-controlled modes of the V/I source. This means that all three modes use the same components for establishing their dominant frequency characteristics. As known to those skilled in the art, however, voltage-controlled modes and current-controlled modes generally have different frequency response characteristics. 
     When using the same integrator components, the open loop gain of the current-controlled loops generally exceeds the open loop gain of the voltage-controlled loop. This is attributable to the need to provide a small full-scale voltage across the shunt  128 , to minimize headroom. The current feedback signal generally requires amplification (by the differential circuit  130  or the summers  118  and  120 ) to boost the small voltage across the shunt  128  and allow one-for-one comparison with the outputs of the DACs  112  and  114 . No such amplification is needed for the voltage-controlled loop. In fact, to achieve high output voltages the feedback must be attenuated. These conflicting requirements generally lead to a situation in which one sacrificially slows the voltage-controlled loop to keep the current-controlled loops stable. 
     The instant invention avoids this sacrifice by allowing separate input resistors to be used in the different modes to drive the control circuit&#39;s integrator. Referring to FIG. 4, the crossover circuit  400  includes a first input resistor  418  for voltage-controlled mode, and a second input resistor  420  for the current-controlled modes. If desired, different input resistors can be provided for each of the two current-controlled modes. The input resistors can be accompanied by additional components coupled in series or in parallel with the input resistors, for establishing various compensating effects. The instant invention thus allows different dominant frequency characteristics to be established for different loops. The different loops can then be individually optimized for stability and response time. 
     Alternatives 
     Having described one embodiment, numerous alternative embodiments or variations can be made. The description has focused primarily on using the crossover circuit  400  in a V/I circuit with one voltage-controlled loop and two current-controlled loops. However, the crossover circuit  400  can alternatively be used in a V/I circuit with one current-controlled loop and two voltage-controlled loops, as shown in FIGS. 8 and 9. With this arrangement, the V/I source can be regarded as a current source with positive and negative voltage clamps. 
     As described above, the crossover circuit  400  selects from among three error signals, for controlling three different feedback loops. It can also be used, however, to switch between two error signals, for controlling only two feedback loops. According to this embodiment, one pair of comparators and a latch of FIG. 4 can be omitted. The remaining S/R latch selects one error signal from its Q output, and the other error signal from a complement of its Q output. 
     The techniques described above are applicable to areas other than V/I sources used in automatic test equipment. In many control systems, voltage and current are used to represent a variety of physical characteristics. The crossover circuit  400  and associated methods described herein are applicable to those systems as well, to select among different feedback modes that are represented with voltages and currents. 
     As described above, the output current(s) and voltage(s) are programmable. However, this is merely an example. Any of these outputs can be fixed rather than programmable. In addition, separate DACs  112  and  114  are described for providing positive and negative programmable currents. Alternatively, one DAC could be used for one polarity, with an electronic inverter driven from the same DAC to provide the other polarity. Using an analogous arrangement, one DAC could be used to provide both voltage clamps for the embodiment of FIGS. 8 and 9. 
     As described above, one threshold voltage V OS−  is supplied to comparators  432  and  434 , and another threshold voltage V OS+  is supplied to comparators  436  and  438 . Alternatively, each comparator could receive a different threshold voltage, optimized for the particular transition that the respective comparator detects. Moreover, the thresholds could be variable instead of fixed, to allow them to be varied based upon output or load conditions. As described above, the non-zero nature of the threshold voltages affords the crossover circuit  400  a type of hysteresis. Feeding back a portion of a comparator output (or the output of an S/R latch) to an input of the respective comparator could also be used to provide hysteresis. 
     As with most electronic circuits, the particular topology of the crossover circuit  400  can be varied based upon known ways of transforming digital and analog circuits, to provide a circuit having a different appearance, but which accomplishes the same results. 
     Each of these alternatives and variations, as well as others, has been contemplated by the inventors and is intended to fall within the scope of the instant invention. It should be understood, therefore, that the foregoing description is by way of example, and the invention should be limited only by the spirit and scope of the appended claims.