Abstract:
Voltage offset of an integrated circuit amplifier is accurately determined for calibration of the device. Circuit connections are established between the integrated circuit and an external device, such as an IC handler. Internal noise gain resistors formed within the integrated circuit are connected by internal switches to the amplifier and the output voltage is measured so that a value of a voltage offset of the amplifier while the noise gain resistors are connected can be calculated. The use of internal noise gain resistors produces a voltage offset component of the output voltage that is significantly greater in magnitude than effects of thermocouple voltages generated by circuit junction connections.

Description:
TECHNICAL FIELD 
   The present disclosure relates to the calibration of integrated circuit amplifiers, more particularly to the accurate measurement of voltage offset for very low offset operational amplifiers to obtain improved calibration accuracy. 
   BACKGROUND 
   Precision integrated circuit amplifiers require a calibration process to null out inherent voltage offset for proper operation. Several common methods of voltage offset calibration can be performed by high volume test systems. Typically, a manual adjustment to the amplifier&#39;s input stage is performed and ultimately made permanent. A known predictable imbalance is introduced to the differential input stage, which counteracts the effect of voltage offset error V OS . One method of amplifier voltage offset adjustment is described by George Erdi, “A Precision Trim Technique for Monolithic Analog Circuits,” IEEE Journal of Solid-State Circuits, Vol. SC-10, No. 6, December 1975. Selective shorting of Zener diodes adjusts for imbalances in the differential resistive load seen by the input stage differential pair. Additional circuitry must be included in the input stage which, when activated, will produce a known predictable imbalance to counteract and cancel the effect of V OS . Another method of introducing known predictable input stage imbalance is to purposely skew the W/L ratio of the differential input pair by means of CMOS switching the W/L ratio. In yet another known method, one can access an on-the-same-die integrated circuit digital to analog converter (DAC) to steer current and control a degree of DC current imbalance to the differential input pair&#39;s load. Since opamp input differential pairs steer the tail (bias) current to a differential load, a digital word in the DAC can counter balance the effect of V OS . 
   Common test environments for measuring voltage offset of an operational amplifier embedded in an integrated circuit generally involves an integrated circuit handler with external relays to configure the amplifier for noise gains in excess of 100. Very low offset and offset drift amplifiers, for example less than 1 mv of voltage offset, present difficulties in obtaining an accurate measurement of V OS  due to thermoelectric voltage error sources in high volume test environments. Thermocouple voltages, produced by the known Seebeck effect, exist wherever two different conductive materials are joined in series, and two junctions of the two conductive materials are at different temperatures. As dissimilar materials exist throughout the testing system, thermocouple voltages are generated that would not be generated during subsequent normal operation of the amplifier with the testing system detached. Each dissimilar junction contributes a thermocouple emf which varies as a function of junction temperature, making it difficult to distinguish between the amplifier voltage offset and unrelated thermocouple potentials. The handler-IC junctions can be a source of voltage offset error that can be comparable in magnitude to the voltage offset that is desired to be calibrated out. 
   A simplified diagram of a typical testing arrangement, such as described above, is depicted in  FIG. 1 . Integrated package  10  includes an operational amplifier  12  having input bond wire and package interface and package interface junctions  14  and  16  and an output bond wire and package interface and package interface junction  18 . The internal voltage offset of the operational amplifier is represented by a voltage V OS    20 . Handler junctions  22  and  24  connect to the input bond wire and package interface and package interface junctions; handler junction  26  connects to the output bond wire and package interface junction. Test relays connect resistor  28 , of value R, across the inverting and non-inverting input and resistor  30 , of value KR, between the inverting input and output, form thermocouple junctions  32  and  34  respectively. Noise gain resistors  28  and  30  are utilized during testing to establish an amplifier gain for calibrating out offset voltage. The non-inverting amplifier input is connected to ground. Each of the voltages V 14 –V 34  produced by the eight illustrated thermocouple junctions has been identified in the diagram. In the illustrated example, the actual voltage measured at V OUT  is as follows:
 
 V   OUT =( V   16   +V   24   −V   14   −V   22   +V   OS )·(1 +K )+ V   32   −K+V   34 +( V   18   +V   26 )·(1 +K )/ A   V  
 
where V OS  is the voltage offset to be measured and calibrated out, A v  is the loop gain of the IC amplifier under test, and V i  is the thermocouple voltage at the i th  junction. Because both the amplifier voltage offset (V OS ), and the various thermal voltages are both gained up by the noise gain (1+K), it is impossible to distinguish the amplifier&#39;s offset voltage contribution by measuring the amplifier&#39;s output voltage when the magnitude of the offset is the same order of magnitude as the thermocouple voltages V i .
 
   The need thus exists for a method and implementation to accurately measure, characterize, and calibrate out voltage offset of precision integrated circuit amplifiers in which thermocouple voltages are significant. 
   SUMMARY OF THE DISCLOSURE 
   The subject matter described herein fulfills the above-described needs of the prior art at least in part by providing a method of measuring for calibration that utilizes noise gain resistance formed within the integrated circuit for testing. Circuit connections are established between the integrated circuit and an external device, such as an IC handler. Internal noise gain resistors formed within the integrated circuit are connected to the amplifier and the output voltage is measured so that a value of a voltage offset of the amplifier while the noise gain resistors are connected can be calculated. By using internal noise gain resistors, a voltage offset component of the output voltage that is greater in magnitude than the effects of voltages generated by circuit junction connections can be produced. V OS  becomes the dominant contributor to V OUT  and makes it possible to distinguish amplifier offset from the accompanying contact thermocouple voltages and then to proceed with adjusting the offset voltage. The resistors form a voltage divider circuit used only during a test mode to provide an output in the test mode that is proportional to a voltage offset of the amplifier. 
   Switches within the integrated circuit are activated to connect the noise gain resistors to any of a plurality of advantageous specific circuit configurations. In a preferred implementation, a first internal switch is activated to connect a resistance of a first value between inverting and non-inverting input nodes of an operational amplifier and a second internal switch is activated to connect resistance of a second value between the inverting input node and an output node of the amplifier. With the non-inverting input of the amplifier connected to ground, the voltage output is measured, either directly or from a junction node between the second noise gain resistor and the second switch. The voltage output measurement provides an indication of amplifier offset voltage that can then be nulled out in a calibration process. A further output voltage measurement can be made with the switches deactivated to remove the noise gain resistors from circuit and with a third internal switch activated to connect the inverting input node to the output node. The difference between the two voltage measurements can be calculated to provide a highly accurate voltage offset indication. As a variation of the above described two measurement procedure, output voltage can be sensed with the noise gain resistors connected while the non-inverting amplifier input node is alternatively connected to positive and negative voltage sources in lieu of connection to ground. Additional internal switches are used to provide these circuit connections. After calibration, additional measurements as described above can be made to determine whether the offset adjustment has met specified requirements. If no further adjustment is necessary, the adjustment can be made permanent, for example by programming a digital code, and then permanently deactivating the switches in circuit with the internal noise gain resistors, for example by a fuse blowing lockout mechanism. 
   Additional advantages will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiments are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Implementations of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements. 
       FIG. 1  is a simplified diagram of a conventional testing arrangement. 
       FIG. 2  is a simplified diagram of an offset measurement arrangement in accordance with the present invention. 
       FIG. 3  is a simplified diagram of a variation of the offset measurement arrangement of  FIG. 2 . 
       FIG. 4  is a simplified diagram of a further variation of the offset measurement arrangement of  FIG. 2 . 
       FIG. 5  is a simplified diagram of an additional variation of the offset measurement arrangement of  FIG. 2 . 
       FIG. 6  is a simplified diagram of another variation of the offset measurement arrangement of  FIG. 2 . 
       FIG. 7  is a flow chart exemplary of the operation in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
   The simplified diagram of  FIG. 2  is illustrative of one approach for circumventing the thermocouple voltage generation problem. Noise gain resistors  28  and  30 , with respective values represented by R and KR, and switches  40  and  42  are integrated into the integrated circuit package  10 . Noise gain resistor  28  is connected at one end with the non-inverting amplifier input and at the other end with switch  40 . Switch  40  is connected to the inverting amplifier input. Noise gain resistor  30  is connected at one end to the amplifier output and at the other end with switch  42 . Switch  42  is connected to switch  40 . While the noise gain resistors remain permanently embedded in the integrated circuit, they are connected to the operational amplifier  12  only when integrated circuit switches  40  and  42  are activated. Switches are activated to the closed state only during a calibration process to access noise gain and are disabled to the open state otherwise. Thermocouple generated voltages can be produced at input bond wire and package interface junctions  14  and  16 , output bond wire and package interface junction  18 , and handler junction connections  22 ,  24  and  26 . 
   During the closed state of switches  40  and  42  V OUT  can be calculated as follows:
 
 V   OUT =( V   OS )·(1 +K )+ V   16   +V   24   +V   18   +V   26 .
 
Thus only the amplifier voltage offset is gained up by the noise gain factor (1+K). When K is large, V OS  becomes the dominant contributor to V OUT  and also makes it possible to distinguish amplifier offset from the accompanying contact thermocouple voltages, and then to proceed with adjusting for the offset voltage. The particular circuit connection of switch  40 , i.e., between the inverting input of the amplifier and the node between resistor  28  and switch  42 , eliminates contribution of the ON resistance of switch  40  to noise gain error.
 
     FIG. 3  depicts a variation of the arrangement of  FIG. 2  wherein noise gain resistor  28  is connected in series with switch  40  across both amplifier inputs and switch  42  is connected to noise gain resistor  28  through switch  40 . Switch  40  can be scaled such that its W/L size is 1/K times the size of switch  2  to produce an accurate noise gain of (1+K). 
     FIG. 4  depicts another variation of  FIG. 2  and differs therefrom in the provision of amplifier output access from a node  19  that joins noise gain resistor  30  and switch  42 . Node  19  is connected to bond wire and package interface junction  21  which, in turn, can be connected via a separate pin to the handler through junction  23 . Access from node  19  occurs only during a calibration process. A more accurate noise gain can be achieved by sensing around the switch  42 , thereby removing any switch  420 N resistance error to obtain a more accurate measurement of V OS . 
     FIG. 5  depicts another measurement arrangement. All elements of  FIG. 2  are arranged in the same configuration in  FIG. 5 , with the addition of integrated switch  44 . Switch  44  is connected between the inverting input of the amplifier and the amplifier output. Calculation of V OS  is made after taking two V OUT  measurements. A first measurement is taken with only switches  40  and  42  closed.
   V   OUT1 =( V   OS )·(1 +K )+ V   16   +V   24   +V   18   +V   26    
A second measurement is taken only with switch  44  closed.
   V   OUT2 =( V   OS )+ V   16   +V   24   +V   18   +V   26    
Subtracting the first measurement from the second measurement yields
   V   OUT1   −V   OUT2 =( V   OS )· K    
Thus the offset voltage V OS  can be obtained from these measurements completely independent of the thermocouple voltages throughout the system.
 
     FIG. 6  depicts a variation of the arrangement of  FIG. 5  and differs therefrom in the following respects. An integrated circuit switch is inserted between the non-inverting input to the amplifier and the bond wire and package interface junction  16 . An additional pin is provided at bond wire and package interface junction  25 . Integrated switch  48  is connected between the non-inverting input of the amplifier and the bond wire and package interface junction  25 . The bond wire and package interface junction  25  can be connected to the handler at junction  27 . 
   The additional circuit provisions permit a measurement of actual noise gain before calculating voltage offset. Such measurement eliminates effects occurring from deviation of the actual noise gain factor (1+K ACTUAL ) from the ideal noise gain factor (1+K). Switch  46  is always in an ON state except when the noise gain is measured. To measure the noise gain, switch  46  is set to the OFF state and switch  48  is set to the ON state. With switch  40  and  42  set to ON and switch  44  set to OFF, voltages V T1  and V T2  from source  48 , and preferably of opposite polarity, are successively applied to the non-inverting amplifier input via switch  48 . The following calculations can then be made:
 
 V   OUT1 =( V   T1   +V   25   +V   27   −V   OS )·(1 +K   ACTUAL )− K   ACTUAL ·( V   24   +V   16 )+ V   18   +V   26  
 
 V   OUT2 =( V   T2   +V   25   +V   27   −V   OS )−(1 +K   ACTUAL )− K   ACTUAL ·( V   24   +V   16 )+ V   18   +V   26  
 
Subtraction of V OUT2  from V OUT1  yields the noise gain factor (1+K ACTUAL ) as a function of four measured variables:
 
 V   OUT1 −V OUT2 =( V   T1   −V   T2 )·(1 +K   ACTUAL ), or
 
(1 +K   ACTUAL )=( V   OUT1   −V   OUT2 )/( V   T1   −V   T2 )
 
Further measurements can be taken as described for the arrangement of  FIG. 5  for an accurate calibration process.
 
     FIG. 7  is a simplified flow chart of the measurement process for the IC handler calibration device. At step  50 , the handler is connected to the appropriate pins of the integrated circuit package. In the arrangement of  FIGS. 2 ,  3  and  5 , connection is made to two input pins and a single output pin. In the arrangement of  FIG. 4 , connection is made to two input pins and two output pins. In the arrangement of  FIG. 6 , connection is made to three input pins and a single output pin. Ground connection is made as shown in each figure. 
   At step  52 , the internal integrated switches  40  and  42  are concurrently activated to the ON state and the output measured at step  54 . For the arrangements of  FIGS. 5 and 6 , steps  52  and  54  also comprise activation of switch  44  to the ON state and output measurement, while switches  40  and  42  are inactive. In addition, in the arrangement of  FIG. 6 , two further measurements are made with switches  40 ,  42  and  48  active and switches  44  and  46  inactive. The two additional measurements are taken with voltages V T1  or V T2  applied to the third input pin. If an unacceptable output measurement has been determined in step  56 , all internal switches are deactivated in step  58 , except for switch  46  of  FIG. 6 , which is maintained active in the ON state. 
   In step  60 , the amplifier voltage offset is calculated based the measurements taken in accordance with the equations specified above and the amplifier is calibrated to null the offset V OS . Calibration may be performed in any of the conventional methods earlier described. For example, an auxiliary differential input stage, not accessible to a user, may be used to steer a small current imbalance to the same load seen by the input differential pair to counteract the effect of V OS . The differential voltage seen by the auxiliary pair may be controlled and adjusted by a voltage digital to analog converter whose voltage output is controlled in a known, predictable way by a stable voltage reference, a binary resistor divider string, and integrated CMOS switches that select the tap in the voltage divider string as a function of the programmed digital word. The digital to analog converter may be programmed by means of a digital serial interface. The digital word controls the digital to analog converter to set the voltage at the input of the auxiliary differential pair, which steers the degree of current imbalance to the main differential input pair load. 
   After calibration, the internal switches again are activated in step  61  and the amplifier output is measured at step  62  to determine whether the offset V OS  is within specified limits. If the offset does not meet specification, as determined in step  64 , the process flow reverts to step  60  so that calibration adjustment is performed. For the exemplified calibration method discussed above, another digital to analog converter code can be tried until the auxiliary differential pair produces enough imbalance to null out the voltage offset. When the measured output of step  62  is found acceptable, as determined in step  64 , the process reverts to step  52  to confirm this result. The internal switches again are appropriately activated and the output measured. After determination at step  56  that the output meets specification, the handler is disconnected from the integrated circuit and internal switches are permanently disabled at step  66 . Switches  40 ,  42 ,  44  and  48 , and integrated die resistors  28  and  30  are permanently locked out, for example, by a polysilicon fuse blowing mechanism and are made inaccessible to the user. Access to a digital to analog converter, if used for the calibration process, may be made inaccessible in a similar manner. 
   In this disclosure there are shown and described only preferred embodiments of the invention and but a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example the measurement process can be performed either at wafer sort or at final post package test.