Abstract:
A method, an algorithm, and circuits for implementation of a high-accuracy voltage divider are described that include a capability of fault detection. The disclosure allows for correction of non-catastrophic faults, such as significant changes of the components&#39; values. The performance of the circuit built as described is vastly superior to operations achievable with the modern-day components utilized in previous standard and known configurations.

Description:
BACKGROUND 
       [0001]    One of the commonly utilized circuits in any electronic device is a voltage divider shown in  FIG. 1 . An input voltage V IN  can be scaled according to the ratio of resistors R 1  and R 2 ; one is able to calculate the output voltage of the divider by well-known formula in  FIG. 1 . 
         [0002]    Two familiar applications for the voltage divider are depicted in  FIG. 2 . 
         [0003]    Shown in (a) is the typical circuit for the voltage regulator; the voltage divider consisting of R FB1  and R FB2  generates a feedback voltage V FEEDBACK  that is used to regulate the output voltage V OUT . The alert reader will immediately recognize that any faults within the voltage divider will produce an abnormal output voltage from the regulator; this may lead to destruction of the whole device that incorporates this voltage regulator, as well as to smoke and fire. 
         [0004]    For example, if resistor R FB2  is opened (due to overheating, failure of the solder, or any other reason), the voltage regulator circuit will assume that the output is low, near 0 V, and will try to increase it uncontrollably. 
         [0005]    Therefore, it is typical that any circuit that may produce dangerous output in case of the divider fault includes some redundancy or an independent control mechanism to limit the maximum output. A characteristic solution for the voltage regulator is to utilize two voltage dividers, with the circuit inside of the voltage regulator being able to select the highest feedback voltage, and thus limit the output; alternatively, a separate over-voltage protection circuit is employed. 
         [0006]    Illustrated in  FIG. 2  ( b ) is a so-called Instrumentation Amplifier (IA), a circuit that is able to produce at the output an amplified difference between the two input signals. 
         [0007]    An important characteristic of the IA is the Common Mode Rejection Ratio (CMRR), an ability to reject changes in input voltages that are common to both inputs, while the difference between the inputs gets amplified and goes though unimpeded. 
         [0008]    With the common-day state-of-the-art components it is possible to create an IA circuit of this configuration that boasts a CMRR value of, perhaps, 20-48 dB (utilizing the 0.1% accurate resistors, that are the best available for practical use). Any further improvement of CMRR is achieved by manually trimming the value of one or several resistors. 
         [0009]    However, even a perfectly adjusted IA will tend to lose the CMRR value when operated over some temperature range, and specifically when operated at a temperature other than at the temperature at which the adjustments were made. This is due to resistors having various temperatures and/or various temperature coefficients. It will be appreciated that the best possible practical performance for this type of the circuit is on the order of 1% or worse. It is only with strict laboratory conditions and very expensive high-accuracy resistors that better performance is achievable. 
         [0010]    On the other hand, the IA constructed according to  FIG. 2  ( b ) is able safely to sense voltages that are many times higher than the supply voltage of the Operational Amplifier, which is a very desirable property. 
         [0011]    In a typical system, it is likely that an Analog-to-digital converter will follow the IA, and the IA output will thus be converted to a digital value. Also it is likely that in a modern-day system a microcontroller will make sense of and act upon the values received from the IA. 
         [0012]    Furthermore, present-day analog-to-digital converters can have an accuracy and resolution that is many times better than the best-possible performance available from the IA circuit in  FIG. 2  ( b ). 
     
    
     
       DESCRIPTIONS OF THE DRAWINGS 
         [0013]    This invention will be described with respect to several drawings, of which: 
           [0014]      FIGS. 1 and 2  depict prior-art circuits, 
           [0015]      FIG. 3  presents an exemplary circuit making use of the invention; 
           [0016]      FIG. 4  describes a method and algorithm, and shows formulas for calculations; 
           [0017]      FIG. 5  demonstrates a multiple-input Instrumentation Amplifier created according to the invention; and 
           [0018]      FIG. 6  illustrates a battery created with series-connected cells; the circuit in  FIG. 5  is superbly suitable for measurements of all the voltages in such a battery. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The current invention teaches a method, an algorithm, and circuits for implementation of a high-accuracy voltage divider that includes a capability of fault detection. 
         [0020]    Presented in  FIG. 3  is the exemplary circuit for the current invention. In addition to R 1  and R 2  that were part of the original circuit for the voltage divider of  FIG. 1 , there are several components (resistors and switches) that make possible the new properties of high accuracy and fault detection. 
         [0021]    Also present is the Analog-to-digital converter that is typically a part of any contemporary electronic device; the closing and opening of the switches would most likely be governed by a microcontroller that is also a typical part of modern-day devices. 
         [0022]    If several measurements are performed according to an algorithm described in  FIG. 4(   a ), an unknown voltage V x  can be calculated from the formula presented in  FIG. 4  ( b ). As will be appreciated, the calculated value of the unknown voltage V x  is independent of the values of the resistors. 
         [0023]    In other words, if one or all resistors changed their values (due to temperature, ageing, mechanical stress, or any other effects), the calculated value of the unknown voltage V x  would still be accurate. 
         [0024]    Using similar formulas (not shown for clarity, but readily derivable by the alert reader), the individual values for all resistors and/or ratios between the resistors can be calculated; deviations in excess of preset limits will be promptly recognized by a microcontroller, and can be reported as a detected fault. 
         [0025]    In addition, such deviations will prompt the microcontroller to behave in a specific way that will place the system in a known and safe state. Such behavior may include (but is not limited to) shutting down the supply voltages, disconnecting the loads, and preventing the human operator from using a faulty device. 
         [0026]    A special case that allows for simplification of the formula is shown in  FIG. 4  ( c ); a necessary condition for this simplification is that R 1 =R 3 . However, the system may still periodically check that condition R 1 =R 3  is in fact precise to the desired accuracy. 
         [0027]    Describing individual steps within the algorithm of  FIG. 4  ( a ):
       In Step #1 switch SW 2  is closed and measurement V 1  is obtained (as a digitally coded result from ADC 1 ); the expected value is shown under heading “ADC 1  V in ” in the table  FIG. 4(   a ).   In Step #2 switch SW 1  is closed and measurement V 2  is obtained.   In Step #3 switches SW 1  and SW 3  are closed and measurement V 3  is obtained.   And finally, in Step #4 switches SW 1  and SW 2  are closed and measurement V 4  is obtained.       
 
         [0032]    Substituting the above measured values into formula  FIG. 4  ( b ) or ( c ), one can find the unknown value V x . 
         [0033]    The exact order of the above four (4) steps is not important, they may in fact be executed in whatever sequence is conductive to and coherent with other system processes. 
         [0034]    The accuracy of the calculated voltage V x  depends only on the accuracy of the Analog-to-digital converter. As described above, the typical A/D unit in a modern device is routinely many times more accurate than the accuracy of an unaided resistive voltage divider. 
         [0035]    Described in  FIG. 5  is a multiple-input (e.g. multiplexed) Instrumentation Amplifier that utilizes voltage dividers according to the invention. 
         [0036]    An alert reader will recognize that the circuit in  FIG. 5  includes two independent voltage divider channels, each similar to the depiction in  FIG. 3 . 
         [0037]    A microcontroller  50  has internal circuits  51 ,  52 ,  55 , and  56 . 
         [0038]    Circuits  51  and  52  are ports typically used for digital input/output, but utilized here instead of switches SW 2  and SW 3  of  FIG. 3 , with exactly the same functionality. 
         [0039]    Circuit  56 , with the aid of circuit  55 , corresponds to an Analog-to-digital converter with multiplexed input; alternatively, and with better performance, a microcontroller with dual on-board A/D converters can be used. 
         [0040]    Alternatively, circuit  55  could be eliminated and nodes  58  and  59  combined, thus permitting the elimination of some other system elements such as driver  52  and resistors R 3b  and R 1b . 
         [0041]    Digital drivers  53  control the voltage on the gates of n-channel MOSFET switches  57 ; only one MOSFET switch connected to R 1a /R 3a  and one MOSFET switch connected to R 1b /R 3b  should be activated (turned On) at the same time. 
         [0042]    It will be appreciated that the gate voltage on the MOSFET switch should be sufficiently large in relation to the voltage on lines  58  and  59 , in order to fully enhance (turn On) the corresponding switch. 
         [0043]    When the MOSFET switch is fully turned On, it still has some residual resistance; however, the action of the algorithm will accommodate this additional resistance, as it will simply manifest itself as slight increase of corresponding resistors R 2-n ; as described above, the algorithm is not sensitive to the actual value of all resistors. 
         [0044]    It will likewise be appreciated that there are obvious limitations, namely, the normal operating input range of the A/D converter  56  should not be violated as the result of the resistance changes. 
         [0045]    The configuration of  FIG. 5  is suited for measurements of voltages that are positive in respect to ground; obvious steps can be taken in order to make this circuit operational with negative voltages as well as with voltages that are both positive and negative in respect to ground; such steps may include utilization of p-channel MOSFETs, or pairs of n-channel or p-channel MOSFETs in order to make all switches bi-directional-blocking 
         [0046]    After the two unknown voltages are calculated (one in each channel of measurements), the difference between the two voltages is obtained digitally, inside microcontroller  50 . Then, the difference voltage can be used in the same manner as if it had been obtained from an actual old-configuration IA of  FIG. 2(   b ). 
         [0047]      FIG. 6  shows one system that is well suited for measurements with the circuit in  FIG. 5 . A battery  60  is created by serially connecting cells  63  through  68 . As a part of the control system for such a battery, it will be necessary to measure voltages V 1  through V N . A circuit such as in  FIG. 5  is ideally suited for the job. 
         [0048]    It is noted that the examples given above refer to a “ground” and to a V REF  that is positive relative to ground. Such designations are, of course, completely arbitrary and are employed merely for economy of description. Every circuit portrayed here could be just as well set up with opposite sense, for example with V REF  being negative relative to ground. 
         [0049]    The alert reader, having learned the teachings given herein, will have no difficulty devising myriad obvious improvements and variants of the invention, all of which are intended to be encompassed within the scope of the claims below.