Patent Publication Number: US-8994356-B2

Title: Method for adjusting a reference voltage based on a band-gap circuit

Description:
This application claims priority from European Patent Application No. 11177618.3 filed Aug. 16, 2011, the entire disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention concerns a method for adjusting a reference voltage of an electronic circuit provided with a band-gap stage. 
     The invention also concerns an electronic circuit for implementing this method for adjusting a reference voltage. 
     BACKGROUND OF THE INVENTION 
     Making electronic circuits including a band-gap stage to provide a reference voltage is generally well known. This reference voltage must, in principle, be regulated to be independent of temperature. 
     As shown in  FIG. 1 , this type of band-gap electronic circuit  1  is formed of a diode, such as a bipolar transistor N 1  in a diode arrangement traversed by a continuous current Ic generated by a current source Sc to define a diode voltage V BE . Generally, this diode voltage V BE  decreases with an increase in temperature, and conversely increases with a decrease in temperature. Current source Sc and the diode-connected bipolar transistor N 1  are series-connected between two terminals of a continuous supply voltage. 
     Since diode voltage V BE  varies inversely with temperature variation, there is also provided a generator  2  of a voltage K·U T , wherein voltage K·U T  varies inversely with diode voltage V BE . This voltage K·U T  is added in an adder  3  to the diode voltage to supply a reference voltage V REF , which is equal to V BE +K·U T . Factor K is thus adapted to obtain a reference voltage V REF  which is independent of temperature. To achieve this, dV BE /dT must be equal to −K·dU T /dT. Reference voltage V REF , which may be a band-gap voltage, has a value substantially equal to 1.22 volts at 0° C. The thermodynamic voltage U T , which is equal to k·T/q, has a value of around 23.5 mV at 0° C., where k is the Boltzmann constant, T is the Kelvin temperature and q is the charge of an electron in absolute value. 
     Generally, for the type of band-gap electronic circuit shown in  FIG. 1 , a default value of factor K is set when the electronic circuit is designed in order to have a temperature-independent reference voltage V REF . This factor K affects the absolute reference voltage and first order temperature dependence. During adjustment of the absolute value of the reference voltage, the factor K variation also affects temperature stability. Since the method for manufacturing this type of electronic circuit may vary for adjusting the reference voltage, this may result in non-optimum temperature stability. This leads to variation from one electronic circuit to another with a reference voltage that is not entirely temperature-independent, which is a drawback. 
     US Patent Application No. 2006/0043957 A1, which discloses an electronic circuit of this type, provided with a band-gap stage, may be cited in this regard. This Patent Application discloses a way of adjusting the temperature coefficient. To achieve this, voltage measurements are taken at different temperatures in order to calculate the slope and thus adjust the reference voltage generated. This band-gap stage therefore supplies a precise reference voltage following different temperature coefficient adjustment measurements. However, the adjustment method requires several measuring steps in order to extract the precise reference voltage adjustment parameters, which is a drawback. Moreover, the reference voltage adjustment is highly dependent on variations in the electronic circuit manufacturing parameters, which is another drawback. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to overcome the drawbacks of the prior art by providing a method for adjusting a reference voltage based on an electronic circuit provided with a band-gap stage, which is simple to implement. The method easily adjusts the generated reference voltage independently of variations in the manufacturing parameters of said electronic circuit and removes first order temperature dependence. 
     The invention therefore concerns a method for adjusting a reference voltage of an electronic circuit provided with a band-gap stage, wherein the band-gap stage includes in a series arrangement between two terminals of a supply voltage source, at least one current source, a first configurable resistor and a first diode, the band-gap stage supplies a band-gap voltage, which is defined by the voltage generated by the current passing through the configurable resistor and the diode, the reference voltage being obtained based on the band-gap voltage supplied by the band-gap stage, and wherein the method includes the step consisting in:
         measuring a first band-gap voltage with a first resistor value configured by a first binary word at a first temperature selected within an operating temperature range of the electronic circuit,   measuring a second band-gap voltage with a second resistor value configured by a second binary word at the first temperature,   measuring a third band-gap voltage with the first resistor value configured by the first binary word at a second temperature which is different from the first temperature and within the operating temperature range of the electronic circuit,   measuring a fourth band-gap voltage with the second resistor value configured by the second binary word at the second temperature, and   determining an appropriate binary word for configuring the configurable resistor based on the four measured band-gap voltage values, so as to obtain a band-gap voltage that is independent of temperature variation.       

     The invention concerns also a method for adjusting a reference voltage of an electronic circuit, which is provided with a band-gap stage, wherein the band-gap stage includes in a series arrangement between two terminals of a supply voltage source of at least one supply voltage source, at least one current source, a first configurable resistor and a first diode, the band-gap stage supplies a band-gap voltage, which is defined by the voltage generated by the current passing through the configurable resistor and the diode, the reference voltage being obtained based on the band-gap voltage supplied by the band-gap stage, and wherein the method includes the step consisting in:
         measuring a first band-gap voltage with a first resistor value configured by a first binary word at a first temperature selected within an operating temperature range of the electronic circuit,   measuring a second band-gap voltage with the first resistor value configured by the first binary word at a second temperature which is different from the first temperature and within the operating temperature range of the electronic circuit,   measuring a third band-gap voltage with a second resistor value configured by a second binary word at the first temperature,   measuring a fourth band-gap voltage with the second resistor value configured by the second binary word at the second temperature, and   determining an appropriate binary word for configuring the configurable resistor based on the four measured band-gap voltage values, so as to obtain a band-gap voltage that is independent of temperature variation.       

     One advantage of the method for adjusting a reference voltage according to the invention lies in the fact that a band-gap voltage is measured at two different temperatures for two resistor values trimmed by two binary words. The appropriate binary calibration word of one or two configurable resistors of the band-gap stage is determined based on four band-gap voltage values to obtain a temperature-independent band-gap voltage. 
     Another advantage of the reference voltage adjusting method is that the reference voltage level may thus be precisely adjusted in a second step based on the adjusted band-gap voltage. The reference voltage adapted to the desired level is also independent of any temperature variation. 
     The invention therefore also concerns an electronic circuit provided with a band-gap stage for implementing the reference voltage adjustment method, wherein the reference voltage is obtained based on a band-gap voltage supplied by a first band-gap stage, wherein the first band-gap stage includes in a series arrangement between two terminals of a supply voltage source, a current source connected to a first branch, which includes a first configurable resistor in series with a first diode, and to a second branch, which includes a second configurable resistor connected to a complementary resistor in series with a second diode, the band-gap voltage being supplied to a connection node between the current source and each branch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, advantages and features of the reference voltage adjustment method and the electronic circuit for implementing the same will appear more clearly in the following description made with reference to at least one non-limiting embodiment, illustrated by the drawings, in which: 
         FIG. 1 , cited above, shows a simplified view of an electronic band-gap circuit of the state of the art, 
         FIG. 2  shows an embodiment of an electronic circuit provided with a band-gap stage for implementing the method for adjusting a temperature-independent reference voltage in accordance with the invention, and 
         FIG. 3  shows a graph representing the variation with temperature of the voltage supplied by the electronic circuit band-gap stage with respect to implementation of the reference voltage adjustment method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, all those elements of the electronic circuit for implementing the reference voltage adjustment method, which are well known to those skilled in the art in this technical field, will only be described in a simplified manner. 
       FIG. 2  shows an embodiment of the electronic circuit, which includes at least a first band-gap stage  11  for supplying a band-gap voltage V 1  and a second stage  12  for adapting the reference voltage V REF  based on band-gap voltage V 1 . In a first step of the adjustment method, band-gap voltage V 1  is adjusted to be independent of any temperature variation. In a second step of the adjusting method, reference voltage V REF  may be adapted to a desired level for powering other electronic components. However, band-gap voltage V 1  may also be used as reference voltage for other electronic components. This reference voltage does not vary with temperature if the band-gap voltage has been properly adjusted in the first stage and in accordance with the adjustment method of the present invention, as explained below. 
     A simple configuration of the electronic circuit with the first band-gap stage may include at least one current source P 1 , a resistor R 1   a  which can be configured by a binary word M 1  and a diode element, such as a diode-connected bipolar transistor N 1 . The current source, resistor and junction diode are series-connected between two terminals of a supply voltage source which is not shown. Current source P 1  is preferably connected to the high potential terminal of the supply voltage source, whereas diode N 1  is preferably connected to the low potential terminal of the supply voltage source. Band-gap voltage V 1 , which may define, in this case, a reference voltage, is thus supplied to the connection node between the current source P 1  and the configurable resistor R 1   a.  However, this band-gap voltage may also be supplied to the connection node between current source P 1  and diode N 1 , if the configurable resistor R 1   a  is directly connected to the low potential terminal of the voltage supply source. This band-gap voltage V 1  is thus the addition of the diode voltage of transistor N 1  and the voltage generated by the current passing through resistor R 1   a.    
     The electronic circuit is generally formed in a semiconductor substrate, such as silicon Si or gallium arsenide GaAs. With an increase in temperature, the value of resistor R 1   a  increases, whereas diode voltage N 1  decreases, and the reverse occurs with a decrease in temperature. The binary word M 1  must therefore be determined so that the band-gap voltage V 1  output by the first stage  11  is independent of temperature variation. As explained below, particularly with reference to  FIG. 3 , the method for adjusting a reference voltage determines the appropriate binary word M 1  for configuring resistor R 1   a . The method for adjusting a reference voltage or band-gap voltage V 1  eliminates first order temperature dependence by adapting factor K, as briefly explained with reference to  FIG. 1 . 
     According to the adjustment method of the present invention, band-gap voltage V 1  must be measured at a first temperature T 1  and at a second temperature T 2  within a temperature range that allows the electronic circuit to operate. This temperature range may, for example, be between −40° C. and at least 85° C. depending on the technology used for integrating the electronic circuit. For example, a first temperature T 1  of 0° C. and a second temperature T 2  of 60° C. may be selected, but other temperatures may also be selected for the adjustment method of the invention. 
     Preferably, the two measuring temperatures T 1  and T 2  may be selected on either side of a median temperature value within the electronic circuit operating temperature range. This also minimises the second order effects. The two temperatures must also be sufficiently far apart without approaching the limits of the temperature range to avoid amplifying measurement imprecisions. 
     Band-gap voltage V 1  is measured at two temperatures at a first resistor value R 1   a  and a second resistor value. Two first band-gap voltage values V 1  are advantageously measured at the first temperature T 1  for the two resistor values R 1   a  configured in succession by the two binary words M 1 . Next, two second band-gap voltage values V 1  are measured at the second temperature T 2  for the two resistor values R 1   a  configured in succession by the two binary words M 1 . The four band-gap voltage values may be stored in storage means in a microprocessor unit, which may be integrated in the same integrated circuit as the electronic circuit or simply be connected to the electronic circuit. 
     The two band-gap voltage values V 1  of the two resistor values at first temperature T 1  may also be stored in a test file during production. This file may be reused when the two band-gap voltage values are tested at the second temperature T 2  for the final calculation of factor K. The production test stores the results of the measurement of the two band-gap voltage values at the first temperature associated with each circuit. In these conditions, it is not necessary for the electronic circuit to have a non-volatile memory. 
     In a variant of the method, two values of band-gap voltage V 1  may be measured with the first resistor value R 1   a  configured by a first binary word M 1 , at the two measuring temperatures T 1  and T 2 . Next, two other band-gap voltage values V 1  may also be measured with the second resistor value R 1   a  configured by a second binary word M 1  at the two temperatures T 1  and T 2 . The four band-gap voltage values V 1  may be stored in the storage means of the microprocessor unit. 
     Based on the four stored band-gap voltage values V 1 , it is immediately possible to calculate the binary word required for configuring said resistor R 1   a . Once resistor R 1   a  is configured by the appropriate binary word M 1 , the band-gap voltage V 1  is independent of any temperature variation. This allows the first order temperature stability to be adjusted. The binary configuration word M 1  for the configurable resistors may be a binary word of at least 4 bits, and preferably 7 or more bits. The current I supplied by the current source may also be adapted as a function of the band-gap voltage value to obtain a determined band-gap voltage level V 1  that takes account of the value of the configured resistor R 1   a.    
     It should also be noted that the variation slopes of the band-gap voltage can be determined for the two values of resistor R 1   a  configured by the two different binary words M 1 , to determine the appropriate binary word M 1 . However, in this case, the equations must take account of the measuring temperature values, which complicates the reference voltage adjustment method. Moreover, the same binary word is always obtained for identical slopes of every measured electronic circuit, which means that advantage cannot be taken of proper temperature adaptation. 
     Next, in a second step, the reference voltage V REF  can be adapted in the second stage  12  of the electronic circuit. This reference voltage V REF  can be precisely adjusted to a higher value or a lower value for example around 0.8 volts, or also to an identical value to that of band-gap voltage V 1 , as explained in more detail below. Since the band-gap voltage adapted in the first stage  11  of the electronic circuit may be different from one circuit to another in the same integrated circuit wafer or in different integrated circuit wafers, the desired reference voltage has to be adapted in the second stage  12 . 
     In a more complete configuration illustrated in  FIG. 2 , the first band-gap stage  11  is first of all formed of a current source P 1 , which is made by means of a PMOS transistor P 1 . The source of PMOS transistor P 1  is connected to a high potential terminal of a supply voltage source (not shown), whereas the drain is connected to a first configurable resistor R 1   a  and to a second configurable resistor R 1   b . In order to make PMOS transistor P 1  conductive, the gate of said PMOS transistor P 1  is controlled by an output voltage of a first operational amplifier A 1  of a current control loop. Thus a controlled current I is supplied by said PMOS transistor P 1  to the first and second configurable resistors R 1   a  and R 1   b . A first current I a  passes through first resistor R 1   a , whereas a second current I b  passes through second resistor R 1   b . The band-gap voltage V 1  output by first stage  11  is defined at the connection node between PMOS transistor P 1  and each configurable resistor R 1   a  and R 1   b.    
     In a first branch, the first resistor R 1   a  is connected on one side to the drain of PMOS transistor P 1  and on the other side to a first diode, which is preferably a first diode-connected bipolar transistor N 1 . This first diode-connected transistor N 1  is formed of n elementary bipolar transistors. This first bipolar transistor may be a PNP transistor with the base and collector connected to the low potential terminal of the supply voltage source. Thus PMOS transistor P 1 , first resistor R 1   a  and the first diode-connected bipolar transistor N 1  are series-connected between the terminals of the voltage supply source. 
     In a second branch, the second resistor R 1   b  is connected on one side to the drain of PMOS transistor P 1  and on the other side to a complementary resistor R 2 , which is then connected to a second diode. 
     This second diode is preferably a second diode-connected bipolar transistor N 2 . This second diode-connected transistor N 2  is formed of m elementary bipolar transistors. The second bipolar transistor may be a PNP transistor with the base and collector connected to the low potential terminal of the voltage supply source. Thus, PMOS transistor P 1 , second resistor R 1   b , complementary resistor R 2  and the second diode-connected bipolar transistor N 2  are series-connected between the terminals of the voltage supply source. 
     The number m of elementary bipolar transistors of the second branch is higher than the number n of elementary bipolar transistors of the first branch. In an advantageous embodiment of the electronic circuit, the number n of elementary bipolar transistors for diode N 1  may be chosen to be equal to 1, whereas the number m of elementary bipolar transistors of diode N 2  may be chosen to be equal to 24. This choice results from the good match required with central symmetry when the elementary transistors are placed on the integrated circuit of the electronic circuit. The elementary bipolar transistor of diode N 1  is arranged at the centre of the  24  elementary bipolar transistors of diode N 2  to give a square-shaped structure. 
     The two configurable resistors R 1   a  and R 1   b  may be similar and configured by the same binary word M 1  supplied via a configuration bus connected to the microprocessor unit. Each configurable resistor may be formed in series of a base resistor and an array of resistors. The resistors of the array may each be short-circuited by means of a respective switch actuated by a respective bit of binary word M 1 . The values of one part of the resistors of the array may be weighted by the power of 2 or each have the same value, for example selected between 15 and 20 kOhm. Preferably, each configurable resistor may vary from 1.8 MOhm (base resistor) to 4.03 MOhm. The default value of each configurable resistor, which is adjusted for example to the design, may be set at 2.94 MOhm. The complementary resistor R 2  may have a set value on the order of 420 kOhm. Of course, other resistor values may be provided to obtain a band-gap voltage V 1  on the order of 1.22 volts at 0° C. 
     It should be noted that instead of the first and second PNP diode-connected bipolar transistors N 1  and N 2 , it is possible to envisage using first and second NPN diode-connected bipolar transistors N 1  and N 2 . In this case, the emitter of each transistor is connected to the low potential terminal of the voltage supply source, whereas the base and the collector are connected to the first resistor R 1   a  for the first resistor and to the complementary resistor R 2  for the second transistor. 
     As mentioned above, current I, which is supplied by the PMOS transistor P 1  to resistors R 1   a , R 1   b  and R 2  and to diodes N 1  and N 2 , is determined in the current control loop. To achieve this, the positive input of the first operational amplifier A 1  receives a first comparison voltage value Vp at the connection node between the first configurable resistor R 1   a  and the first diode-connected PNP transistor N 1 . The negative input of the first operational amplifier A 1  receives a second comparison voltage value Vm at the connection node between the second configurable resistor R 1   b  and the complementary resistor R 2 . The output of this first operational amplifier A 1  controls the gate of the PMOS transistor P 1  so as to control current I a  passing through the first configurable resistor R 1   a  and current I b  passing through the second configurable resistor R 1   b.    
     The first stage  11 , which supplies band-gap voltage V 1 , thus adjusts the first order temperature stability. Conversely, second stage  12  enables the desired reference voltage value V REF  to be adjusted without altering temperature stability, via a simple offset adjustment, as explained in more detail below. 
     The value of band-gap voltage V 1 , which is output by first stage  11 , is defined by the following equation:
 
 V 1 =Vp+R 1 a·In ( m/n )· U   T   /R 2
 
where Vp is the diode voltage V BE  of the first diode-connected PNP transistor N 1 , which is formed of n elementary bipolar transistors. Factor K for adjusting the first order temperature stability is thus R1 a ·In(m/n)/R2.
 
     It is therefore easy to calculate factor K in order to obtain a band-gap voltage V 1  which is temperature stable by applying the equation K=(V1−Vp)/U T . It is clear that this result may vary from one electronic circuit to another with variations in the manufacturing method. Configurable resistor R 1   a  and configurable resistor R 1   b  both therefore allow adjustment of factor K. 
     As shown in  FIG. 3 , if the value of these configurable resistors varies from a minimum value to a maximum value by the i-bit binary configuration word M 1 , the variation with temperature of band-gap voltage V 1  is represented by the straight lines p b  and p m . For a maximum value of the configurable resistors, a first band-gap voltage value V 1HT1  can be measured at a first temperature T 1 , and a second band-gap voltage value V 1HT2  at a second temperature T 2 . The slope of line p m  for a maximum configurable resistor value is a positive slope, which means that the band-gap voltage increases with a temperature increase. For a minimum value of the configurable resistors, a first band-gap voltage value V 1LT1  can be measured at a first temperature T 1 , and a second band-gap voltage value V 1LT2  at a second temperature T 2 . The slope of line p b  for a minimum configurable resistor value is a negative slope, which means that the band-gap voltage decreases with a temperature increase. 
     For the factor K adjustment it is therefore simply necessary to measure two band-gap voltages V 1  values at two different temperatures. This enables the appropriate binary word M 1  for configuring resistors R 1   a  and R 1   b  of  FIG. 2  to be determined, to obtain a temperature-independent band-gap value V 1 . The temperature-independent band-gap voltage V 1  is represented by the dotted line p n  in  FIG. 3 . This line p n  is parallel to the temperature axis x. 
     In a practical case of determining the appropriate binary word, the configurable resistors are configured between the minimum and maximum values. They are configured at a first resistive value by a first binary word and at a second resistive value by a second binary word. The first resistive value may be, for example, higher than the second resistive value. The first line p 1  relating to the first resistive value is shown with a positive slope, whereas the second line p 2  is shown with a negative slope. However, it is also entirely possible for both slopes to be positive or for both slopes to be negative for determining the appropriate binary word. It is, however, imperative that the electronic circuit is devised to have a positive slope with a maximum configurable resistor value and a negative slope with a minimum configurable resistor value. This is necessary to determine the appropriate zero temperature variation binary word of the band-gap voltage. 
     A first band-gap voltage value V 11T1  can be measured at first temperature T 1  with the first resistive value of the configurable resistors. A first band-gap voltage value V 12T1  can be measured at first temperature T 1  with the second resistive value of the configurable resistors. A second band-gap voltage value V 11T2  can be measured at the second temperature T 2  with the first resistive value of the configurable resistors. Finally, a second band-gap voltage value V 12T2  can be measured at the second temperature T 2  with the second resistive value of the configurable resistors. The four band-gap voltage values are stored in storage means of the microprocessor unit for determining the appropriate binary word. 
     The appropriate i-bit binary word M 1  for configuring resistors R 1   a  and R 1   b  is thus given by the following equation:
 
 M 1 [i− 1:0]=(2 i −1)·( V   12T1   −V   12T2 )/( V   11T2   −V   12T2   −V   11T1   +V   12T1 )
 
     It should be noted that the aforementioned formula relies on very good differential non-linearity (DNL) and very good integral non-linearity (INL). Between the values V 1HT1  and V 1LT1 , and between values V 1HT2  and V 1LT2 , all the adjustment steps (LSB) must, if possible, be equal to each other. If the function V1=f(M1) is not linear, the above formula must, in principle, be adapted to this non-linearity. 
     Differential non-linearity focuses on the adjustment steps. This differential non-linearity is the relation between each adjustment step and the theoretical step. For an adjustment ranging from 0 to 15 (16 steps), which is encoded on 4 bits, there is a theoretical step of 1. To characterize this means, the value of each step can be measured and compared to the theoretical result. For a theoretical step (LSB=1) of the series 0, 1, 2 up to 15, a series from 0, 1.1, 1.9, 3.2 up to 15 is measured for example. The differential non-linearity is calculated for each step, and gives DNL(0)=0, DNL(1)=(1.1−0)/LSB−1=0.1, DNL(2)=(1.9−1.1)/LSB−1=0.2, DNL(3)=(3.2−1.9)/LSB−1=0.3 and so on. Thus the differential non-linearity (DNL) of this system is the maximum absolute value between all the steps DNL(i) which are defined by the formula (f(i)−f(i−1))/LSB−1. 
     The integral non-linearity (INL) represents the accumulation of the differential non-linearity (DNL). This integral non-linearity shows the deviation of the adjustment function relative to the theoretical curve. For each step, this gives INL(0)=DNL(0)=0, INL(1)=DNL(0)+DNL(1)=0.1, INL(2)=DNL(0)+DNL(1)+DNL(2)=−0.1, INL(3)=DNL(0)+DNL(1)+DNL(2)+DNL(3)=0.2 and so on. The integral non-linearity (INL) of this system is the maximum absolute value between all the INL(i). 
     If the differential non-linearity of this type of system is poor, this means that there is typically a broad deviation in the distribution of the steps around the theoretical value. Poor integral non-linearity means that the adjustment curve is not far from the theoretical curve. On portions of the curve, the mean value of the steps is not equal to the theoretical value of the steps. This also means that on this portion, the mean value of the DNL(i) is not equal to 0. 
     For a DNL smaller than 0.5, this means that the system is monotonous and of high quality. For a DNL larger than 0.5, each step has to be analysed. For an INL smaller than 0.5, this means that the adjustment function never moves away from the theoretical curve by more than 0.5 LSB. This is a very good result. 
     For determination of the electronic circuit reference voltage, the factor K adjustment range must always be broad enough to obtain a variation slope for band-gap voltage V 1  which is always positive for Kmax and always negative for Kmin. The binary configuration word M 1  is thus minimum for Kmin and maximum for Kmax. With a minimum value Kmin, each configurable resistor R 1   a  and R 1   b  may have a value of 1.8 MOhm. However, with a maximum value Kmax, each configurable resistor R 1   a  and R 1   b  may have a value of 4.03 MOhm. 
     It should be noted that an optimum factor K does not necessarily give an optimum absolute value result. This is due in particular to variations in the electronic circuit manufacturing method. 
     With the method for adjusting the reference voltage and particularly the band-gap voltage according to the invention, it is possible to perform an immediate calculation with two pairs of band-gap voltage values to be measured. Two first band-gap voltage values are measured with the resistors configured with two different binary words at a first temperature T 1 . The two measurements are taken in very close time periods and with a stable first temperature T 1 . The junction temperature of diodes N 1 , N 2  therefore does not have time to change. Next, two second band-gap voltage values are measured with the resistors configured by the two binary words at a second temperature T 2 . Again, the two measurements are performed in very close time periods and with a stable second temperature T 2 . With this manner of determining the four band-gap voltage values, each absolute temperature value does not need to be selected very precisely. 
     As previously indicated, it is also possible to envisage calculating the temperature variation slopes of band-gap voltage V 1 . However, this measuring method requires absolute temperature precision for the first and second measuring temperatures. This measuring method is difficult to put into practice in producing electronic circuits. Thus this method does not offer any great advantage for determining the appropriate binary word in order to provide a temperature-independent band-gap voltage. 
     Set slopes may also be calculated. This only makes sense if it is not possible to store the measurement results in a file or non-volatile memory. The slope calculation takes place during the design characterization phase and is then set for all the integrated circuits according to the characterization. 
     The absolute value of the reference voltage V REF  output by the electronic circuit is adjusted by the second stage  12 . In this second stage, a second operational amplifier A 2  is arranged as a voltage follower to input band-gap voltage V 1  from first stage  11 . This voltage follower avoids affecting the adaptation of band-gap voltage V 1  in first stage  11 . A third configurable resistor R 3  is provided for lowering the voltage before the amplification unit. This third resistor R 3  is connected between the voltage follower output A 2  and the low potential terminal of the supply voltage source. The third resistor R 3  includes a low part and a high part, which may be configured by means of a second binary adaptation word M 2  supplied via an offset bus. This binary word may also be an at least 4-bit binary word, preferably of 7 or more bits. The low part of the third resistor R 3  may have a value equal to 1.66 MOhm, whereas the high part may be configured by the binary word to vary from 0 to 720 kOhm. 
     The amplification unit includes a third operational amplifier A 3 , the positive input of which is connected to an intermediate configured part of third resistor R 3 . This gain of this amplification unit is fixed by fourth and fifth resistors R 4  and R 5 . The fourth resistor R 4  is connected between the negative input and the output of the third operational amplifier A 3 . This fourth resistor may be selected with a value of 862 kOhm. The fifth resistor R 5  is connected between the negative input of the third operational amplifier and the low potential terminal of the supply voltage source. This fifth resistor R 5  may be selected with a value of 1.57 MOhm. According to this electronic circuit configuration, no voltage is defined as negative. Thus the third amplifier A 3  must be connected with a positive gain. 
     In a variant of the second stage  12  of the electronic circuit, the third operational amplifier A 3  may be arranged as a voltage follower without the fourth and fifth resistors. The high part of the third resistor R 3  may be adjusted for example to the design at a value of 363 kOhm. 
     Since band-gap voltage V 1  may have a higher value than the desired reference value V REF , the overall gain of the second stage must be smaller than 1. Band-gap voltage V 1  may be on the order of 1.22 volts, whereas reference voltage V REF  may be set at 0.8 volts. To achieve this, band-gap voltage V 1  is decreased by the resistive divider formed by the third configurable resistor R 3  prior to entering the final amplification unit with the third amplifier A 3  of the second stage. 
     The method for adjusting the reference voltage in second stage  12  may be achieved in several ways depending on the design selected for the second stage. If the differential and integral linearities of the adjustment assembly of the second stage are good (&lt;LSB), the reference voltage can be adjusted in a simple manner. A minimum value and a maximum value can be measured. Next, the binary adjustment word M 2  can be calculated, so that it is proportional to the difference between the two measurements min and max, and the desired target value. If only the differential linearity is good, the reference voltage can be adjusted by using a dichotomy method. However, if linearity is not guaranteed, a refined search must be carried out after the dichotomy method has been performed. For all the possibilities chosen for adjusting reference voltage V REF  in second stage  12 , the binary adjustment word M 2  has to be determined to configure the third resistor R 3  so as to obtain the desired target value. This binary adjustment word M 2  may of course be different from one electronic circuit to another electronic circuit, given that the stabilised band-gap voltage V 1  output by the first stage may be different from one circuit to another. 
     From the description that has just been given, several variants of the method for adjusting the reference voltage of an electronic circuit can be devised by those skilled in the art without departing from the scope of the invention defined by the claims. The current source may be connected to the low potential terminal of the supply voltage source, whereas the series arrangement of the junction diode with the configurable resistor of the first band-gap stage may be connected to the high potential terminal of the supply voltage source. The first and second configurable resistors of the first band-gap stage of the electronic circuit can each be configured separately at a different resistive value.