Abstract:
The invention relates to analog integrated electronic circuits using differential pairs. The proposal is for a method of automatic correction of offset voltage. The inputs (V 1 , V 2 ) of the differential circuit are short circuited during a calibration phase distinct from the normal usage phase. A capacitor is charged through the difference of the output currents of the branches of the differential pair in this phase. The voltage at the terminals of the capacitor is compared with at least one threshold. During the normal usage phase following the calibration phase, the result of the comparison is kept in memory. In the normal usage phase, a correction is applied depending on the result kept in memory to a current source of a follower stage upstream of the differential pair.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present Application is based on International Application No. PCT/EP2006/068688, filed on Dec. 12, 2006, which in turn corresponds to French Application No. 0512837 filed on Dec. 16, 2005, and priority is hereby claimed under 35 USC §119 based on these applications. Each of these applications are hereby incorporated by reference in their entirety into the present application. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to analog integrated electronic circuits using differential pairs making it possible to convert a difference of two input voltages into a difference of two output currents. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a differential pair, the difference of output currents depends on the difference of input voltages, and if a null potential difference is applied between the inputs, for example by short circuiting the inputs, the difference of the output currents should be null. 
         [0004]    The defects of matching the components that constitute the pair mean that the pair has an offset input voltage, more commonly called the offset voltage, that is not null; this offset voltage represents the intrinsic imbalance of the pair and is reflected by a difference of non-null output currents in the presence of a null input differential voltage. In practice, the offset voltage is the compensation voltage that needs to be applied between the inputs so that the difference of output currents is null. 
         [0005]    A differential pair conventionally comprises two branches supplied by one and the same reference current source Io, each branch comprising a transistor and a charge. The input voltages are applied to the bases of the transistors, the emitters are connected to the common current source, and the collectors are connected to the charges. The current of the source is shared between the two branches depending on the difference of input voltages applied to the bases. 
         [0006]    The differential pairs may be used individually or grouped into associations of pairs when the installations are more complex. For example, two pairs may share one and the same charge, the collector of a transistor of one pair being connected to the collector of a transistor of another pair. The pairs may be mounted in cascade, a current output of one pair being connected to a current output of another pair which itself has another current output connected to a third pair, and so on. Depending on the use of the differential pairs, it is possible to find different associations of pairs. The invention applies in general to all these uses. 
         [0007]    Such differential pairs are used notably in analog-digital converters. They are used for example to constitute comparators, each comparator comprising a differential pair receiving as an input, on the one hand, a voltage to be converted, on the other hand a reference voltage; they are also used, still for converters, in folding circuits; a folding circuit comprises several folding cells each constituted of at least one differential pair, the current outputs of the cells being connected in cascade to one another in order to establish a voltage or an analog output current that varies in a bell or in a sinusoid depending on the input voltage to be converted; the different cells each receive the input voltage and a respective reference voltage. 
         [0008]    In precision analog circuits using several differential pairs, it is realized that the lack of precision of operation (notably lack of precision of conversion in the converters) may result from the presence of non-null offset voltages; from a physical point of view, these offset voltages result above all from the fact that the emitter-base voltages of the various transistors of the pairs are not exactly identical even when identical currents pass through them. 
         [0009]    Specifically, the technologies are not perfect and two transistors manufactured simultaneously, having at least theoretically the same emitter surfaces, and even placed side by side in an integrated circuit therefore having every chance of being identical, do not have strictly identical features. This results from an inevitable dispersion of manufacturing. In addition, not only do the two transistors of a pair generate an offset voltage in this pair, but, due to this very dispersion, the various differential pairs of an integrated circuit inevitably have offset voltages that are different from one another. 
         [0010]    It has already been proposed to reduce this disadvantage by using differential pairs having larger transistors. They have less dispersion because their size is better controlled. But then, the capacitors are bigger and the circuits are therefore slower, which is not desirable in applications such as fast analog-digital converters. For the latter, it would be better to have smaller transistors in the differential pairs. In addition, the circuits are more bulky if the transistors are bigger, and they consume more current. 
         [0011]    Individual calibration solutions have also been proposed a posteriori: a manufactured converter is tested and the conversion errors are stored in an EPROM memory of the integrated circuit to be used for the compensation of the errors during use. Laser adjustment solutions also exist, notably for individually adjusting the resistances present in the differential branches. This technique requires an individual test and an individual correction of each integrated circuit depending on the conversion errors noted. The method is therefore extremely costly industrially, each circuit having to be tested and corrected individually. 
       SUMMARY OF THE INVENTION 
       [0012]    The object of the invention is to propose a solution reducing these disadvantages in order to offer a better compromise between the quality, the cost of the circuit and its consumption. 
         [0013]    The invention proposes to incorporate in the circuit that comprises a differential pair or a group of associated pairs an automatic calibration circuit associated with this pair or with this group of pairs; during an automatic calibration phase distinct from the normal usage phase, this circuit examines the value of the differential output current produced by the pair in the presence of a null input differential voltage; it compares this values with at least one threshold, and, depending on the result, it keeps in memory and may or may not apply a correction to the input of the pair during the normal usage phase of the latter. The correction is applied to a follower stage placed upstream of an input of the differential pair, in the following manner: the correction is applied to a current source which passes through a transistor of the follower stage; the base-emitter voltage will vary depending on this current and this variation is transferred to the input of the differential pair whose offset is therefore partially or totally corrected. 
         [0014]    The measurement of the differential current that results from the application of a null differential voltage to the input is preferably taken by applying successive pulses of charge and discharge currents to a capacitor during a series of timeslots during the calibration phase. The charge current is the current of one of the branches of the differential pair, the discharge current is the current of the other branch. At the end of n pulses, the capacitor is charged in proportion to the difference of current between the branches. There are preferably two symmetrical capacitors for reasons of technological construction of the capacitors. 
         [0015]    In summary, the method for automatic correction of offset according to the invention, applied to a differential integrated circuit comprising at least one differential pair with upstream at least one follower stage having a transistor and a current source, comprises the following steps:
       cancelling the differential voltages on the inputs of the differential circuit during a calibration phase distinct from the normal usage phase,   charging a capacitor through the difference of the output currents of the branches of the differential pair in this phase,   comparing the voltage at the terminals of the capacitor with at least one threshold,   during the normal usage phase following the calibration phase, keeping the result of the comparison in memory,   applying to the current source (SC 1 ) in the normal usage phase a correction depending on the result kept in memory.       
 
         [0021]    The electronic integrated circuit with differential pairs according to the invention therefore comprises at least two inputs to receive two voltages, two current outputs applied to charges, and at least one pair of differential branches connected to these charges and, upstream of the differential pair, at least one follower stage comprising a transistor and a current source connected to the emitter of this transistor; the integrated circuit is characterized in that it comprises
       a sequencer to establish an offset voltage calibration phase and a phase of usage of the circuit after calibration,   at least one capacitor associated with the differential pair,   a set of commutators actuated by the sequencer to direct the output currents to the charges during the usage phase and in order, on the one hand, to apply a null differential voltage to the inputs and, on the other hand, to alternatively direct one and then the other of the output currents to the capacitor during the calibration phase, by alternating charge and discharge, during a series of timeslots supplied by the sequencer,   a circuit for detecting voltage thresholds at the terminals of the capacitor, actuated by the sequencer at the end of the series of timeslots, this circuit supplying a status signal representing the overshooting or not overshooting of a threshold and the sign of the voltage at the terminals of the capacitors,   a circuit for keeping the status signal at the output of the detection circuit, in order to keep, during the usage phase, a status value that was determined during the calibration phase,   a circuit for modifying the current of the current source of the follower stage depending on the status value present at the output of the retention circuit.       
 
         [0028]    If there are two capacitors instead of one, the circuit comprises commutators for charging during an alternation one of the capacitors by one of the output currents of the differential pair while the other is discharged by the other output current, after which, during the next alternation, it is the first capacitor that is discharged by the current of the second output while the second capacitor is charged by the current of the first output. The capacitors are then placed in series in order to add their charges and compare the total with the threshold. 
         [0029]    Provision can be made for there to be several thresholds, in order to determine several possible corrections, or alternatively for there to be two (or more) successive cycles, a first cycle establishing a first current correction on the follower stage and a second cycle establishing an additional correction, without deleting the first correction. 
         [0030]    The invention is most particularly applicable to differential pairs that are more sophisticated than a simple differential pair with two transistors, and notably to interleaved pairs comprising four differential branches sharing a common charge two by two. In particular, the invention is applicable to an analog-digital converter which comprises a signal folding stage comprising several folding cells themselves constituted by interleaved differential pairs with four branches. 
         [0031]    Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention 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 aspects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein: 
           [0033]      FIG. 1  represents a differential pair with follower stages upstream of the inputs; 
           [0034]      FIG. 2  represents the diagram of the invention applied to the pair of  FIG. 1 ; 
           [0035]      FIG. 3  represents the differential current measurement circuit and the associated threshold detector; 
           [0036]      FIG. 4  represents a conventional constitution of a signal folding cell comprising an interleaved differential pair with four differential branches; 
           [0037]      FIG. 5  represents the application of the principle of the invention to the cell of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The general principle of the invention will first be explained concerning a simple differential pair, before it is shown how it is possible to apply it to a group of associated differential pairs, notably a group of two interleaved differential pairs that form part of a signal folding cell designed for an analog-digital converter. 
         [0039]    The diagram of  FIG. 1 , shows the principle of a simple differential pair which comprises, upstream of each input of the actual pair, a voltage follower stage which controls this input. Such installations with a follower stage are extremely conventional, the follower stage having notably the role of bringing the input voltages applied to the pair to a desired common mode level. 
         [0040]    The actual pair comprises two branches; each branch comprises a transistor, T 1  for the first branch, T 2  for the second branch, and a respective charge CH 1 , CH 2 . The input of the pair is constituted by the bases of the transistors T 1  and T 2 ; the outputs are constituted by currents  11 ,  12  in the charges. The emitters of the transistors are connected to a constant current source SC 0  supplying a current Io. The voltage on the base of T 1  is Vi 1 , the voltage on the base of T 2  is Vi 2 . The assembly is connected between a ground M and a supply voltage Vcc. 
         [0041]    Upstream of the transistor bases are the follower stages. The follower stage that controls the voltage Vi 1  comprises, in a simple configuration given as an example, a follower transistor Ts 1 , and a constant current source SC 1  connected between the emitter of Ts 1  and the ground; the collector of Ts 1  is connected to Vcc. Similarly, the follower stage that controls the base of the transistor T 2  of the other differential branch comprises a follower transistor Ts 2  and a current source SC 2  which are connected exactly like the first follower stage. The input voltages of the whole differential circuit (differential pair and follower stages) are voltages V 1  and V 2  applied to the bases of the follower transistors Ts 1  and Ts 2 . The emitters of these transistors are connected to the bases of the transistors T 1  and T 2  of the differential pair. The difference between the input voltage V 1  of the circuit and the input voltage Vi 1  of the pair is the base-emitter voltage drop of the follower transistor TS 1 . It is determined by the current of the source SC 1 . The same applies for the V 2 -Vi 1  voltage drop. The transistors T 1  and T 2  are identical; the transistors Ts 1  and Ts 2  are identical; the current sources SC 1  and SC 2  are identical. Only the technological dispersions affect this similarity. 
         [0042]      FIG. 2  represents the schematic diagram of the automatic offset correction proposed in this instance. 
         [0043]    The correction uses an automatic calibration circuit that is activated during a calibration phase, separate from the normal usage phase. For example, the calibration phase is carried out automatically on each powering-up of the circuit. A sequencer not shown produces the logic signals necessary for the running of this phase. These logic signals notably control switches visible in  FIG. 2 . 
         [0044]    First of all, the calibration circuit comprises switches that short circuit the inputs of the differential circuit during the calibration phase. For example, there is a switch K 1  connecting the base of Ts 1  to a fixed voltage reference Vr and a switch K 2  connecting the base of Ts 2  to the same voltage reference Vr, which is a way of short circuiting the inputs. During this time, the bases of the transistors Ts 1  and Ts 2  are isolated from the normal inputs of the differential circuit. 
         [0045]    Then, there is a switch K 1   a  and a switch K 2   a  which isolate the collector of T 1  or T 2  from the respective charge CH 1 , CH 2  during the calibration phase, and a switch K 1   b  and a switch K 2   b  that connect these collectors each to an input of a differential current measurement circuit MCD, the detail of which will be discussed below. This circuit produces a voltage value representative of the signed value of the differential current present when the differential input voltage is null. The output of the current measurement circuit MCD is applied to a comparison circuit CMP. 
         [0046]    The comparison circuit CMP has two outputs S 1  and S 2 ; the voltages on these outputs are status signals representing an offset correction value to be applied to the differential pair to reduce or cancel out its offset voltage. In the version that is simplest, but sufficient in certain cases, the outputs are binary outputs having only two possible states and the comparison circuit is a comparator with threshold; no correction is applied if the threshold is not exceeded; a single correction value is applied if the threshold is exceeded, either by the output S 1  or by the output S 2  depending on the sign of the voltage produced at the output of the current measurement circuit MCD. 
         [0047]    In more sophisticated versions, provision could be made for the comparison circuit to comprise a larger number of outputs in order to apply a more precise correction depending on the exceeding of several thresholds. This will be necessary for example for high resolution converters (12 bits for example). 
         [0048]    A correction by a single value may suffice if the offset to be corrected is small by nature and if the precision of the converter is limited (8-bit converter for example). The diagram of  FIG. 2  corresponds to this case: the output S 1  is binary, it remains at zero if the measured differential current remains below the threshold, and it switches to 1 if the threshold is exceeded. The same applies for S 2  but for an opposite differential current sign, therefore an opposite offset voltage sign. 
         [0049]    The output S 1  controls one of the follower stages, and the output S 2  controls the other. This control comprises the addition of an additional constant current in parallel with the current of the source SC 1  or SC 2 . The constant current is that of a low value auxiliary source SC 1   a  (or respectively SC 2   a ). The output S 1  therefore controls the placing in conduction or the blocking of a transistor T 1   a  in series with the source SC 1   a . The assembly of the source SC 1   a  and of the transistor T 1   a  is placed in parallel on the main source SC 1  of the follower stage. The source SC 1  is usually constituted by a transistor in series with a resistor. The assembly SC 1   a +T 1   a  may be placed in parallel on the resistor (preferably) or on the assembly of the source SC 1 . The installation for the other follower stage is the same, with an auxiliary source SC 2   a  and a control transistor T 2   a.    
         [0050]    The comparator CMP is constituted so as to keep in memory, after the calibration phase, the status taken via the outputs S 1  and S 2  during the calibration phase. A retention circuit is therefore provided in the comparator or inserted between the comparator and the transistors T 1   a  and T 1   b.    
         [0051]    The correction of the offset results from modifying the base-emitter voltage drop of the follower transistor Ts 1  or Ts 2 , which itself results from the passage of a larger emitter current in the transistor. If the differential current measured during the calibration phase exceeds the acceptable threshold, then an additional voltage drop on the follower transistor Ts 1  or Ts 2  is applied during the usage phases and it compensates for the offset that exists naturally in the pair. 
         [0052]    It will be noted that the overall offset of the pair may result from matching defects of the transistors T 1  and T 2 , or of the transistors Ts 1  and Ts 2  or of their current sources. 
         [0053]      FIG. 3  represents the constitution of the assembly of the differential current measurement circuit MCD and of the comparison circuit CMP. 
         [0054]      FIG. 3  shows the switches K 1   b  and K 2   b  by which the output currents of the differential pair may be brought to the circuit MCD during the calibration phase. These currents would in principle be equal if there was no offset. In the presence of an offset, it is considered that there is a current Ir on the output of the first differential branch and a current Ir+i on the second, where Ir=(Io−i)/2, the current i being the differential current that results from the offset and that will be measured. 
         [0055]    The circuit MCD comprises two capacitors C 1  and C 2  into which the current i will be progressively inserted during the calibration phase. It will be noted that it is not obligatory to have two capacitors, only one would be sufficient, but the technology of the capacitors is such that it is preferable to have two of them and to discharge one while charging the other to prevent a dissymmetry induced by the technology for manufacturing the capacitors (typically, the bottom armature of the capacitor has a not inconsiderable interference capacity relative to the substrate unlike the top armature). 
         [0056]    The first capacitor C 1  is associated with four switches that are represented by MOS transistors. 
         [0057]    The first capacitor C 1  is associated with four commutators which are represented by MOS transistors. The capacitor C 2  is associated with four other commutators. These commutators are actuated in alternation by two clock signals F and F* that are complementary and have the same duration T, and this occurs for n successive clock periods. During a timeslot F, certain commutators are open and others closed; during the next timeslot F*, it is the contrary. The commutators closed during the timeslot F cause a current to flow that can be called the charge current Ir+i originating from the switch K 2   a  (second differential branch of the pair) in the capacitor C 2 , and a current that will be called the charge current Ir originating from the switch K 1   a  (first branch) in the capacitor C 1 . During the following timeslot F*, the commutators are inverted and cause a discharge current Ir to flow (originating from the first branch) in the capacitor C 2  and a discharge current Ir+i (second branch) in the capacitor C 1 . 
         [0058]    The capacitors C 1  and C 2  take a charge (Ir+i).T and −Ir.T respectively if T is the duration of the timeslot F. The duration T is chosen to be sufficiently small so that the voltage at the terminals of the capacitors does not come close to the values that would saturate the transistors that carry the currents. During the following timeslot F*, the capacitor C 2  loses a charge Ir.T and the capacitor C 1  gains a charge (Ir+i).T 
         [0059]    At the end of a pair of consecutive timeslots F, F*, the capacitors C 1  and C 2  each have a charge i.T with opposite signs. 
         [0060]    The capacitors C 1  and C 2  are in principle equal to a common value C. It will be noted that the matching of two capacitors may be much better (by a factor of 4 to 10) than the matching of resistors or of transistors in the same manufacturing process. 
         [0061]    At the end of n periods of alternating charges and discharges F and F*, the charge in the capacitors is n.i.T. The number n of pairs of alternations is chosen to be sufficiently large (T being small) so that the voltage n.i.T/C at the terminals of the capacitors is much greater than the offset voltage that has given rise to this measurement and than the offset voltage of the comparison circuit, and so that it is easily comparable to a threshold in the comparison circuit CMP. The product n.T must be limited in order not to saturate the switches via the voltage n.i.T. The duration T may be approximately 2 nanoseconds, the capacity approximately 1 picofarad and the number n approximately 20, in order to obtain a charge voltage of approximately 100 mV for a current variance of 2.5 microamperes. A voltage of 100 millivolts can easily be compared with a threshold. 
         [0062]    After n timeslots F and F*, the sequencer places the two capacitors in series to supply a voltage that is double the voltage at the terminals of each capacitor, and this voltage is applied to the comparison circuit CMP. Three switches K 1   c , K 2   c  and K 3   c  are rendered on-state in order to be used, on the one hand, for this placing in series and, on the other hand, for the connection of the sum voltage between the inputs of the circuit CMP. 
         [0063]    In this example, the circuit CMP comprises two high input impedance follower stages which comprise two MOS transistors Q 1  and Q 2  (inputs Ec 1  and Ec 2  on the grilles) with a respective resistor bridge that is connected to the source of each of these transistors and that is supplied by a current source. The voltages applied to the grilles of the transistors are transferred to the sources and define the supply voltages of the resistor bridges. Two intermediate points of these bridges are connected to two comparators CM 1 , CM 2  according to an installation such that
       the comparators are both in a first state if the voltage difference between their terminals is less than a threshold,   the comparator CM 1  toggles if the voltage difference, in a first direction, exceeds a threshold,   the comparator CM 2  toggles if the voltage difference, in an opposite direction, exceeds the same threshold; by construction, only the comparator concerned by the exceeding of the threshold can toggle.       
 
         [0067]    A memory toggle MEM, connected to the outputs of the comparators CM 1  and CM 2 , is actuated by the sequencer after the comparators have played their part, and it retains in memory, after the end of the calibration phase, the output state of the comparators; it supplies on its own outputs S 1  and S 2  the logic values representing this state. The outputs S 1  and S 2  control the auxiliary current sources SC 1   a , SC 2   a  as explained with reference to  FIG. 2 . 
         [0068]    It will be understood that a more sophisticated comparison could be made, a set of comparators supplying a larger number of outputs to control a larger number of auxiliary current sources, equal with one another or weighted with one another (in binary for example) on each follower stage upstream of the inputs of the differential pair. 
         [0069]    Equally, it will be understood that it is possible to envision, still with a larger number of additional current sources on each follower stage, that the calibration phase is carried out on several occasions with only the two comparators of  FIG. 3 , provided however that the memory MEM has several couples of outputs S 1   a , S 2   a , S 1   b , S 2   b , etc. in order to store the status values taken by the comparators during each cycle and in order to control the larger number of auxiliary current sources. The sequencer of the calibration phase is then programmed to execute two successive cycles or more if there are more than two auxiliary current sources per follower stage:
       cycle A: a measurement is taken at the end of a first series of n pairs of timeslots F and F*; this culminates in a first state of the comparators, stored in the memory in the form of values S 1   a  and S 2   a . The memory retains these values for the rest of the calibration phase (in particular during the second cycle B) and subsequently during the phase of normal usage of the differential circuit; for all this time, the values S 1   a  and S 2   a  control the auxiliary current sources SC 1   a  and SC 2   a;      cycle B: a second measurement is taken at the end of a second series of pairs of timeslots F and F*; this culminates in a new state of the comparators that may be different than the first state since this new measurement is taken while one of the auxiliary sources SC 1   a  or SC 2   a  is perhaps connected; the state of the outputs is stored in the form of values S 1   b , S 2   b  in the memory MEM without losing the content of the values S 1   a , S 2   a . The stored values S 1   b , S 2   b  control the supplementary auxiliary current sources SC 1   b , SC 2   b  in parallel with the auxiliary sources SC 1   a  and SC 2   a , and the correction of the second sources is added to the correction of the first ones. A weighting, for example binary, may be made between the two cycles, for example by giving the auxiliary sources SC 1   b , SC 2   b  a current value that is half of that of the sources SC 1   a , SC 2   a  and by doubling the number n of clock periods during the second cycle in order to be able to continue using the same thresholds in the comparator.       
 
         [0072]    An application particular to an association of two differential pairs will now be described, this association constituting a folding cell of a folding circuit of an analog-digital converter. The folding circuit comprises several cells placed in cascade with one another. Each cell comprises two inputs to receive a differential voltage Vin-Vip to be converted, in the form of two voltages Vin, Vip (the same voltage Vin, Vip for all the cells), and two inputs to receive a reference differential voltage Vrefn-Vrefp in the form of two reference voltages Vrefn and Vrefp (different reference voltages for each cell of the folding circuit). 
         [0073]      FIG. 4  represents a basic folding cell, which does not comprise the enhancement according to the invention. It comprises a first differential pair (transistors T 1   n , T 2   n ) respectively receiving as inputs Vin and Vrefn and a second pair (transistors T 1   p  and T 2   p ) receiving as inputs Vip and Vrefp. 
         [0074]    A charge CH 1  serves as a charge common to the branch T 1   n  of the first pair, to the branch T 2   p  of the second pair and also to two branches of a preceding cell not shown (connection of this preceding cell via a terminal E 1  of the cell shown, E 1  being connected to the charge CH 1 ). A charge not shown, because it forms part of the following cell, is connected to a terminal O 2  of the cell; it serves as a common charge for the branch T 2   n  of the first pair, for the branch T 1   p  of the second pair, and for two branches of the following cell. 
         [0075]    The charge may conventionally be constituted by a cascade transistor and a follower transistor. 
         [0076]    The current sources that supply the two transistors of each pair (the equivalent of the sources SC 0  of  FIG. 1 ) have been shown here in the form of transistors with emitter resistor, which corresponds to a conventional practical implementation: transistor T 3   n  for the first pair and resistor of value R 0 , identical transistor T 3   p  and resistor of the same value R 0  for the second pair. 
         [0077]    Also represented is a follower stage upstream of the base of the transistor T 2   n  (input Vrefn) and an equivalent stage upstream of the base of the transistor T 2   p  (input Vrefp); no follower stage has been shown upstream of the bases that receive Vin and Vip, although such stages are usually provided (these stages are common to all the cells of the folding circuit that receive the same voltages Vin and Vip). The follower stage upstream of the first differential pair comprises a follower transistor T 4   n  and a current source (transistor T 5   n  and emitter resistor of value R). The follower stage upstream of the second pair comprises a follower transistor T 4   p  with current source constituted by a transistor T 5   p  and an emitter resistor of the same value R. 
         [0078]    The inputs of the differential circuit thus constituted are therefore on the one hand the voltages Vin and Vip (or follower stage inputs upstream of these voltages) and on the other hand the voltages VRn and VRp on the bases of the follower transistors T 4   n  and T 4   p ; the input voltages VRn and VRp are transferred, with a base-emitter voltage drop, to the emitters of the transistors T 4   n  and T 4   p , emitters that are connected to the bases of T 2   n  and T 2   p  respectively. 
         [0079]    A constant voltage Vr 0  controls the bases of all the transistors that are used as current sources: T 3   n , T 3   p , T 5   n , T 5   p.    
         [0080]    The embodiment thus described is the simplest possible; more complex embodiments may be provided, without this changing what has been said on the use of the invention in such cells. For more details on the constitution of the cells, reference will be made to the documents dealing conventionally with analog-digital converters with folding circuits. 
         [0081]      FIG. 5  shows how the cell of  FIG. 4  can be modified to incorporate advantageously the principles of the present invention, by correcting the overall offset presented by the group of two interleaved differential pairs (sharing the same charges on their collectors), rather than the individual offset of each pair. It will now be seen how, in view of the offset correction, the difference of the two output currents of the cell will be measured, each output current in this instance being the sum of two output currents sampled in two pairs respectively. 
         [0082]    Four switches K 1  and K 2 , K′ 1  and K′ 2  make it possible to connect to two respective voltage references Vr and Vr′, during the calibration phase, the inputs of the differential circuit so that the differential voltage seen between the bases of T 1   p  and T 2   p  on the one hand, and between T 1   n  and T 2   n  on the other hand, is null. If individual follower stages precede the inputs Vin and Vip, the switches must be placed on the bases of these followers and in this case the voltages Vr and Vr′ may be identical (supposing that the followers are identical to those that precede Vrefn and Vrefp). This boils down to short circuiting together all the inputs of the interleaved pair during the calibration phase and in any case to applying null differential voltages to the input pairs of the interleaved differential pair. 
         [0083]    An assembly of two switches K 3  and K′3, open during the calibration phase, closed the rest of the time, makes it possible to
       disconnect the cell from the preceding cell during this calibration phase,   disconnect the charge CH 1  from the collectors of the differential pairs.       
 
         [0086]    In  FIG. 5 , for example, K 3  is connected between the terminal E 1  and the charge CH 1 , and the switch K′ 3  is connected between the charge CH 1  and the point A 1  which joins the collectors of T 1   n  and T 2   p.    
         [0087]    A switch K 1   b  connects the junction point A 1  to a first input of the differential current measurement circuit MCD. Another switch K 2   b  connects a second input of this circuit MCD to the junction point A 2  which unites the collectors of the transistors T 1   p  and T 2   n.    
         [0088]    The switches K 1   b , K 2   b , K 1  and K 2 , K′ 1  and K′ 2  are closed during the calibration phase, open the rest of the time. 
         [0089]    The current measurement circuit MCD, whose inputs are connected to the switches K 1   b  and K 2   b , may be strictly identical to that of  FIG. 3 , with two identical capacitors C 1  and C 2 , commutators for directing the currents into the capacitors alternately originating from the switch K 1   b  then from the switch K 2   b , so as to accumulate in the capacitors, during n pairs of timeslots F and F*, a voltage proportional to the difference of currents flowing in the two switches K 1   b  and K 2   b , a difference that should be null in the absence of offset in the differential pairs. 
         [0090]    The commutators K 1   c , K 2   c , K 3   c  which make it possible to connect the capacitors C 1  and C 2  to the comparison circuit CMP are identical to those of  FIG. 3 . The comparison circuit CMP may be the same and has been represented in the form of a rectangle. The memory MEM at the output of the comparison circuit supplies two outputs S 1  and S 2  which control the auxiliary current sources that are added in parallel with the current sources T 5   n , R and T 5   p , R of the associated follower stages to the reference inputs VRn, VRp. 
         [0091]    The auxiliary sources SC 1   a  and SC 2   a  are controlled by transistors T 1   a  and T 2   a , as in  FIG. 2 , and the assembly in series of SC 1   a  and T 1   a  (or symmetrically SC 2   a  and T 2   a ) is in parallel on the emitter resistor R of the respective follower transistor. 
         [0092]    The auxiliary current sources may be embodied by a MOS transistor having its base controlled by a constant voltage, this MOS transistor having a dimension chosen depending on the auxiliary current desired to make the correction. 
         [0093]    The auxiliary current source may be simply a resistor if it is in parallel on the resistor R. It will be understood that this arrangement increases the current of the main current source, which is the equivalent of placing an auxiliary current source in parallel with the main current source. However, it is preferable for the auxiliary source to be constituted in the same manner as the main source in order to have the same variation depending on the temperature. 
         [0094]    The choice of the value of auxiliary current is in practice dictated by the technology: for a given technology (and dimensions of transistors of given differential pairs), the expected dispersion of offset voltages is known, and it is possible to estimate what is the offset voltage increment that makes it possible to limit this dispersion to a narrower value. This offset voltage increment makes it possible, knowing the size of the transistor of the follower stage, to determine what is the auxiliary current increment that must be provided to at least partially compensate for an offset voltage that is too great. 
         [0095]    It will be understood that in a folding cell in which the two differential pairs are tightly mixed, the output currents that have to be considered to measure a differential current due to an offset are not the individual currents of the two branches of a pair, but the output currents of the group of two interleaved differential pairs, that is to say of the sums of currents of the branches of the two pairs. It would be possible however to provide additional switches that separate the two interleaved pairs, in order to correct each pair individually, on condition that a separate measurement circuit for each pair and follower stages with auxiliary current sources on the two inputs of each pair are provided. 
         [0096]    This correction would have no value unless individual followers preceded Vip and Vin. The collectors of T 1   p  and T 2   n  on the one hand, T 1   n  and T 2   p  on the other hand, being connected in normal operation, a self-calibration retaining these connections and considering all four transistors to be a single differential pair is sufficient and the compensation may be made only on one input Vrefn or Vrefp depending on the direction of the offset. 
         [0097]    The foregoing description shows differential pairs with bipolar transistors, but the invention is applicable if the transistors are field effect transistors and it will be considered that the terms base, emitter and collector also cover the equivalent electrodes of source grid and drain of field effect transistors. 
         [0098]    It will be readily seen by one of ordinary skill in the art that the present invention fulfils all of the objects set forth above. After reading the foregoing specification, one of ordinary skill in the art will be able to affect various changes, substitutions of equivalents and various aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by definition contained in the appended claims and equivalents thereof.