Abstract:
A feedback circuit for an operational amplifier is provided, the circuit comprising a first impedance element in a current flow path between an output of the operational amplifier and a first node, wherein a plurality of impedance elements are, in response to a control signal, selectively connectable either between the first node and a first input of the operational amplifier, or between the first node and a further node, and the further node and the first input of the operational amplifier are at the same potential such that a voltage at the first node is independent of the control signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a feedback circuit for an operational amplifier, and such a circuit finds application in current to voltage converters as may be found, for example, in digital to analog converters.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is often necessary to fabricate high accuracy analog integrated circuits. Generally it is desirable to be able to control the gain of such a circuit or its transfer characteristic when performing current to voltage conversion or voltage to current conversion.  
         [0003]     It is known to use thin film resistors in such high accuracy analog integrated circuits because of their accuracy and stability over temperature and with respect to time. However variations and imperfections in the fabrication process mean that adjustments may be needed to the resistance provided by these resistors. Often these resistors are laser trimmed to improve their accuracy. However laser trimming has several disadvantages. Firstly, the on-chip resistor which is to be laser trimmed must be relatively large in order to give the laser an easy target to aim at. Secondly laser trimming must be done before the device is encapsulated in its package. Once the component (integrated circuit) has been laser trimmed, its accuracy may still not be fully guaranteed. This is because placing the component in the package, which is usually plastic, can cause further changes in the resistor accuracy and these cannot be trimmed out by the laser. During packaging the chip is normally immersed in the molten plastic that will form its package. The plastic exhibits thermal contraction as it cools and this places stress upon the semiconductor substrate forming the component. It is this stress which causes variations in the component values.  
       SUMMARY OF THE INVENTION  
       [0004]     According to a first aspect of the present invention there is provided a feedback circuit for an operational amplifier, the feedback circuit comprising a first impedance element in a current flow path between an output of the operational amplifier and a first node, and  
         [0005]     a plurality of impedance elements which are, in response to a control signal, selectively connectable either between the first node and a first input of the operational amplifier, or between the first node and a further node, and the further node and the first input of the operational amplifier are at the same potential such that a voltage at the first node is independent of the control signal.  
         [0006]     It is thus possible to provide a feedback circuit in which, assuming that the output voltage of the operational amplifier is held steady, changes to a switchable network of a plurality of impedance elements does not give rise to changes in voltage at an input node to the switchable network because, when viewed from the first node, the impedance of the switchable network from the first node to a reference voltage, usually ground, is unaffected by the configuration of the switchable network. This has the advantage that adjustments to the switchable network result in a linear and predictable change in the transfer characteristic of an amplifier associated with the feedback network.  
         [0007]     Preferably the plurality of impedance elements within the switchable network are resistors. The resistors may be arranged to form a digital to analog converter core and, in this regard, an R-2R configuration is advantageous. The R-2R configuration has having a single input and each “2R” resistor extends from adjacent nodes of a series chain of “R” resistors to form an output node, and each output node is selectively connectable to either a first output or a second output of the switchable network. This ensures that, for a given voltage at an input node of the R-2R network, the current passing through the network does not depend on the digital code controlling the network provided that both the first and second outputs are held at a common voltage. The first and second outputs can be held at a shared voltage if they are connected to an operational amplifier as the action of the operational amplifier within a properly formed feedback loop is to hold the potential at its inverting and non-inverting inputs the same. Advantageously the operational amplifier is configured to operate in a “virtual earth” mode.  
         [0008]     Advantageously an input, for example the inverting input, of the amplifier is arranged to receive a current from a circuit up-stream of the amplifier, and the feedback network around the amplifier causes the output of the amplifier to assume a voltage such that the entirety of the current can pass through the feedback network to the amplifier output. Thus the amplifier acts as a current to voltage converter.  
         [0009]     Advantageously the current to voltage converter may be formed as an output stage within a digital to analog converter.  
         [0010]     According to a second aspect of the present invention there is provided a current to voltage converter having an adjustable transfer characteristic, the converter comprising: 
        a first element having a first impedance and having first and second terminals;     a current steering device having a first, second and third terminals and controllable in response to a control signal to steer a proportion of a current flowing at the first terminal to the second terminal, and a remainder of the current to the third terminal thereof;     an operational amplifier having an output and an inverting input, and a feedback element having a second impedance connected between the output of the amplifier and the inverting input;     and wherein the first element and the current steering device are arranged in series between the output of the amplifier and the inverting input, and one of the second and third terminals is connected to the inverting input of the amplifier and, in use, the second and third terminals are held at the same voltage.        
 
         [0015]     According to a third aspect of the present invention there is provided a digital to analog converter including a feedback network according to the first aspect of the present invention.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0016]     The present invention will further be described by way of example with reference to the accompanying drawings, in which:  
         [0017]      FIG. 1  schematically illustrates a feedback circuit for an operational amplifier constituting an embodiment of the present invention;  
         [0018]      FIG. 2  schematically illustrates a current steering arrangement, in the form of an R-2R ladder which is suitable for use in the feedback circuit of  FIG. 1 ;  
         [0019]      FIG. 3  illustrates an alternative current steering network in the form of a segmented R-2R ladder;  
         [0020]      FIG. 4  is a schematic diagram showing an embodiment of the feedback network as incorporated within a monolithic integrated circuit;  
         [0021]      FIG. 5  schematically illustrates a digital to analog converter including a current to voltage converter constituting an embodiment of the present invention; and  
         [0022]      FIG. 6  schematically illustrates the amplifier gain as a function of trim code supplied to the trim network. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  schematically illustrates a feedback network constituting an embodiment of the present invention. The feedback network, generally designated  2 , is associated with an operational amplifier  4 . In the arrangement shown in  FIG. 1  the operational amplifier  4  receives a current from a digital to analog converter  6  and the action of the operational amplifier  4  and its feedback network  2  is to convert the current from the digital to analog converter  6  into an output voltage at an output  8  of the operational amplifier  4 . For simplicity of the description it is assumed that the digital to analog converter sinks a current, and hence current flows from the output of the amplifier and into the digital to analog converter via the feedback network. In practice it makes no difference whether the current is sunk by or flows from the input circuit (e.g. digital to analog converter) to the amplifier.  
         [0024]     The operational amplifier  4  has a non-inverting input  10  and an inverting input  12 . The non-inverting input  10  is generally held at a constant voltage, in this example ground voltage. In use, we can also assume that the voltage an the inverting input  12  of the operational amplifier will also be zero volts. The inverting input  12  is connected to an output terminal of the digital to analog converter  6 .  
         [0025]     In use, the circuit  6  sinks a current I which is to be converted into a voltage at the output  8  of the operational amplifier, given that no current (theoretically) flows into the non-inverting input  12  of the operational amplifier  4 , we can assume that all of the current I must flow through the feedback network  2 , and that the output voltage at the output  8  of the operational amplifier will assume whatever voltage is necessary in order to match the current flow through the feedback network  2  to be equal to the current flow to the device  6 .  
         [0026]     A conventional current to voltage converter would merely comprise a feedback resistor  20  connected between the output  8  of the operational amplifier  4  and its inverting input  12 . The performance of the current to voltage converter would then be determined solely by the resistance of the feedback resistor  20 . However, as explained above, in monolithically integrated circuits the act of packaging the circuit can create stresses upon the circuit which in turn can effect the value of components therein and can change the value of the feedback resistor  20  from its nominal value. The present invention overcomes this by providing a digitally controllable trimming network as part of the feedback network  2 . This is implemented as a gain trimming network, generally designated  22 , which is formed in parallel with the feedback resistor  20 . The mere act of placing this trimming network  22  in parallel with the resistor  20  immediately reduces the impendence between the output  8  and the inverting input  12 , and consequently a correction resistor  24  is added in series with the feedback resistor  20  so as to return the impedance to its nominal value. The trimming network  22  comprises a first impedance  26  in series with a current steering network  28 . In this example the first impedance  26  is connected between an input terminal of the current steering network  28  and the output  8  of the operational amplifier. The current steering network, as will be explained in more detail later, effectively has an input terminal  32  connected to a node  30  formed between the network  32  and the first impedance  36  and has first and second output terminals, the first of which, designated  34 , is connected to the inverting input  12  of the amplifier  4 . The second output terminal, designated  36  and shown in  FIG. 2 , is connected to the same potential as the non-inverting input  10  of the amplifier  4 . Such a current steering arrangement can be implemented by the R-2R ladder schematically illustrated in  FIG. 2 .  
         [0027]     The feature of the current steering network  28  is that, although the proportion of the current passing from the input  32  to the first output  34  varies in accordance with a control word applied to the current steering network, the impedance of the network, when viewed from its input terminal  32 , is invariant with respect to the control word that it receives. As a consequence, if the output voltage of the operational amplifier was held constant, then the voltage occurring at node  30  would also be constant irrespective of the control word supplied to the current steering network. The fact that the current steering network presents a constant impedance when viewed from node  30  means that the gain trim network  22  can trim the gain of the current to voltage converter in a consistent and predictable manner, and more importantly, that the step size of the gain adjustment is linear.  
         [0028]     Advantageously a further resistor, in the form of a shunting resistor  36  extends between the first node  30  and the ground connection. It can be seen that the resistors  26  and the parallel combination of the resistor  36  and the current steering network  28  effectively forms a resistive potential divider and hence the value of the shunting resistor  36  can be used to set the step size of the gain correction applied by the current steering network  28 .  
         [0029]      FIG. 2  schematically illustrates an R-2R network. Such a network is commonly used as a digital to analog converter core where a reference voltage is provided at the input terminal  32 , which results in a current flowing into the R-2R network, and then that current is divided between the first output  34  and the second output  36  in proportion to a digital control word presented to the digital control lines  50 - 0  to  50 -N which control electronic switches S 0  to SN within the converter core. The R-2R topology is well known to the person skilled in the art, but it can be seen to be composed at a string of resistors  60 - 1  to  60 -N. The connections between the resistors define nodes  61 - 1  to  61 -N and connected to each node  61 - 1  to  61 -N is a further resistor  62 - 1  to  62 -N having a value 2R which in turn connect to switches S 1  to SN for steering current to the first output  34  or the second output  36 . In this scheme, the node between the input  32  and the first resistor  60 - 1  is also connected to a resistor  62 - 0  having a value 2R which connects to switch S 0  and the final node  61 -N is terminated by a further resistor  64  having a value 2R which is connected to ground. It can be seen in this arrangement that the current flowing through the resistor  62 - 0  is twice the current flowing through the resistor  62 - 1 , which in turn is twice the current flowing through the resistor  62 - 2 , and so on. However, because the outputs  34  and  36  are both held at ground potential by the operation of the amplifier  4  forming a virtual earth, then it can be seen that the current drawn through the R-2R ladder is invariant of the states of the switches S 0  to SN. This feature is particularly useful when forming the current steering network  28  because it means that current flow through the resistor  26  ( FIG. 1 ) and the voltage at node  30  are not perturbed by the digital word controlling the current steering network  28  but that the proportion of the current that is admitted into the feedback loop via the first output  34  is dependent upon the digital word supplied to the current steering network  28 .  
         [0030]     The R-2R ladder configuration shown in  FIG. 2  is not the only way of performing current steering in a manner which presents a constant impedance at a notional input terminal.  FIG. 3  shows an alternative configuration in which a plurality of current steering switches are effectively connected in parallel to the input node  32  via their respective resistors  70  to  73  and the current is steered to the first output  34  or the second output  36  dependent upon the state of the switches SA 1  to SAN. As shown the resistors  70  to  73  have all been drawn as being the same size and hence this scheme is suitable for use with a thermometer decoding driving scheme. However it is also apparent that the resistors do not all have to be the same size and that they could, for example, be scaled in a binary weighted manner if desired. This scheme can be used alone or (as shown) in conjunction with a conventional R-2R ladder network, designated  90 , if desired to form a digital to analog converter core or a current steering network (as appropriate) encoding a large number of bits.  
         [0031]      FIG. 4  schematically illustrates the representation of the trim network  22  suitable for implementation within a monolithic integrated circuit. It can be seen, in comparison with  FIG. 1 , that the shunting resistor  36  is formed by three unit value resistors in parallel, that the resistor  26  is formed by two unit value resistors in series, and that in the R-2R network  28  the resistors  62 - 0  to  62 -N are formed by two unit value resistors arranged in series, as is the terminating resistor  64 . It can also be seen that, as the second output  36  is connected to ground then the terminating resistor  64  can be connected to the second output  36 . It can also be seen that, for ease of implantation, the change over switches S 0  to SN are presented as pairs of field effect transistors, spanning between the respective end of the 2R resistor, and either the first output  34  or the second output  36 , with the transistors receiving complimentary control signals such that, for each pair, one transistor is on whilst the other is off or vice versa.  
         [0032]     In use, the control signals for the transistors within the R-2R ladder forming part of the current steering trim array are provided from a trim memory  100 . After fabrication and encapsulation the performance of the current voltage converter/or gain of the feedback network is characterised and gain adjustment is effected by changing the trim code supplied to the various transistors within the current steering network. Once the performance of the feedback network, and hence the gain of the amplifier has been adjusted to an acceptable level of performance, the trim code is written into the trim memory. The trim memory may be a rewritable memory, and preferably a non-volatile rewritable memory, such as EEPROM, or it may be a write once non-volatile memory, for example formed by fuses which are blown in order to set the trim code permanently into the trim memory  100 .  
         [0033]     The current to voltage converter shown in  FIG. 1  has utility at an output stage of a digital to analog converter. Such a converter is schematically shown in  FIG. 5 . The digital to analog converter shown in  FIG. 5  is formed using a R-2R core of the type shown in  FIG. 2  or  3 , and therefore has outputs I OUT1  and I OUT2 . The converter is also provided with a pin, labelled RFB, which corresponds to the node labelled RFB in  FIG. 1 . Therefore the components  20 ,  24 , and  22  shown in  FIG. 1  can be integrated within the digital to analog converter  110  of  FIG. 5 . The operational amplifier  4  could also be integrated within the converter or, as shown in  FIG. 5 , can be provided as an external component. In use the microcontroller  112  controls the operation of the digital to analog converter and in particular loads the digital word which is to be converted. It should be noted that, if the user wishes to vary the gain of the converter from that determined by the manufacturer, they could introduce resistors R 1  and R 2  in the positions shown in order to provide a user definable gain. However, if the user is happy to accept the gain determined by the manufacturer, then the resistors R 1  and R 2  of  FIG. 5  can be replaced by short circuit links.  
         [0034]     Where the feedback network is, as shown in  FIG. 5 , being used in conjunction with a DAC core, then a FET switch may be placed in series with the feedback resistor  20  (see  FIG. 1 ) and configured to be permanently on. This matches the thermal performance of the feedback network to that of the DAC core which also uses FET switches.  
         [0035]     It is thus possible to provide a trimming feedback circuit suitable for use in a current to voltage converter wherein the current drawn by the trimming arrangement does not vary with a digital trim code, and consequently, as shown in  FIG. 6 , the gain of the current to voltage converter varies in a linear manner with respect to changes in the trim code.