Abstract:
The invention relates to a method and to an electronic circuit for converting a current signal Isignal to a voltage signal Vout) comprising:—a first resistor Rgain and a second resistor Rconversion,—means ( 2 ) for generating a first current Igain based on a reference voltage Vreference applied over the first resistor,—means ( 3, 4 ) for generating a second current, the magnitude of the second current being determined on the basis of the multiplied magnitude of the first current and the current signal,—means ( 5 ) for applying the second current to the second resistor for generating the voltage signal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to the field of converting a current signal to a voltage signal, and more particularly to the usage of an integrated resistor for the purposes of the current-voltage conversion.  
       BACKGROUND AND PRIOR ART  
       [0002]     A variety of manufacturing methods for integrated resistors is known from the prior art. It is convenient to make these resistors in the same process steps as the rest of the integrated circuit. Therefore, the exact way in which the resistors are made depends on which process is used to produce the integrated circuit.  
         [0003]     Integrated circuits are generally fabricated with polysilicon resistors that are formed on the semiconductor substrate. Such resistors generally have a resistor body, formed of doped polysilicon and having metallic leads coupled to opposing ends of the resistor body, generally though contacts in field oxide. The contacts are connected to metal interconnect. The resistor body can be formed concurrently with polysilicon transistor gate electrodes, such resistor body generally doped and resting on the field oxide.  
         [0004]     Integrated circuits that require passive resistors often have tight tolerances on the resistance value of these resistors. However, these prior art semiconductor resistors are subjected to variations in resistance value. Sources of variation in the resistance value of theses resistors include process fluctuations that result in physical changes to the resistor properties such as physical dimension or resistivity and changes in temperature. The sources of change in temperature can be either external to the resistor itself or internal due to the self-heating effects associated with power dissipation. As the resistor temperature changes, the value of resistance of the resistor also changes.  
         [0005]     The general prior art method utilized for minimizing the resistance alteration effects due to the temperature coefficient of resistance (TCR) of semiconducting resistor (a resistor formed of semiconductor material) is to increase the doping concentration in the resistor body to a sufficiently high level such that the TCR of the resistor body is at a minimum. Then the resistors are built with dimensions that make the resistor head resistance a small percentage of the resistor body resistance. As a result, the resistor heads TCR has little effect on the overall resistor temperature characteristics.  
         [0006]     A corresponding integrated circuit with an integrated resistance having a minimized temperature coefficient is shown in U.S. Pat. No. 6,333,238. Another method for producing a semiconductor integrated circuit with an integrated resistor is known from U.S. Pat. No. 6,329,262.  
         [0007]     Further it is known to use integrated resistors for digital to analogue converters. U.S. Pat. No. 5,604,501 shows a digital-to-analog converter including a resistor string having intermediate taps at resistor junctions as well as resistor-potential junctions. Switching transistors are coupled between a respective intermediate tap and an output node. Decode circuits are capable of switching at least two transistors to be in the on state at the same time to electrically couple more than one intermediate tap to the output node to produce at least one analog output. One row select line can be energized simultaneously with at least two column select lines. Alternatively, at least two row select lines can be energized simultaneously with one column select line.  
         [0008]     U.S. Pat. No. 6,278,393 shows an integrated circuit including a digital-to-analog converter in which a resistor string is adapted to be coupled to a reference source. The resistor string includes a plurality of serially coupled impedances defining intermediate taps at the junctions thereof. A first plurality of switches are coupled between a first output node and respective ones of the intermediate taps.  
         [0009]     A first selection circuit receives a first digitally codes signal and is coupled to each switch in the first plurality of switches. The first selection circuit selectively switches the first plurality of switches to predetermined states depending upon a first digitally coded signal provided thereto, to generate a first analog output. A second plurality of switches are coupled between a second output node and respective ones of the intermediate taps. A second selection circuit coupled to each switch in the second plurality of switches selectively switches the second plurality of switches to predetermined states depending upon a second digitally codes signal provided thereto, to generate a second analog output.  
         [0010]     Generally the resistance tolerance of an integrated resistor is within about ±20%. Integrated resistors having a high square resistance have even higher tolerances. The lack of a method to produce integrated resistors having a precise resistance value has limited the field of use of integrated resistors in the prior art.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of the present invention to provide an improved electronic circuit for converting a current signal to a voltage signal by means of a resistor with a high degree of precision and which is suitable for monolithic integration.  
         [0012]     The present invention provides an electronic circuit for converting a current signal to a voltage signal comprising: 
        a first resistor and a second resistor,     means for generating a first current based on a reference voltage applied over the first resistor,     means for generating a second current, the magnitude of the second current being determined on the basis of the multiplied magnitudes of the first current and the current signal,     means for applying the second current to the second resistor for generating the voltage signal.        
 
         [0017]     Further embodiments of the electronic circuit of the invention are given in the dependent claims.  
         [0018]     Further the invention provides for a method for converting a current signal to a voltage signal comprising the steps of: 
        generating a first current by means of applying a reference voltage to a first resistor,     generating a second current, the magnitude of the second current being determined on the basis of the multiplied magnitudes of the first current and the current signal,     applying the second current to the second resistor for generating the voltage signal.        
 
         [0022]     Further embodiments of the method of the invention are given in the dependent claims.  
         [0023]     The invention is particularly advantageous as it allows to convert a current signal to a voltage signal by means of first and second resistors whereby the conversion is performed with a high degree of precision. To obtain a high degree of precision it is not necessary to provide precision resistors having exact resistance values.  
         [0024]     Rather the precision of the conversion in accordance with the present invention is determined by the precision of the ratio of the resistance values of the first and second resistors. This makes the electronic circuit of the present invention particularly suitable for monolithic integration as it is possible to produce a pair of integrated resistors having a precise resistance ration without difficulty.  
         [0025]     One field of application of the present invention is for CMOS integrated circuits, such as processors with analogue current outputs. Typically such an analogue current output needs to be converted to a voltage for example for the purposes of amplification. The problem of providing a precise and cost efficient interface between the processor and the following amplification stage is solved by means of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  shows a block diagram of a first embodiment of an electronic circuit in accordance with the invention,  
         [0027]      FIG. 2  shows a block diagram of a second embodiment of an electronic circuit in accordance with the present invention,  
         [0028]      FIG. 3  shows a block diagram for the generation of the circuit Igain in the embodiments of  FIGS. 1 and 2 ,  
         [0029]      FIG. 4  shows a detailed embodiment of the electronic circuit of  FIG. 1 ,  
         [0030]      FIG. 5  shows a detailed embodiment of the electronic circuit of  FIG. 2 ,  
         [0031]      FIG. 6  to  9  show a variety of detailed embodiments for the voltage conversion in the embodiments of  FIGS. 1 and 2 . 
     
    
     DETAILED DESCRIPTION  
       [0032]      FIG. 1  shows a block diagram of an electronic circuit  1  for converting a current signal Isignal to a voltage signal Vout. The electronic circuit  1  has two integrated resistors Rgain and Rconversion. A reference voltage Vreference is provided to the electronic circuit  1  or generated within the electronic circuit  1 . By means of the reference voltage Vreference and the integrated resistor Rgain a current Igain is generated in block  2  of the electronic circuit  1 . The current Igain equals Vreference divided by Rgain.  
         [0033]     The current Igain is provided to a divider  3 . Further a reference current Ireference is provided to the divider  3 . The purpose of the provider  3  is to divide the current Igain by the reference current Ireference. The resulting current Igain/Ireference is provided to multiplier  4 . Further the current signal Isignal to be converted is applied to multiplier  4 . The multiplier  4  provides an output current having a magnitude which is determined on the basis of the magnitudes of the current signal Isignal and the current Igain divided by the reference current Ireference. In the example considered here the output current provided by the multiplier  4  is the product of Isignal and Igain divided by Ireference.  
         [0034]     The output current of the multiplier  4  is provided to block  5  of the electronic circuit  1  which contains the integrated resistor Rconversion. The block  5  serves to convert the output current of the multiplier  4  to the voltage signal Vout. This is done by applying at least a portion of the output current of the multiplier  4  to the integrated resistor Rconversion. As a consequence the voltage signal Vout is as follows:  
       Vout   =       Isignal   Ireference     ⁢   Vreference   ⁢     Rconversion   Rgain     ⁢   N         
 
 Where N is a constant and depends on the configuration of the implementation. Convenient choices for N are N=½ or N=1. 
 
         [0036]     As it appears from the above formula the value of the voltage signal Vout does not depend on the absolute values of Rgain and Rconversion but on the ratio of Rconversion and Rgain. Hence, even though the absolute values of the resistances of Rgain and Rconversion may have large tolerances, the ratio of the resistances of Rgain and Rconversion can be manufactured with a high degree of precision such that the current to voltage conversion of the present invention is also accomplished with a high degree of precision even though integrated resistors are utilized.  
         [0037]      FIG. 2  shows an alternative embodiment of the electronic circuit  1  where like elements are denoted by the same reference numerals as in the embodiment of  FIG. 1 .  
         [0038]     In the embodiment of  FIG. 2 a  current signal Isignal is applied to the divider  3  as well as the reference current Ireference. The output provided by the divider  3  is Isignal/Ireference. This value is inputted into the multiplier  4 .  
         [0039]     Further the multiplier  4  receives the current Igain from the block  2 . This current Igain is multiplied by Isignal/Ireference in the multiplier  4 . The mutliplier  4  outputs the product of Igain and Isignal divided by Ireference.  
         [0040]     The output of the multiplier  4  is provided to the block  5  for current to voltage conversion in the same way as in the embodiment of  FIG. 1 .  
         [0041]      FIG. 3  shows a detailed embodiment of the block  2  of the embodiments of  FIGS. 1 and 2 . The reference voltage Vreference is either externally applied to the electronic circuit  1  or generated within electronic circuit  1 . The reference voltage Vreference is applied over the integrated resistor Rgain. One of the terminals of the resistor Rgain is connected to current mirror  6  which is connected to ground. This way the current Igain is obtained.  
         [0042]      FIG. 4  shows a more detailed embodiment of the electronic circuit of  FIG. 1 . The current signal Isignal is split by means of the differential pair of bipolar transistors Q 3  and Q 4 . A differential base-emitter voltage is provided to the differential pair of bipolar transistors Q 3  and Q 4  from the circuit arrangement comprising transistors Q 1 , Q 2 , Q 5  and Q 6 .  
         [0043]     The bases of the transistors Q 1  and Q 2  are coupled to a common potential which is also common to the collectors of Q 1  and Q 2 . The reference current Ireference is applied to this common potential. The emitter of the transistor Q 1  is coupled to the collector of the transistor Q 5 . The emitter of the transistor Q 5  is coupled to Rgain.  
         [0044]     The emitter of the transistor Q 2  is coupled to the collector of the transistor Q 6 . The emitter of the transistor Q 6  is coupled to a further reference voltage Vreference.  
         [0045]     The base of the transistor Q 6  is coupled to the emitter of the transistor Q 1 ; the base of the transistor Q 5  is coupled to the emitter of the transistor Q 2 . The differential base-emitter voltage for the differential pair of bipolar transistors Q 3  and Q 4  is provided between the potential of the emitter of transistor Q 2  and the emitter of transistor Q 1 .  
         [0046]      FIG. 5  shows an embodiment which corresponds to the embodiment of  FIG. 2 . In comparison to the embodiment  FIG. 4  the currents Isignal and Igain are interchanged. As a result the current Igain is split rather than the current Isignal. The voltage Vin determines the input voltage at the emitter of Q 5 . The resulting current in the collector branches of the transistors Q 3  and Q 4  is the same as in the embodiment of  FIG. 4 .  
         [0047]      FIG. 6  shows one way of converting the compensated current signal to voltage by means of the integrated resistor Rconversion. The collector branches of the transistors Q 3  and Q 4  (cf.  FIG. 4  and  FIG. 5 ) are connected to current mirror  7 . The resistor Rconversion is connected between the collector potential of the transistor Q 4  and VDD to produce the voltage Vout.  
         [0048]      FIG. 7  shows an embodiment of block  5  of  FIGS. 1 and 2  with feedback. To provide feedback an operational amplifier  8  is used for producing the voltage Vout. A voltage reference Vx is connected to the non-inverting input of the operational amplifier  8 . The inverting input of the operational amplifier  8  is coupled to the collector potential of the transistor Q 4 . The resistor Rconversion is coupled between the inverting input of the operational amplifier  8  and the output of the operational amplifier  8 .  
         [0049]      FIG. 8  shows an alternative embodiment where only one of the collector currents is used for the conversion. For this purpose the resistor Rconversion is coupled between VDD and the collector potential of the transistor Q 4  in order to produce the output voltage Vout. This corresponds to a choice of N= 12 .  
         [0050]      FIG. 9  shows a further embodiment for producing a differential output voltage Vout 1  and Vout 2 . A resistor Rconversion is coupled between VDD and collector potential for both transistors Q 3  and Q 4 . The differential voltage Vout=Vout 1 −Vout 2  can be used for input to an amplifier in a computer monitor or for similar applications requiring a differential output voltage.  
         [0051]     In essence it is possible to implement the present invention by means of an extra line per input current signal and a separate line to provide the reference current Ireference. For an audio-mono signal this means that two lines are required—one for the current signal Isignal and one for the reference current Ireference. For an audio-stereo signal an additional line for the additional audio channel is required. For the case of RGB-video three signal lines for the three channels are required in addition to the line for the reference current Ireference.  
         [0052]     Rather than inputting the signal current(s) and the reference current on separate lines a differential input can be provided. In the case of an audio-mono signal this can be implemented by providing a signal Ireference−Isignal on one line and Ireference+Isignal on the other line. Likewise this can be implemented for more than one channel.  
         [0053]     Alternatively the number of input lines can be restricted to the number of channels. In other words a separate line for Ireference can be avoided. This can be accomplished by utilizing the current signal itself as a basis to form the reference current Ireference. This can be done by using a maximum, a minimum, an average value or the RMS of Isignal as a reference. For example, in the case of a video signal the black signal of each video frame or line can be utilized as the reference Ireference.  
         [0054]     In case of more than one channel one or more or all of the channels can be used to form the reference current Ireference.  
         [0055]     As a further alternative the input line for the signal Isignal is time multiplexed to provide the reference current Ireference. In case of more than one channel one or more or all of the channels can be utilized for time multiplexing the reference current.  
       LIST OF REFERENCE NUMERALS  
       [0056]    
       
          electronic circuit  1   
          block  2   
          divider  3   
          multiplier  4   
          block  5   
          current mirror  6   
          current mirror  7   
          operational amplifier  8