Abstract:
A resistive ladder has first, second and third resistors coupled in series between first and second voltage terminals. A first node of the first resistor is coupled to the first voltage terminal and a first node of the third resistor is coupled to the second voltage terminal. A voltage selection unit has a first input coupled to a first node of the second resistor and a second input coupled to a second node of the second resistor and is adapted to selectively couple one of the first and second inputs to an output node of said resistive ladder. The resistive ladder also includes a first switch coupled between a second node of the third resistor and the second voltage terminal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit of French Patent Application number 13/60285, filed on Oct. 22, 2013, the contents of which is hereby incorporated by reference in entirety to the maximum extent allowable by law. 
       FIELD 
       [0002]    The present application relates to the field of resistive ladders, and also to a resistive ladder of a programmable amplifier circuit. 
       BACKGROUND 
       [0003]    Amplifiers having a programmable gain are known in the art. For example, in an amplifier circuit formed of an operational amplifier with negative feedback, the gain of the amplifier is a function of the resistance in the feedback path. It has been proposed to use a resistive ladder to provide a potential divider in the feedback path of such an amplifier, such that by setting the output level of the potential divider, the gain of the amplifier can be programmed. 
         [0004]    However, a drawback of existing resistive ladder circuits is that performance is poor and/or the circuits are complex, leading to a relatively high manufacturing cost and large silicon area. 
         [0005]    There is thus a need in the art for an improved resistive ladder circuit. 
       SUMMARY 
       [0006]    It is an aim of embodiments of the present disclosure to at least partially address one or more needs in the prior art. 
         [0007]    According to one aspect, there is provided a resistive ladder comprising: at least first, second and third resistors, coupled in series between first and second voltage terminals, a first node of the first resistor being coupled to the first voltage terminal and a first node of the third resistor being coupled to the second voltage terminal; a voltage selection unit having a first input coupled to a first node of the second resistor and a second input coupled to a second node of the second resistor, wherein the voltage selection unit is adapted to selectively couple one of the first and second inputs to an output node of the resistive ladder; and a first switch coupled between a second node of the third resistor and the second voltage terminal. 
         [0008]    According to one embodiment, the second resistor has the same resistance as the third resistor within a 5 percent tolerance. 
         [0009]    According to one embodiment, the resistive ladder further comprises: a fourth resistor coupled in series with the first, second and third resistors and having a first node coupled to the second resistor and a second node coupled to the third resistor; and a second switch coupled between the first node of the fourth resistor and the second voltage terminal. 
         [0010]    According to one embodiment, the resistive ladder comprises at least one further resistor coupled between the second and fourth resistors. 
         [0011]    According to one embodiment, the resistive ladder further comprises a control circuit adapted to generate one or more control signals for controlling the voltage selection unit and the first switch. 
         [0012]    According to one embodiment, the voltage selection unit comprises a multiplexer having a selection input receiving the one or more control signals. 
         [0013]    According to one embodiment, the control circuit is adapted to select one of: a first voltage level of the resistive ladder by controlling the voltage selection unit to couple the first node of the second resistor to the output node and activating the first switch; and a second voltage level of the resistive ladder by controlling the voltage selection unit to couple the second node of the second resistor to the output node and deactivating the first switch. 
         [0014]    According to a further aspect, there is provided an integrated circuit comprising the above resistive ladder. 
         [0015]    According to a further aspect, there is provided a programmable gain amplifier comprising the above resistive ladder; a differential amplifier having a positive input node coupled to an input voltage node, an output node coupled to the first voltage terminal of the resistive ladder, and a negative input node coupled to the output node of the resistive ladder. 
         [0016]    According to a further aspect, there is provided a method comprising: selecting, by a control circuit, a voltage level of a resistive ladder having at least first, second and third resistors coupled in series between first and second voltage terminals, a first node of the first resistor being coupled to the first voltage terminal and a first node of the third resistor being coupled to the second voltage terminal, wherein the voltage level is selected by: selectively coupling one of first and second nodes of the second resistor to an output node of the resistive ladder; and selectively activating a first switch coupled between a second node of the third resistor and the second voltage terminal. 
         [0017]    According to one embodiment, selecting the voltage level comprises selecting one of: a first voltage level by coupling the first node of the second resistor to the output node and activating the first switch; and a second voltage level by coupling the second node of the second resistor to the output node and deactivating the first switch. 
         [0018]    According to a further aspect, there is provided a method of programming the gain of an amplifier circuit comprising selecting a voltage level of a resistive ladder according to the above method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The foregoing and other features and benefits will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0020]      FIG. 1  schematically illustrates an amplifier circuit with negative feedback according to an example application of embodiments of the present disclosure; 
           [0021]      FIG. 2  schematically illustrates an example of a resistive ladder; 
           [0022]      FIG. 3  schematically illustrates a resistive ladder according to an example embodiment of the present disclosure; 
           [0023]      FIG. 4  schematically illustrates a resistive ladder according to a further example embodiment of the present disclosure; and 
           [0024]      FIG. 5  illustrates an electronic device according to an example embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    While in the following description an application of a resistive ladder in a programmable amplifier circuit is described, there are other possible applications of the resistive ladder, such as for the generation of reference voltages, the generation of regulated supply voltages, or as part of a digital-to-analog converter (DAC). 
         [0026]      FIG. 1  schematically illustrates a programmable amplifier circuit  100  according to an example embodiment. 
         [0027]    Circuit  100  comprises a differential amplifier  102 , which is for example an operational amplifier. The differential amplifier  102  has a positive input  104  coupled to an input voltage node providing the input voltage that is to be amplified. In one embodiment, the input voltage is a reference voltage V REF , such as a supply voltage level, which is to be adjusted, and the amplifier circuit provides the function of an output regulator. 
         [0028]    The differential amplifier  102  provides, at an output  106 , an output voltage V O . A feedback path is provided between the output  106  and a negative input  108  of the differential amplifier  102 , the feedback path comprising a potential divider  110  that divides the voltage V O  to provide a feedback voltage VFB. The potential divider has a resistance R 1  between the output  106  and the negative input  108  of the differential amplifier, and a resistance R 2  between the negative input  108  of the differential amplifier and a ground voltage. The gain of the amplifier circuit is thus: 
         [0000]        V   O   /V   REF =1+( R 1/ R 2) 
         [0000]    The gain of the amplifier circuit can be modified by varying the ratio between the resistance values R 1  and R 2 . 
         [0029]      FIG. 2  schematically illustrates an example of a resistive ladder  200  that could be used to implement the potential divider  110  of  FIG. 1 . As illustrated, the resistance ladder  200  comprises seven resistors  201  to  207  coupled in series with each other between the output  106  of the differential amplifier  102  and a ground terminal (GND). The resistor  201  is connected to the output  106  and has a resistance R TOP  and the resistor  207  is connected to the ground terminal and has a resistance R BOT . The other five intermediate resistors  202  to  206  each have a resistance R STEP . Each of the six nodes between the resistors  201  to  207  provides a corresponding voltage level VFB 1  to VFB 6 . By selecting one of these voltages to be provided as the feedback voltage VFB to the negative input of the differential amplifier  102 , a particular gain of the amplifier can be selected. 
         [0030]    For example, if the voltage level VFB 6  between the resistors  206  and  207  is selected as the feedback voltage VFB, the gain will be equal to 1+(R TOP +5R STEP )/R BOT . Alternatively, if the voltage level VFB 5  between resistors  205  and  206  is selected as the feedback voltage VFB, the gain will be equal to 1+(R TOP +4R STEP /(R STEP +R BOT ). 
         [0031]    While the resistive ladder of  FIG. 2  provides a relatively simple solution for varying the resistances R 1  and R 2  of the potential divider, a drawback is that the step size in the gain will not be constant. In particular, the step size in the gain between selecting the feedback voltage VFBn and VFBn+1, for n between 1 and 5, will vary for different values of n. While this drawback could be overcome by replacing the resistors  202  to  206  of resistance R STEP  by resistors having carefully selected resistance values that are each different, this would lead to a broad variation in the resistance values and thus a circuit layout that is far from optimal. 
         [0032]      FIG. 3  schematically illustrates a resistive ladder  300  according to an example embodiment. In one embodiment, the resistive ladder  300  implements the potential divider  110  of the programmable amplifier circuit  100  of  FIG. 1 , although other applications would be possible. 
         [0033]    As illustrated in  FIG. 3 , the resistive ladder  300  comprises a number of resistors coupled in series between an input voltage terminal, for example the node  106  providing the voltage V O  at the output of the amplifier circuit of  FIG. 1 , and a ground terminal. In the example of  FIG. 3 , four resistors  301  to  304  are illustrated coupled in series. Furthermore, the resistive ladder  300  comprises a voltage selection unit  310 , for selecting one of the two or more intermediate nodes between the resistors  301  to  303  to be coupled to the negative input  108  of the differential amplifier  102  of  FIG. 1 . In the example of  FIG. 3 , the voltage selection unit  310  is implemented by a multiplexer. 
         [0034]    The resistor  301  has a resistance R TOP , and one of its nodes is coupled to the node  106 . As shown by a dashed line, optionally this node is also coupled to an input of the multiplexer  310 . The resistors  302  to  304 , and any intermediate resistors between these resistors, each for example have the same resistance value R STEP , within a 5 percent tolerance. For example, this means that the resistance of resistance  302  is between 95% and 105% of that of resistor  304 . The resistor  302  has one of its nodes coupled to the resistor  301 , and its other node coupled to a further node  305 , optionally via one or more intermediate resistors as represented by a dotted line. The node  305  is further coupled to a node  306  of the resistor  303 , optionally via one or more further intermediate resistors as represented by another dotted line. The other node  307  of resistor  303  is coupled to a node  308  of resistor  304 , optionally via one or more intermediate resistors represented by yet another dotted line. 
         [0035]    The voltage levels at each of the nodes of resistor  302  are labelled VFB 1  and VFB 2  respectively, and each is coupled to a corresponding input of the multiplexer  310 . The voltage levels at the nodes  305  and  306  are labelled VFBM and VFBN respectively, and are also for example coupled to corresponding inputs of the multiplexer  310 . As represented by dots between the input lines of the multiplexer  310 , in the case that there are additional resistors between resistors  302  and  303 , the multiplexer  310  may comprises further inputs coupled to the corresponding intermediate nodes between these additional resistors. 
         [0036]    The resistive ladder  300  further comprises a switch block  312  for selectively coupling one or more of the intermediate nodes between the resistors  302  to  304  to the ground voltage terminal. For example, the switch block  312  comprises a switch  314  coupled between the node  308  and ground. Optionally, the switch block  312  further comprises a switch  315  coupled between the node  307  and ground, a switch  316  coupled between the node  306  and ground, and/or a switch  317  coupled between the node  305  and ground. As represented by dots between the switches  314  and  315 , and between the switches  316  to  317 , one or more further switches may be provided in the case that there are additional resistors between resistors  302  and  304 . 
         [0037]    Thus the intermediate nodes between the resistor  301  and the node  305 , and the node  305  itself, provide voltage levels VFB 1  to VFBM, which are coupled to corresponding inputs of the multiplexer  310 . Furthermore, the node  306  and all other intermediate nodes between the node  306  and resistor  304  are coupled by a corresponding switch of the switch block  312  to ground. The nodes  305  and  306 , and any intermediate nodes, correspond to overlapping nodes that are for example coupled to both a corresponding input of the multiplexer  310  and to a corresponding switch of the switch block  312 . 
         [0038]    The resistive ladder  300  further comprises a control block  320  for generating one or more control signals SEL A  for controlling the voltage selection unit  310 , and one or more control signals SEL B  for controlling the switch block  312 . The control block  320  for example generates the control signals SEL A  and SEL B  based on an input signal G indicating a desired level of the output signal. For example, in the case that the resistive ladder  300  is used as a potential divider  110  of the amplifier circuit  100  of  FIG. 1 , the signal G indicates a gain to be applied by the circuit. 
         [0039]    There are many different possible combinations of the selection signals SEL A  and SEL B  resulting in a broad range of voltage levels at the output. However, the selection signals SEL A  and SEL B  are for example generated such that the resistance R 2  between the selected voltage level VFBn and ground remains constant. For example, the lowest feedback signal VFB is for example selected by the multiplexer  310  as the voltage level VFBN at node  306 , and in this case all of the switches  314  to  317  are deactivated, so that the resistance R 2  is equal to the sum of the resistances of resistors  303  and  304 , and any intermediate resistors. This resistance level is for example maintained when any of the other voltage levels VFB 1  to VFB(N−1) is selected, by controlling one of the switches of the switch block  312  to bypass an appropriate number of the resistors closest to ground. For example, calling the switches  314  to  317   1  to K, when the voltage level VFB(N−1) is selected, the switch  1  is activated, when the voltage level VFB(N−2) is selected, the switch  2  is activated, etc. 
         [0040]    While the resistive ladder  300  is shown as comprising a minimum of four resistors, in its simplest form, two gain levels could be provided using only the three resistors  301 ,  302  and  304 . In such a case, the multiplexer  310  would comprise only two inputs coupled to the voltage levels VFB 1  and VFB 2 , and the switch block  312  would comprise a single switch  314 . 
         [0041]    The optional connection of the node  106  to an input of the multiplexer  310  as shown by a dashed line in  FIG. 3  for example permits a unitary gain of the amplifier circuit to be selected. 
         [0042]      FIG. 4  schematically illustrates a resistive ladder  400  according to a particular example embodiment in the case that there are eight resistors  401  to  408  coupled between the input voltage terminal V O  and the ground terminal. The intermediate nodes between resistors  401  and  405  provide voltage levels VFB 1  to VFB 4  respectively, and are coupled to corresponding inputs of a 4-input multiplexer  410 . A switch block  412  comprises three switches  413  to  415  respectively coupled between the intermediate nodes between resistors  408  to  405  and ground. In the embodiment of  FIG. 4  there is no overlap between the nodes coupled to the multiplexer  410  and the nodes connected to the switches of the switch block  412 . In such a case, the resistor separating the two groups of resistors, which is resistor  405  in  FIG. 4 , has a resistance R BOT  that is for example different from the resistance R STEP  of the other resistors  402  to  404  and  406  to  408 . 
         [0043]    In the example of  FIG. 4 , the multiplexer  410  is for example controlled by a 4-bit selection signal SEL[3 . . . 0], in which the code “1000” selects the voltage level VFB 1 , the code “0100” selects the voltage level VFB 2 , the code “0010” selects the voltage level VFB 3 , and the code “0001” selects the voltage level VFB 4 . The switch block  412  is for example controlled by a 3-bit signal SEL[3 . . . 1] composed of the three most significant bits of the signals SEL[3 . . . 0], in which the code “100” activates the switch  415 , the code “010” activates the switch  414 , and the code “001” activates the switch  413 , and the code “000” does not activate any of the switches  413  to  415 . Thus there are four gain levels that may be selected by the selection signal, and for each gain level the resistance R 2  between the node  108  and ground is equal to R BOT +3R STEP . 
         [0044]      FIG. 5  illustrates an electronic device  500 , which is for example an integrated circuit, in which a power source (PS), such as a battery, provides a voltage supply level V S  to an output regulator (OR), which in turn provides an output voltage level V O . The output regulator is for example implemented by the programmable amplifying circuit  100  of  FIG. 1 , and comprises the resistive ladder  300  of  FIG. 3 , controlled by a signal G. The control signal G is for example an error signal generated by comparing the output voltage level V O  of the output regulator to a reference level. The output voltage level V O  is for example used to power one or more circuit blocks C of the electronic device. 
         [0045]    An advantage of the embodiments described herein is that the output level of a potential divider can be programmed in a simple fashion, with substantially equal step sizes between each level, while permitting the use of resistors having substantially the same resistance. Furthermore, given that each of the switches of the switch block is coupled to the same supply level, e.g. ground, very little noise is introduced when selecting the different voltage levels. 
         [0046]    While a number of specific embodiments have been described herein, it will be apparent to those skilled in the art that there are numerous variations and modifications that could be applied. 
         [0047]    For example, it will be apparent to those skilled in the art that the ground voltage could be replaced by a voltage level different from 0 V, which could be negative or positive.