Abstract:
A system (e.g., a programmable resistive device) including switches and resistors. Respective ones of each of the switches is connected in series with respective ones of each of the resistors to form switch-resistor branches. The switch-resistor branches are connected in parallel between first and second nodes. Respective ones of the switches receive control signals to turn them ON, which forms an equivalent resistor having an equivalent resistance. A resistance of the switch is substantially insignificant compared to a resistance of the resistor in each of the switch-resistor branches making the equivalent resistance substantially linear.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is related to programmable switched-resistor networks.  
         [0003]     2. Background Art  
         [0004]     Many chips have devices that require a different resistance during different operations of the chip. For example, programmable gain amplifiers require different resistance in feedback paths from their output to their inverting and non-inverting input nodes during different operations.  
         [0005]     Typically, programmable resistive devices are used to most effectively allow for variable resistance in these devices. Programmable resistive devices usually have an array of resistors coupled in series between first and second nodes. A switch is associated with each resistor. A particular switch or number of switches that are ON control how many of the resistors are in a signal flow path (e.g., a current path) for a particular operation. The total number of resistor in series forms an equivalent resistance equaling a desired resistance value for the programmable resistive device.  
         [0006]     Problems can arise when transistors (e.g., MOSFETS) are used for the switches because they exhibit non-linear behavior. This can result in a non-linear equivalent resistance.  
         [0007]     Therefore, what is needed is a system and method that substantially reduce non-linearity in programmable resistive devices.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     Embodiments of the present invention provide a system (e.g., a programmable resistive device) including switches and resistors. Respective ones of each of the switches are connected in series with respective ones of each of the resistors to form switch-resistor branches. The switch-resistor branches are connected in parallel between first and second nodes. Respective ones of the switches receive control signals to turn them ON, which forms an equivalent resistor having an equivalent resistance. A resistance of the switch is substantially insignificant compared to a resistance of the resistor in each of the switch-resistor branches making the equivalent resistance substantially linear.  
         [0009]     Other embodiments of the present invention provide a method including at least the following steps. Connecting respective switches in series with respective resistors to form switch-resistor branches. Connecting in parallel the switch-resistor branches between first and second nodes. Individually controlling turning respective ones of the switches ON to form an equivalent resistor having an equivalent resistance between the first and second nodes. Using resistance values for the resistors orders of magnitude larger than resistive values for the switches, such that an equivalent resistor formed between the first and second nodes is substantially linear.  
         [0010]     Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
       [0011]     The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.  
         [0012]      FIG. 1  shows symbolic schematic diagram of a programmable resistor.  
         [0013]      FIGS. 2-3  show schematic diagrams of programmable resistors having series connected resistors.  
         [0014]      FIGS. 4-5  show schematic diagrams of various programmable resistors having parallel connected resistors according to various embodiments of the present invention.  
         [0015]     The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number may identify the drawing in which the reference number first appears. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [heading-0016]     Overview  
         [0017]     While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the pertinent art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the present invention. It will be apparent to a person skilled in the pertinent art that this invention can also be employed in a variety of other applications.  
         [0018]      FIG. 1  shows a programmable resistor  100  coupled between a node A and a node B.  
         [heading-0019]     Programmable Resistive Devices with Series Connected Resistors  
         [0020]      FIGS. 2-3  show two configurations for programmable resistive devices  200  and  300  having series connected resistors coupled between nodes A and B.  
         [0021]     With reference to  FIG. 2 , programmable resistive device  200  includes series resistors  202 - 1  to  202 - n  (n is an integer greater than 1) and switches  204 - 1  to  204 - n . Switches  204  are coupled between nodes  206 - 1  to  206 - n  ( 206 - 1  to  206 - n  and node A are a same node) and  208 - 1  to  208 - n.    
         [0022]     If p1 turns  204 - 1  ON, and all other switches are OFF, then all resistors  202  are coupled in series between node A and node B to form an equivalent resistor having an equivalent resistance. If p2 turns switch  204 - 20  ON, and all other switches are OFF, then only resistors  202  to the right of nodes  206 - 2  and  208 - 2  are coupled in series between node A and node B to form an equivalent resistor having an equivalent resistance. Thus, if px (x is an integer greater than 1) turns switch  204 - x  ON, and all other switches are OFF, all resistors  202  to the right of  206 - x  and  208 - x  are coupled in series between Node A and Node B to form an equivalent resistor having an equivalent resistance. All portions of programmable resistive device  200  to the left of nodes  206 - x  and  208 - x  are excluded, such that their resistors  202  are not included in the equivalent resistance between Node A and Node B.  
         [0023]     With reference to  FIG. 3 , programmable resistive device  300  includes series resistors  302 - 1  to  302 - n  (n is an integer greater than 1) and switches  304 - 1  to  304 - n . Switches  304  are coupled between adjacent nodes  306 - 1  to  306 - n  ( 306 - 1  and node A are a same node).  
         [0024]     If p1 to pn turn switches  304 - 1  to  304 - n  OFF, all resistors  302  are coupled in series between node A and node B forming an equivalent resistor having an equivalent resistance. If p1 turns switch  304 - 1  ON, and switches p2 to pn are OFF, then a portion with resistor  302 - 1  is shorted, and all resistors  302  to the right of  306 - 2  are coupled in series between node A and node B forming an equivalent resistor having an equivalent resistance. If p1 and p2 turn switches  304 - 1  and  304 - 20 N, and switches  304 - 3  to  304 - n  are OFF, then portions with resistors  302 - 1  and  302 - 2  are shorted, and only resistors  302  to the right of node  306 - 3  are coupled in series between node A and node B forming an equivalent resistor having an equivalent resistance. Thus, if p1 to px (x is an integer greater than 1) turn switches  304 - 1  to  304 - x  ON, and switches  304 -( x + 1 ) to  304 - n  are OFF, then all resistors  302  to the right of  306 -( x + 1 ) are coupled in series between Node A and Node B forming an equivalent resistor having an equivalent resistance. All portions of programmable resistive device  300  to the left of nodes  306 -( x + 1 ) are shorted, such that those resistors  302  are not included in the equivalent resistance between Node A and Node B.  
         [0025]     Transistors or metal oxide silicon field effect transistors (MOSFETS) can be used as switches  204  or  304 . MOSFETs exhibit non-linearities in regards to their resistance. When their resistances cross a threshold percentage value compared to resistors  202  and  302 , the non-linearities can make the equivalent resistance between node A and node B unacceptably non-linear.  
         [0026]     For example, programmable resistive devices  200  and  300  can have ten (10) resistors  202  or  302  of about 1 KΩ each and switches  204  or  304  exhibiting resistance of about 10 Ω. In this case, the resistance of switches  204  or  304  is around 1% of the resistance of resistors  202  or  302 . Depending on an application of programmable resistive device  200  or  300 , this can cross a threshold, making the equivalent resistance between nodes A and B unacceptably non-linear.  
         [0027]      FIGS. 4 and 5  show programmable resistive devices  400  and  500  that exhibit substantially linear behavior.  
         [heading-0028]     Systems for Providing Substantially Linear Programmable Resistive Devices  
         [0029]      FIG. 4  is a schematic diagram of a programmable resistive device  400  according to embodiments of the present invention. Programmable resistive device  400  includes an array of resistors  402 - 1  to  402 - n  coupled in series with an array of respective switches  404 - 1  to  404 - n  between nodes  406 - 0  to  406 - n  and  408 - 0  to  408 - n . The series connected switches  404  and resistors  402  form switch-resistor branches  410 - 1  to  410 - n . Switch-resistor branches  410  are coupled in parallel to each other and between nodes A and B. So, all nodes  406 - 0  to  406 - n  are the same node as node A; and all nodes  408 - 0  to  408 - n  are the same node as node B.  
         [0030]     At all times, resistor  402 - 0  is coupled between nodes A and B. If p1 turns switch  404 - 1  ON, resistors  402 - 0  and  402 - 1  are coupled between nodes A and B, and in parallel to each other, forming an equivalent resistor with an equivalent resistance. Thus, if px turns switch  404 - x  ON, all resistors  402  below nodes  406 - x  and  408 - x  are coupled between nodes A and B, and in parallel to each other, forming an equivalent resistor with an equivalent resistance.  
         [0031]     Resistors in parallel must be larger than resistors in series in order to form a same equivalent resistance. For example, parallel resistors can be orders of magnitude larger than series resistors in order to form a same equivalent resistance. However, switch  404  will have substantially a same resistance value as switch  204  or  304 . Thus, with the increased resistance required of resistors  402 , a percentage of a resistance of switch  404  compared to resistance of resistors  402  will be orders of magnitudes lower.  
         [0032]     For example, if resistors  402  are about 100 KΩ and switches  404  are about 10 Ω, then the resistance of switches  404  would be 0.1% of the resistance of resistors  402 . This substantially eliminates non-linearity in an equivalent resistance.  
         [0033]      FIG. 5  is a schematic diagram of a programmable resistive device  500  according to embodiments of the present invention. Programmable resistive device  500  functions substantially the same as device  400 , except device  500  allows for a situation when no current flow may exist between nodes A and B. This is because a switch  404 - 0  is provided in switch-resistor branch  410 - 0 . If switch  404 - 0  and all other switches  404  are OFF, then no resistor  402  is coupled between nodes A and B. Also, when each switch-resistor branch  410  has different valued resistors  402 , this configuration allows for any of those resistors  402  to be placed between nodes A and B by turning on their respective switches  404 . Again, all nodes  406 - 0  to  406 - n  are the same node as node A; and all nodes  408 - 0  to  408 - n  are the same node as node B.  
       Conclusion  
       [0034]     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.