Abstract:
Resistors may be more accurately tuned by using an external resistor and comparing the value of an internal resistor to the value of the external resistor. The value of the internal resistor may be adjusted to match the value of the external resistor. Any number of on-chip resistors may then be matched and adjusted using the information obtained with respect to the first internal resistor.

Description:
BACKGROUND 
   This invention relates generally to tuning resistors to produce more accurate resistors. 
   Termination impedances are often used at the transmit and receiving ends of devices passing high speed electrical signals over a transmission line media to reduce reflections and the subsequent signal degradation caused by those reflections. Reflections may degrade the signals at both the transmitter and receiver. The closer the termination impedance matches the impedance of the transmission line, the smaller the reflections. The reflections increase as the speed of the signals gets higher relative to the round trip delay of the media. 
   Termination resistors are sometimes implemented on integrated circuits to minimize these reflections when high speed signals are being transmitted or received. Accurately matching the termination resistors to the value of the impedance of the attached transmission line reduces reflections and resultant signal degradation. Thus, it is desirable that the termination resistor be very accurate across a range of temperatures and power supply voltages. 
   Termination resistors may be implemented in commonly available integrated circuit technologies materials such as in a diffusion, polysilicon, or metal. The resulting resistors are not very accurate and vary substantially with temperature and power supply voltage. This variation in resistance may result in a significant mismatch between the on-chip termination resistor and off chip transmission line, and hence significant reflections may result. This level of signal degradation may not be acceptable in some applications. 
   One method used to make more accurate termination resistors on an integrated circuit is to use an integrated circuit process that makes very accurate resistive elements. However, making accurate resistor elements using integrated circuit process techniques substantially increases the cost of the process. 
   A second method to implement more accurate termination resistors on an integrated circuit is to trim an inaccurate resistor to an accurate value. This can be done by fuse trimming or laser trimming the resistor to the desired value at manufacturing. The drawback of this approach is that it consumes silicon area and only adjusts absolute value. This approach is not able to compensate for resistor variations with temperature and power supply voltage. 
   Thus, there is a need for better ways to improve the accuracy of resistors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a flow for one embodiment of the present invention; and 
       FIG. 2  is a circuit diagram in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In accordance with one embodiment of the present invention, a typical on-chip resistor may automatically be adjusted to an accurate value across a broad range of manufacturing processes, temperatures, and power supply voltages. In one embodiment, an accurate on-chip termination resistor may be provided. However, the present invention is applicable to more accurate resistors for any application. 
   In one embodiment, shown in  FIG. 1 , an internal resistor value is compared to an external resistor value as indicated at block  100 . Then the value of the internal resistor is adjusted digitally to match the value of the external resistor as indicated in block  200 . Those digitally adjusted resistor values may then be used to adjust the value of another on-chip resistor as indicated in block  300 . The internal resistor may then be as accurate as the external resistor, given an ideal circuit. 
   The method is further illustrated using the equivalent circuit  10  shown in FIG.  2 . Assume that the voltage on node B starts at a higher voltage than that of node A and the switches  30  in series with the internal resistors  26  are all open. The combination of the amplifier  12  and the transistor  14  causes the amplifier  12  reference voltage (Vref) to be impressed across the external resistor  20  on the node A. The reference voltage Vref can come from any source; in one embodiment, it may be generated by a bandgap reference circuit. The current in the external resistor  20  is then equal to the voltage Vref divided by the resistance (Rext) of the external resistor  20  (Vref/Rext). This same current flows in transistor  16  in series with the transistor  14 . 
   Since the transistors  16  and  18  are arranged as a current mirror, the same current flows through the transistors  16  and  18  as well as the internal resistor  24  having a resistance Rint. Thus, the voltage on node B is Vref*(Rint/Rext). 
   The comparator  22  then compares the voltages of nodes A and B and sends the result to the logic block  34 . If the voltage of node B is greater than the voltage of node A, the logic block  34  increments a digital output value of SW[n: 0 ] by one count. This signal closes the switch  30   b  on resistor  24   a  having a resistance Rint 0  decreasing the total resistance across node B to the resistance of the resistors  24  and  26   a . As a result, the voltage at node B decreases to Vref*((Rint+Rint 0 )/Rext). 
   If the voltage on node B is still greater than the voltage on node A, the process continues switching in more resistors  26  using switches  30  until the voltage on node B is less than the voltage on node A. When this point is reached, the process stops and the voltages and digital values on the SW[n: 0 ] stay constant unless the value of an internal resistor changes. The hysteresis in the comparator  22  prevents the circuit  10  from toggling between two values and offers some noise immunity. 
   The digital value SW[n: 0 ] then can be distributed to another part of the chip and used to adjust any number of on-chip resistors, made by the same process such as the resistor  36 . If the internal resistor  36  was made by the same process to have the same value as the resistor  24 , the same digital values SW[n: 0 ] may be used again to set the second internal resistance value using the resistors  36   a-n  and switches  40   a-n.    
   The value of the equivalent resistor formed with the resistors  36  and  36   a-n  is as accurate as the external resistor  20 , assuming ideal circuit elements and an infinite array of switches/resistors. In practical applications, the circuit  10  error and a finite switch/resistor array can achieve a value for a termination resistor  36 ,  36   a-n  to be within 5% of the external resistor  20  value in some embodiments. This is much better than can be achieved with normal resistor elements. 
   While the above description refers to a way to adjust a termination resistor to accurate value, the present invention is not limited to termination resistors and can be used to make any resistor more accurate. 
   In one embodiment all of the components shown in  FIG. 2 , except the external resistor  20 , are integrated in the same integrated circuit. A connection may be made to the external resistor  20  through a pin at node A. 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.