Self-tracking integrated differential termination resistance

[0002] This application claims benefit of provisional application No. 60/327,114 filed Oct. 3, 2001.

[0003] The present invention relates in general to integrated circuitry, and in particular to methods and circuitry for implementing integrated differential termination resistance with a self-tracking feature that results in a relatively constant resistance value.

[0004] Advances in semiconductor processing technology and high speed circuit techniques have made gigabits per second (Gb/s) data rates possible at the integrated circuit level. At the same time reduction in power consumption continues to be a great concern not only for battery-operated portable systems, but for reduction of overall system costs related to packaging and cooling systems. This combination poses a challenging task for the circuit designer, especially when it comes to the interface circuitry. Various data signaling approaches have been developed to address some of these concerns. Low-voltage differential signaling (LVDS), for example, is one signaling technique that provides a low-power, low-voltage and high-speed alternative. LVDS, however, places relatively stringent requirements on the tolerance affecting the input/output (I/O) levels. It requires, for example, that the input termination resistance to stay relatively constant within ±10% of tolerance and under a specific operating range. Termination is needed to prevent unwanted reflections that would corrupt the signal when transmitting signals over distances appreciable with respect to the signal period. It is desirable to provide a self-terminating circuit that incorporates the termination resistance on the same integrated circuit for a more cost-efficient implementation.

[0005] To implement such an integrated resistor in the LVDS input buffer using a standard CMOS process technology has been a challenging task and the focus of many research efforts. One approach uses appropriate passive devices such as diffusion or implant resistors, but suffers from process variations, which is normally greater than 15% in both directions. The use of a relatively higher accuracy polysilicon resistor may be restricted as the oxide layer under the polysilicon may not sustain a specified electrostatic discharge (ESD) at an LVDS receiver's inputs. Another approach employs active circuits including pass transistors or transmission gates. The on-resistance of the transistors, however, fluctuates which makes it difficult to meet the relatively wide range of input common-mode voltage level (0 to 2.4V). The on-resistance of the devices is also influenced by the fluctuations in processing, power supply voltage and temperature. Improvements in this regard have been made through dynamic on-chip trimming, but that has also come at the expense of more complexity and device overhead.

[0006] There is therefore a need to implement a relatively constant integrated termination resistor that satisfies the requirements of differential signaling with minimum circuit complexity.

[0007] The present invention addresses problems associated with implementation of an integrated termination resistor required by differential interconnect signaling technologies such as LVDS. In a specific embodiment, the invention presents a circuit technique for implementing a self-tracking differential termination resistor in standard CMOS process technology. The circuit employs a mechanism that automatically tracks variations on input common-mode voltage level as well as varying device characteristics due to fluctuations in processing, power supply voltage and temperature. It then adjusts the resistance across the two differential input signals in a self-calibrating manner to obtain a substantially constant termination resistance for a given application. In a specific embodiment, the actual resistance is defined with an external reference resistor, independent of other operating conditions.

[0008] Accordingly, in one embodiment, the present invention provides an integrated differential termination resistance coupled between a first node and a second node, including: a first resistive element coupled to the first node; a first field effect transistor (FET) coupled between the first resistive element and a common node; a second resistive element coupled to the second node; a second FET coupled between the second resistive element and the common node, wherein source terminals of the first and second FETs couple to the common node; and a termination control circle coupled to gate terminals of the first and second FETs and the common node, and configured to track variations in a resistance value of the termination resistance and to compensate for the variations by adjusting a control voltage at the gate terminals of the first and second FETs. In a more specific embodiment, the termination control circuit includes a replica biasing circuit that adjusts the value of the control voltage in response to variations in a common-mode voltage at the common node.

[0009] In another embodiment, the present invention provides a method of implementing a differential termination resistance including coupling source and drain terminals of a field effect transistor (FET) across differential terminals, respectively; operating the FET in a triode region such that its channel provides an adjustable termination resistance; tracking variations in a nominal value of the termination resistance via a replica termination resistance; and adjusting a resistance value of the FET by varying a gate voltage of the FET to compensate for the variations.

[0010] The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the self-tracking integrated differential termination according to the present invention.

[0011] FIG. 1 depicts one implementation of a termination resistor according to an exemplary embodiment of the present invention;

[0012] FIG. 2 illustrates an exemplary implementation for a replica biasing circuit according to an embodiment of the present invention that generates the control signal for the termination resistor of FIG. 1;

[0013] FIGS. 3 presents an exemplary implementation for a reference circuit used in connection with the replica biasing circuit of FIG. 2 and the termination resistor of FIG. 1; and

[0014] FIG. 4 is a simplified diagram of an alternative embodiment for the differential termination resistance according to the present invention.

[0015] The present invention provides various embodiments for implementing an integrated self-tracking differential termination resistance. Referring to FIG. 1, there is shown an integrated circuit 100 that includes a termination resistor R_TERM102 connected across differential terminals V+ and V−. In the exemplary embodiment shown in FIG. 1, R_TERM102 includes two serially-connected n-type transistors T1 and T2 that respectively connect to V+ and V− via resistive elements R1 and R2 in a symmetrical fashion as shown. Resistive elements R1 and R2 are passive resistors made of, for example, diffused or implanted material. Transistors T1 and T2 are preferably identical in size and formation as are resistors R1 and R2. The source terminals of transistors T1 and T2 connect together at node 103 and carry a common mode signal V_COM, while the gate terminals of transistors T1 and T2 are shorted together at node 105 driven by a gate control signal V_GC. A control circuit 104 connects to nodes 103 and 105 and generates gate control signal V_GCbased on the common mode signal V_COMas described below in connection with FIGS. 2 and 3. Integrated circuit 100 further includes input/output circuit 106 that connects to differential terminals V+ and V−. For illustrative purposes, the invention is described in the context of an exemplary input termination resistance. It is to be understood, however, that the termination resistance according to the present invention can be used either as an input termination resistor or an output termination resistor.

[0016] In operation, control circuit 104 controls the voltage levels at V_GCin reference to and V_COMto maintain transistors T1 and T2 in the triode region. As thus constructed, the value of R_TERMis given by R_TERM=R1+R(T1)+R(T2)+R2, where, R(T1) and R(T2) correspond to the channel resistance of transistors T1 and T2, respectively. In one embodiment, the combined value of the two resistors is chosen so that transistors T1 and T2 have relatively small source-drain voltages (V_DS) in order for them to remain in the triode region for a maximum differential input voltage. It is to be understood, however, that in alternative embodiments it is possible to construct the termination resistance without the inclusion of resistors R1 and R2.

[0017] The differential termination resistance according to the present invention is particularly advantageous when employed in circuit applications using low voltage differential signaling (LVDS). Each of the two differential LVDS input signals VRX+and VRX−can be described as having two components: common-mode voltage (VCOM) and differential mode voltage (VDIFF) defined as follows:

[0018] V_RX+=V_COM+V_DIFF/2;  (1)

[0019] V_RX−=V_COM−V_DIFF/2  (2)

[0020] Where

[0021] V_COM=(V_RX++V_RX−)/2;  (3)

[0022] V_DIFF=V_RX+−V_RX−  (4)

[0023] Referring back to FIG. 1, the on-resistance of transistors T1 and T2 is determined, to a first order approximation, by the gate-source voltage, [V_GC-V_COM]. This is based on the assumption of small drain-to-source voltages (V_DS) for transistors T1 and T2 (as described above). Common-mode voltage, V_COM, also approximately corresponds to the middle-point of the two differential inputs of the circuit. Transistors T1 and T2 will exhibit a slight on-resistance difference and V_COMwill slightly shift away from the middle-point voltage when the differential voltage (V_DIFF) is large. However, these are considered second order effects with negligible impact on operation of the circuit as described below.

[0024] There are several factors that may cause appreciable deviations in the target resistance value of R_TERM. Among these are variations in the common-mode voltage V_COMas well as variations in process (P), power supply voltage (V) and temperature (T) (PVT variations). Common-mode voltage V_COMof the input signals V_RX+ and V_RX− depends on the far-end output driver's characteristics that drives the signal onto the transmission line, the voltage induced on the transmission line and the ground plane difference between the input and output buffers. According to the present invention, control circuit 104 is designed such that when V_COMchanges, V_GCtracks the change in order for a constant gate-source voltage and hence a constant resistance for transistors T1 and T2.

[0025] Referring to FIG. 2, there is shown an exemplary circuit implementation for a replica biasing 200 portion of control circuit 104 that generates V_GCaccording to the actual level of V_COM. Replica biasing circuit 200 includes transistor T3 and resistor R3 that are serially connected between nodes A and B and replicate one half of R_TERM102. For area saving purposes, in one embodiment, transistor T3 is made k₂times smaller (k₂<1) in terms of the width-to-length ratio compared to transistor T1 (or T2) in FIG. 1. Similarly, resistor R3 is formed with k₂multiples of R1 (or R2). Being one half of R_TERM102, the tracking resistance across nodes A to B should thus be equal to (k₂×R_TERM)/2. It is to be understood that T3 and R3 need not be scaled and can be direct duplicates of T1(T2) and R1(R2). The gate terminal of transistor T3 is driven by the output of an operational amplifier (opamp) 202, while the source terminal (node B) of transistor T3 is connected to the inverting input of opamp 202 in a feedback arrangement. The common-mode voltage V_COM(node 103 in FIG. 1) connects to the other (non-inverting) input of opamp 202. Opamp 202 is preferably a wide-band amplifier. The output of opamp 202 driving the gate terminal of transistor T3 generates control voltage V_GCthat also drives the gate terminals of transistors T1 and T2 in FIG. 1. As thus constructed, the gate and source terminals of transistor T3 are biased identically to those of transistors T1 and T2 in FIG. 1, respectively connecting to V_GCand V_COM. The feedback action of opamp 202 ensures that as V_COMvaries, V_GCfollows, thereby maintaining a constant gate-to-source voltage V_GCacross transistors T1, T2 and T3. Thus, circuit 200 generates a voltage level at V_GCthat is in reference to the common-mode voltage V_COMessentially eliminating variations in R_TERMdue to variations in V_COM.

[0026] The portion of the control circuit as described thus far addresses variations in the common-mode voltage V_COM. As described above, other sources of variations that impact the value of R_TERMinclude process, power supply voltage and temperature variations and input offset variations. Control circuit 104, according to the present invention, also compensates for these and other types of variations by controlling the level of voltage V_GCin a manner that adjusts the on resistance of transistors T1 and T2 in order to maintain the total value of the termination resistance R_TERMconstant. This is achieved, according to an exemplary embodiment of the present invention, through the same replica biasing circuit given in FIG. 2 together with the reference circuit shown in FIG. 3. Referring back to FIG. 2, circuit 200 further includes a p-type transistor T4 that has its gate terminal connected to V_COMand is biased by a current source I₀connected to its source terminal forming a source-follower as shown. The source terminal of transistor T4 also connects to node A of the replica termination resistor. Circuit 200 further includes a first current source 204 that connects between node A and the positive power supply V_CCand a second current source 206 that connects between node B and ground. Current source 204 is controlled by signal V_Pto generate a current equal to k₁I_REF, while current source 206 is controlled by signal V_Nto generate the same amount of current k₁I_REF. Control signals V_Pand V_Nare generated by reference circuit 300 shown in FIG. 3.

[0027] Referring to FIG. 2, the value of the tracking resistance R′_TERM(=k₂×R_TERM/2) can be given in terms of the gate-to-source voltage V_GSPof the p-type transistor T4 and the reference current I_REF. Opamp 202 operates such that the voltage at its two inputs are virtually the same. Therefore, the voltage across the source and gate terminals of transistor T4 (i.e., V_GSP) is equivalent to the voltage across nodes A and B (i.e., V_AB). as follows. Accordingly, R′_TERMcan be give by:

[0028] R′_TERM=(k₂R_TERM)/2=V_GSP/(k₁I_REF)  (5)

[0029] From the above, the value of R_TERMcan be derived as follows:

[0030] R_TERM=2V_GSP/(k₁k₂I_REF)  (6)

[0031] The present invention next renders R_TERMentirely independent of internal circuit parameters V_GSPand I_REFby generating an I_REFthat tracks V_GSPso that these terms are canceled from the above equation (6). In a specific embodiment, this is accomplished by first generating a replica V_GSP(i.e., V′_GSP) and then generating the I_REFbased on V′_GSP. Referring to FIG. 3, there is shown an exemplary circuit implementation for a reference circuit 300 that accomplishes this task and controls the biasing of circuit 200. Reference circuit 300 includes a p-type transistor T5 that is connected in a source-follower configuration in a similar fashion as p-type transistor T4 of circuit 200 and is biased by the current I₀such that its gate-to-source voltage Vgs is the same as that of transistor T4, i.e., V_GSP=V′_GSP. A reference signal V_REFis connected to the gate terminal of transistor T5. The reference voltage V_REFis arbitrarily set to V_CC/2 but may be any voltage around the mid-point between ground and V_CC. The voltage V′_GSPis applied across an external resistor R_EXTconnected to nodes A′ and B′ via circuit 302. Circuit 302 includes a first buffer 304 and a second buffer 306 that essentially convert a voltage signal to a current signal. Buffers 304 and 306 are both biased by a constant current source I₀, but transistor T7 in buffer 304 connects to the source terminal of transistor T5, while transistor T9 in buffer 306 connects to the gate terminal of transistor T5. This results in current I₁flowing in transistor T7 that is different than current I₂flowing in transistor T9. As thus constructed, the operation of circuit 300 yields the following:

[0032] V′_GSP=V_GSP=V_A′B′=I_XR_EXT  (7)

[0033] where, I_Xis the current flowing through R_EXT, which can be derive as follows:

[0034] I₁=I₀+I_X  (8)

[0035] I₂=I₀−I_X  (9)

[0036] From equations (8) and (9).

[0037] I_X=(I₁−I₂)/2  (10)

[0038] Reference circuit 300 further includes a current difference circuit 308 that includes transistors T12 and T13 that are biased so that they carry currents I₁and I₂, respectively. The drain terminals of transistors T12 and T13 are connected at node 310 so that the difference between currents I₁and I₂flows out of node 310. This current is then used as the reference current I_REFin replica bias circuit 200 of FIG. 2. That is, node 310 connects to transistors T14, T15 and T16 that generate bias voltages V_Nand V_Pthat are then applied to the control inputs of current sources 206 and 204, respectively, as show in FIG. 2. This results in:

[0039] I_REF=(I₁−I₂)=2I_X  (11)

[0040] Combining equations (7) and (11) results in:

[0041] I_REF=2(V_GSP/R_EXT)  (12)

[0042] Going back to equation (6) and replacing I_REFwith the expression in equation (12) yields:

[0043] R_TERM=2V_GSP/[2(k₁k₂)(V_GSP/R_EXT)]  (13)

[0044] Which can be simplified as:

[0045] R_TERM=R_EXT/(k₁k₂)  (14)

[0046] Thus, the value of termination resistance R_TERMis made dependent on the value of the external resistor R_EXTand the constants k₁and k₂. That is, the termination resistance R_TERM, according to this embodiment of the present invention, is defined by an external resistor, regardless of other conditions internal to the circuit or those of the signals received by the circuit. Suitable values for k₁and k₂may be selected in order to achieve an optimum trade-off between area and power consumption. The value of R_EXTmay vary depending on the application. For LVDS applications, for example, termination resistance R_TERMis 100 Ohms, allowing the designer to select, e.g., K=k₁k₂=500 and R_EXT=50 Kilo Ohms. It is to be understood that different differential I/O standards may require different termination resistance, and that R_EXTwould therefore vary accordingly. In one embodiment, a variable resistor implements R_EXTto allow for fine tuning and tweaking of the value of R_TERM. In another embodiment, the capability to adjust the value of the termination resistance based on an external resistor provides for a finely controlled variable differential termination resistance that is suitable for, e.g., multi-standard I/O applications. The accuracy for R_EXTalso varies depending on the I/O requirements. For LVDS applications, for example, R_EXTis selected such that it results in an R_TERMwith ±10% tolerance.

[0047] In some applications, an integrated circuit may include multiple differential I/O terminals. In such applications, a replica biasing circuit 200 shown in FIG. 2 may be used for each pair of differential input signals. However, one replica biasing circuit 200 may be shared by two or more of I/O differential pairs that have the same common-mode voltage. The same common-mode voltage may be shared by, for example, multiple input signals that are driven by the output buffers of the same kind and carried by the transmission lines with the same characteristics. Reference circuit 300 shown in FIG. 3 provides a tracking current for any number of the replica biasing circuits used for multiple different differential input pairs.

[0048] In an alternative embodiment of the present invention, the voltage range of operation for the circuit is extended by adding p-type transistors in parallel with the n-type termination transistors. For those input signals with a high-level common-mode voltage close to the maximum value of, e.g., 2.4V (for LVDS), the gate control voltage V_GCmay saturate at the power supply voltage V_CC. This could happen especially when V_CCis low, e.g. V_CC=3.3V. In such a case, it is desirable to employ additional p-type transistors in parallel with the n-type transistors for extended control. FIG. 4 is a simplified diagram of the termination resistance R_TERMshowing the addition of p-type transistors T₁′ and T₂′ in parallel with n-type transistors T1 and T2. In this embodiment, the tracking termination resistance (R′_TERMin FIG. 2) would also include p-type transistor connected in parallel with the n-type transistor. The gate control signals of p-type transistors T₁′ and T₂′ can be derived from a similar but separate feedback arrangement. However, for less complexity and reduced specifications, the gates of the p-type transistors can alternatively be connected to ground.

[0049] In conclusion, the present invention provides a termination resistance the value of which is defined accurately with an external reference resistor, independent of variations in circuit parameters and signal conditions including input signal common-mode voltage, as well as processing, power supply voltage and temperature variations.The invention provides for a self-tracking mechanism that tracks these variations and automatically makes adjustment to keep the termination resistance unchanged. The invention therefore eliminates any need for expensive process and post-process tweaking. Furthermore, by adjusting the value of the external reference resistor, the invention allows the termination resistance to easily be adapted to match different line resistances. This has been accomplished with minimum device count and circuit complexity. While the above provide detailed description of specific embodiments, it is to be understood that various alternatives, equivalents and modifications are possible. Therefore, the scope of the invention should not be limited to the illustrative embodiments described, and should instead be determined by the following claims and their full breadth of equivalents.