Abstract:
The differential input receiver provides constant symmetrical hysteresis over a wide input signal range. The differential input receiver includes a pair of complementary differential comparators having common input terminals, a pair of series connected complementary current mirrors each having source terminals driven by the output terminals of the corresponding differential comparator, a pair of transistors connected in series across each differential pair transistor in each differential comparator to form a potential divider across it, and a pair of series connected inverting buffers connected to a common output of the differential comparators to provide the final output. The individual buffer outputs are fed back to the control terminals of the series connected transistors in a manner that provides positive feedback thereby providing equal rise-time, fall-delay and transition times in the output signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of high speed differential signaling, and, more particularly, to a differential input receiver with hysteresis. 
   BACKGROUND OF THE INVENTION 
   Hysteresis characteristics are used in digital logic circuits to reduce/eliminate input noise to prevent false signals and glitches. A differential input receiver is essentially used for high-speed differential signaling. As the signaling speed is very high, the potential for noise induced interference is higher than for the normal case. Further differential IO standards generally require a low voltage input swing, e.g. for Low Voltage Differential Signals (LVDS) the minimum input voltage swing is 100 mV, as a result any small noise in the input can have a significant effect. For this reason, IEEE standard 1596.3-1996, for LVDS for Scalable Coherent Interface (SCI) recommends a minimum of 25 mV hysteresis in the LVDS receiver. Since LVDS signaling can operate over a wide range of input signals, it is necessary that the width of the hysteresis should be almost constant over that range. 
     FIG. 1  shows a differential input receiver in accordance with U.S. Pat. No. 6,275,073. This differential input circuit does not incorporate any hysteresis characteristics. The circuit can operate over a wide input range and can be used to support differential standards such as LVDS. The differential input circuit of  FIG.1  includes a current mirror constant current source comprising a PMOS differential amplifier, and an NMOS differential amplifier. P_in and N_in are the two differential inputs to the circuits. PMOS transistors, Tr 1  Tr 2  and Tr 3  and NMOS transistors, Tr 9  and Tr 10  constitute a PMOS differential amplifier while NMOS transistors Tr 4 , Tr 5  and Tr 6  and PMOS transistors Tr 7  and Tr 8  constitute an NMOS differential amplifier. 
   Transistors Tr 2 , Tr 3 , Tr 5  and Tr 6  are input transistors as their gates are connected to the input signals P_in and N_in. The source of transistors Tr 2  and Tr 3  are connected to the drain of transistor Tr 1  whose gate is connected to VSS and source to VDD. VDD and VSS are the lower power supply terminals. The drain terminals of transistors Tr 2  and Tr 3  are connected to the drains of transistors Tr 9  and Tr 10  respectively. The source terminals of transistors Tr 9  and Tr 10  are connected together to the VSS. The gates of transistors Tr 9  and Tr 10  are connected to the drain of input transistor Tr 2 . The source terminals of transistors Tr 5  and Tr 6  are connected together to the drain of transistors Tr 4 , which has its gate connected to VDD while its source is connected to VSS. The drains of transistors Tr 5  and Tr 6  are connected to the drains of transistors Tr 7  and Tr 8  respectively whose source terminals are connected to VDD. The gates of transistors Tr 7  and Tr 8  are connected to the drain of input transistor Tr 5 . A resistance R is connected between the drains of transistors Tr 2  and Tr 5  while the drain terminals of transistors Tr 3  and Tr 6  are connected together to the output terminal Out. 
   The operation of the differential input receiver of  FIG.1  can be understood as follows. When P_in is greater than N_in i.e. P_in&gt;N_in, the output Out is HIGH. On other hand when P_in is less than N_in (P_in&lt;N_in), Out is LOW. In this manner, the differential input receiver acts as a comparator and the switching point of the circuit is the cross-over point of the two differential inputs. If there is noise in any of the input signals that results in the crossing of two inputs, the output can switch to a false state. This is significant because the differential input receiver normally works at high frequencies where the potential noise influence is much greater. The output characteristics of this circuit are shown in  FIG. 3  by the curve OUT_prior. 
     FIG. 1A  shows the another prior art input receiver for Gunning Transceiver Logic (GTL) standard which is shown in U.S. Pat. No. 5,666,068 titled GTL Input Receiver With Hysteresis. This input receiver incorporates hysteresis to support GTL standard. This circuit is basically a PMOS differential amplifier where Vin 1  and Vin 2  are the two inputs and OUT is the output. PMOS P 3  and PMOS P 4  are the input transistors which are connected to the inputs. P 8  and P 9  transistors connected in parallel with P 3  and P 4  respectively are used to provide hysteresis. This circuit supports GTL standard only and can not be used to support LVDS standard. 
   Therefore, it has been observed that there is a need to develop an input receiver that incorporates hysteresis properties to eliminate the influence of noise signals while operating with low voltage swing and wide range of inputs. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to obviate the above drawbacks. To achieve the objective this invention provides a differential input receiver providing constant symmetrical hysteresis over a wide input signal range, including a pair of complementary differential comparison means having common input terminals, a pair of complementary current source means each having its current supply/sink terminals driven by the output terminals of the corresponding differential comparison means, a switched controlled resistance connected across each comparison element in each differential comparison means having its control terminal connected to the input of its corresponding comparison element, and a pair of series connected inverting buffers connected to a terminal common to the output of the differential comparison means to provide the final output, the individual buffer outputs being fed back to the switch terminals of the switched resistance in a manner that provides positive feedback, thereby providing equal rise-time, fall-delay and transition times in the output signal. 
   The differential comparison means is a differential comparator. The switched resistance is a transistor. 
   The present invention also provides a method for improving a differential input receiver to provide constant symmetrical hysteresis over a wide input signal range, comprising connecting together the common input terminals of a pair of complementary differential comparators, attaching the current supply/sink terminals of a pair of series connected complementary current sources to the first output terminals of the corresponding differential comparator, connecting a switched controlled resistance across each differential pair transistor in each differential comparator having its control terminal connected to the output terminal of its corresponding differential pair transistor, and connecting a pair of series connected inverting buffers to a terminal common to the second output of the differential comparators to provide the final output, the individual buffer outputs being fed back to the switch terminals of said controlled resistance in a manner that provides positive feedback, thereby providing equal rise-time, fall-delay and transition times in the output signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the accompanying drawings. 
       FIG. 1  shows a differential input receiver in accordance with the U.S. Pat. No. 6,275,073. 
       FIG. 1A  shows the prior art differential input receiver of patent, U.S. Pat. No. 5,666,068. 
       FIG. 2  shows a first embodiment in accordance with the present invention. 
       FIG. 3  shows the output characteristics of the input receivers according to the present invention and prior art of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the present invention will be described with reference to  FIG. 2 . The differential input receiver includes an NMOS differential amplifier and a PMOS differential amplifier. The NMOS differential amplifier includes NMOS transistors N 1 , N 2  and N 3  and PMOS transistors P 1  and P 2 . The transistors N 1  and N 2  are the input transistors since their gates are connected to the two differential inputs IN 1  and IN 2  respectively. The source terminals of transistors N 1  and N 2  are connected together to the drain of transistor N 3 . The gate of transistor N 3  is connected to NET 1  while its source is connected to ground. 
   Transistors N 3  with P 6  and N 6  together act as a current source circuit that sinks the desired amount of current in the NMOS differential amplifier. The drains of transistors N 1  and N 2  are connected to the drains of transistors P 1  and P 2  respectively. The source terminals of transistors P 1  and P 2  are connected to the supply VDD while their gates are connected to NET 1 . The PMOS differential amplifier includes PMOS transistors P 3 , P 4 , P 5  and NMOS transistors N 4  and N 5 . P 3  and P 4  are the input transistors as their gates are connected to the inputs IN 1  and IN 2  respectively. The source terminals of P 3  and P 4  are connected to the drain of P 5 . The gate of P 5  is connected to NET 1  while its source is connected to VDD. The drain terminals of P 3  and P 4  are connected to the drains of N 4  and N 5  respectively. 
   Transistors N 4  and N 5  have their source terminals connected together to the ground voltage while their gates are also connected together to NET 1 . P 5  with P 6  and N 6  forms a current source circuit and sources the desired amount of current in the PMOS differential amplifier. P 6  and N 6  have their gates and drains shorted together to NET 1 . The source of P 6  is connected to NET 2  which is the drain of N 1  while the source of N 6  is connected to NET 4  which is the drain of P 3 . P 7  and N 7  form an inverter in which their gates are connected to NET 1  while the source of P 7  is connected to NET 3  which is the drain terminal of N 2  and the source of N 7  is connected to NET 5  which is the drain of P 4 . The drain terminals of transistors P 7  and N 7  is the OUT terminal of the receiver. Inverters INV 1  and INV 2  form a buffer circuit, where Y and X are respectively the true and complement value of the output of the receiver. 
   The transistors from N 8  to N 11  and from P 8  to P 11  are used to provide hysteresis in the receiver. In the NMOS differential amplifier, transistors N 8  and N 9  are connected in the series with each other and in parallel with transistor N 1  while N 10  is in series with transistor N 11  and they are in parallel with transistor N 2 . The gates of transistors N 8  and N 9  are connected to the input IN 1  and true value of the output Y respectively. On other hand, the gate of transistor N 10  is connected to the other input IN 2  while the gate of transistor N 11  is connected to the complement value of the output X. Transistors N 1 , N 8  and N 9  form a branch  21  while transistors N 2 , N 10  and N 11  form branch  22 . 
   In the PMOS differential amplifier, transistors P 8  and P 9  are in series with each other and this combination is in parallel with input transistor P 3 . The gates of P 8  and P 9  are connected to IN 1  and Y respectively. Transistors P 10  and P 11  are in series with each other and are in parallel with transistor P 4 . The gates of transistors P 10  and P 11  are connected to IN 2  and X respectively. Transistors P 3 , P 8  and P 9  form branch  23  and transistors P 4 , P 10  and P 11  form branch  24 . 
   The operation of the circuit can be understood as follows. 
   Case 1: LOW to HIGH Transition 
   Initially IN 1 &lt;IN 2  and OUT=LOW so X=1 and Y=0. In this case, for NMOS differential amplifier, transistor N 9  is OFF, cutting off the current path to transistor N 8 . Also transistor N 11  is ON, hence transistor N 10  is in parallel with transistor N 2 . As IN 1  is less than IN 2 , the resistance of N 2  is less than N 1 . As transistor N 10  is in parallel with transistor N 2 , the effective resistance of branch  22  becomes much smaller than that of branch  21 . As a result, the current through branch  22  is much greater than the current through branch  21 . Now if IN 1  increases and IN 2  decreases, the resistance of N 1  decreases and resistance of N 2  increases. When IN 1  and are equal IN 2 , the resistance of transistors N 1  and N 2  are equal but still X=1 and Y=0, this keeps transistor N 10  in parallel with N 2  and N 8  is cut off, so the effective resistance of branch  22  is still less than branch  21 . On further increase in IN 1 , say IN 1  is just less than VT H  (IN 1 ˜VT H ), the resistance of branch  21  becomes equal to the resistance of branch  22 , the current through the two branches become equal and any slight increase in IN 1  (say IN 1 =VT H ), makes output OUT, HIGH as at this point the current through branch  21  is greater than the current through branch  22 . This makes X=0 and Y=1. This causes transistor N 9  ON and transistor N 11  OFF. This makes transistor N 8  in parallel with transistor N 1  while transistor N 10  becomes ineffective. This further decreases the resistance of branch  21  and increases the resistance of branch  22  thereby further increasing the current through branch  21 . 
   The PMOS differential amplifier of differential input receiver operates similarly. For IN 1 &lt;IN 2  or even IN 1 =IN 2 , the effective resistance of branch  23  is less than that of branch  24 . So current through branch  23  is greater than that thru branch  24 . Only when IN 1 =VT H , the effective resistance of branch  23  becomes greater than that of branch  24 . At this moment the current through branch  23  becomes less than the current through branch  24 , and OUT goes HIGH. This makes X=0 and Y=1 which makes P 9  OFF and P 11  ON. This will further increase the current through branch  24  as compared to the current through branch  23 . 
   Hence a LOW to HIGH transition at the output takes place only when IN 1 =VT H  or greater. 
   Case 2: HIGH to LOW Transition 
   Assuming, IN 1 &gt;IN 2  initially and OUT=HIGH. So X=0 and Y=1. As IN 1 &gt;IN 2 , in NMOS differential amplifier the resistance of transistor N 1  is less than that of transistor N 2 . Moreover as Y=1 and X=0, which makes transistor N 9  ON and transistor N 11  OFF, so that transistor N 8  comes in parallel with transistor N 1  while transistor N 10 &#39;s path is cut off. So the effective resistance of branch  21  is much less than that of branch  22 . Hence the current through branch  21  is greater than that in branch  22  which keeps OUT at HIGH, X=0 and Y=1. 
   If IN 1  decreases and IN 2  increases, the resistance of transistor N 1  increases while that of transistor N 2  decreases. But still X=0 and Y=1, which holds transistor N 9  ON and transistor N 11  OFF. This keeps transistor N 8  in parallel with transistor N 1  while transistor N 10 &#39;s path is cut off. So the effective resistance of branch  21  is still less than that of branch  22  and current through branch  21  is still greater than branch  22 . 
   On further decrement in IN 1 , e.g. IN 1  becomes just less than VT L  (i.e. IN 1 ˜VT L ), the effective resistance of branch  21  becomes equal to that of branch  22  and an equal amount of current flows through both the paths. Any further increase in IN 1 , e.g. at IN=VT L , the resistance of transistor N 1  is further increased and become greater than that of transistor N 2  such that the effective resistance of branch  21  becomes greater than that of path  22 . So the current through path  22  becomes greater than the current through path  21  which makes OUT LOW and X=1 and Y=0. This results in transistor N 9  OFF and transistor N 11  ON. Now transistor N 10  comes in parallel with transistor N 2  while transistor N 8 &#39;s path is cut off. This will further increase the effective resistance of  21  and decreases the effective resistance of  22 . This further increases the current through  21 , and keeps OUT at LOW state. 
   Similarly in the case of the PMOS differential amplifier when IN 1  is greater than IN 2 , and OUT=HIGH, X=0 and Y=1, transistor P 9  is OFF while transistor P 11  is ON. This makes transistor P 10  come in parallel with transistor P 4  while transistor P 8 &#39;s path is cut off. So the effective resistance of branch  23  is much greater than that of branch  24  which results in increased current flow through  24  as compared to  23 . Hence output OUT remains HIGH and therefore X=0 and Y=1. Only when IN 1  is reduced to VT L  i.e. IN 1 =VT L , the effective resistance of  23  becomes less than that of  24  and the current through  23  becomes greater than that in  24 . At this point, output OUT become LOW, X=1 and Y=0. Hence a HIGH to LOW output transition takes place only when IN 1 =VT L  or less. 
   The PMOS transistor P 6  and NMOS transistor N 6  form a potential divider while transistors P 7  and N 7  form an inverter. The trip point of this inverter should be adjusted such that it can detect whatever small variation occurs on NET 1 . When IN 1 &gt;IN 2  and IN 1  is greater than or equal to VT H , the voltage at NET 2  is reduced while the voltage at NET 3  is increased. Also for this case, the voltage at NET 4  is reduced while that at NET 5  is increased. For the potential divider formed by transistors P 6  and N 6 , hence the voltage at NET 2  and NET 4  is decreased, the voltage at NET 1  is reduced. Also, since the gate voltage for the inverter formed by transistors P 7  and N 7  at NET 1  is reduced while the voltage at NET 3  and NET 5  is increased the inverter gives a HIGH output i.e. OUT become HIGH. 
   For the other case, i.e. IN 1 &lt;IN 2  and IN 1  is less than or equal to VT L , the voltage at NET 2  and NET 4  is increased while the voltage at NET 3  and NET 5  is decreased. This results in an increase in the voltage at NET 1  which is the potential divider&#39;s output. As NET 1  is increased and NET 3  and NET 5  are reduced, the output of the inverter formed by transistors P 7  and N 7  gives a LOW output i.e. OUT is LOW, X=1 and Y=0. 
   The inverters INV 1  and INV 2  are used to improve the swing of the differential amplifier and restore the logic levels. The INV 1  and INV 2  provide the complement value of the output X and true value of the output Y respectively. 
   As explained above, the differential input receiver of present invention has two different trip points for two different transition i.e. VT H  for LOW to HIGH transition and VT L  for HIGH to LOW transition, so a noise of width VT H –VT L  can be eliminated. Hence it provides improved noise immunity. 
     FIG. 3  shows the simulation results. It is clear from the graph that the present invention provides an input receiver with hysteresis as compared to the prior art receiver. The differential input receiver of the prior art has only one trip point VT for both HIGH to LOW and LOW to HIGH transitions. VT is basically the crossing point of two inputs signals. Whereas the input receiver according to the present invention provides two trip or switching points, VT H  for LOW to HIGH transition and VT L  for HIGH to LOW transition. This means that the receiver makes a LOW to HIGH transition only when inputs cross at VT H  and HIGH to LOW transition when inputs crosses each other at VT L . Hence a noise of width VT H –VT L  can be eliminated. 
   Thus it is clear that the present invention provides an improved differential input receiver with hysteresis that can operate at low input voltage and over a wide range of input swings. Moreover the different embodiments provide hysteresis characteristics such that the width of hysteresis is almost constant for a wide range of input signal. This makes it possible for the differential input receiver of the present invention to support various differential IO standards e.g. LVDS, LVPECL, HSTL etc. with improved noise margin.