Differential receiver

[0001] The present application claims priority from Indian Patent Application No. 1133/Del/2004 filed Jun. 15, 2004, the disclosure of which is hereby incorporated by reference.

[0002] 1. Technical Field of the Invention

[0003] The present invention relates to an improved differential receiver.

[0004] 2. Description of Related Art

[0005] Techniques have been developed for facilitating high-speed electrical communication over buses. High-speed communication uses low voltage swing signals that require a differential signaling technique.

[0006] To support differential signaling in integrated circuits (IC's), the circuit is designed to convert internal full swing signals to external low voltage differential signals and external low voltage differential signals to core acceptable signals. Generally differential receivers are used to accept external low voltage swing differential signals, but some noise is induced in the differential receivers at the time of reception that leads to power dissipation in the circuit.

[0007] FIG. 1 shows a circuit schematic of a conventional differential receiver circuit used in differential signaling applications. In FIG. 1, N11 and N12 are input NMOS transistors having their gates connected to differential input pads PAD and PADbar. PMOSs P11 and P12 are load transistors. NMOSs N13 and N14 and resistor R11 form a current sink.

[0008] The operation the differential amplifier is not explained herein. Details of one such differential amplifier is explained in book “CMOS Circuit Design, Layout and Simulations” by R. Jacob Baker, Harry W. Li and David E. Boyce.

[0009] In the schematic, reference 100 is not a part of the differential receiver, but represents an electrical parasitic appearing between I/O pads and I/O pins due to packaging. RT is a terminating resistor placed outside the device. It is shown here just for reference.

[0010] To get a good high-speed performance, the differential receiver shown is generally provided with high current, high gain and high bandwidth. But such a receiver will introduce propagation of noise and glitches because the receiver is so fast that even a small and short time disturbance at the input is amplified and presented at the output. So the main problem with the differential receiver shown is its noise susceptibility.

[0011] The majority of the noise problem comes at the time of transition of differential signals. As the operating frequencies of digital electronic devices increases, the signal lines used to route signals between components begin to behave like transmission lines because of the faster edge rates of the signals. If the impedance of the transmission line and the receiver are not matched, a portion of the incoming signal is reflected back. Reflections cause distortion in the received signal, which may lead to false interpretation of the logical value of the incoming signal.

[0012] In case of differential input signals, as the transitions are in opposite direction, noise due to reflections is also in opposite directions. That is, if one line of the differential pair is having a noise overshoot, the other line will have an undershoot at the same time. So, in this case noise on the differential pair is differential in nature instead of common mode noise. If sufficiently large, this differential noise is easily accepted by the differential receiver.

[0013] FIG. 2 shows HSpice post-layout simulation results of the prior art receiver in FIG. 1 with package parasitic 100 (package stub). (Here parameters of package parasitic are considered for typical BGA package with L=4.5 nH, R=0.45 ohm, C=1.45 pf). In FIG. 2, IN and INbar are the differential input signals applied having time period of 4 ns and signal swing of 0.4V. PAD and PADbar are the actual input waveforms looked by the differential receiver. OUT1 is the output of the differential receiver. It can be seen that the noise is differential in nature instead of being common mode and because of this, there are glitches on output signal OUT1.

[0014] A resistive “termination” technique is often applied to reduce signal reflections. One of the resistive “termination” techniques used for differential signals is shown in FIG. 1. Here a terminating resistor RT is placed external to the IC, between the differential lines. But the external termination does not take into consideration the package stub 100. The package stub 100 may cause the external input signals IN/INbar to ring and thus the actual input signal looked by the receiver at nodes PAD/PADbar is distorted.

[0015] FIG. 3 shows the simulation results of FIG. 1, with noisy input at PAD and PAD bar. Output OUT1 shows a glitch, due to the noise at the input of receiver.

[0016] One solution to this can be provided by using on-chip termination instead of external termination. Here termination resistor RT is provided between PAD and PADbar. Signal quality is improved by the on-chip resistor due to removal of the ‘package stub’ 100. But on-chip termination would be undesirable in multi-point and multi-drop configurations of data transmission.

[0017] A second solution is adding a Schmitt trigger at the input thus introducing DC hysteresis. To add hysteresis characteristics in input receivers, the ranges of low and high level input voltages are changed depending upon the direction of transition in the input signal. Generally adding hysteresis increase the receiver delays and therefore degrades the performance in high-speed I/O operations.

[0018] FIG. 4 shows a prior art differential receiver with hysteresis as suggested in U.S. Pat. No: 5,796,281. The architecture of the receiver of FIG. 4 is the same as that of FIG. 1 except that NMOS N3 is used to provide hysteresis. The gate of N3 is connected to the output OUT1. Here the sizes of N11 and N12 are not same. Resistance offered by N11 in parallel with N3 (when OUT1 is at logic high, means N3 is on) should be equal to the resistance offered by N12. So the size of N11 is smaller than that of N12. When IN1=0, IN2=1, OUT1=0, transistor N3 is off. When IN1 rises from 0 to 1 and IN2 falls from 1 to 0, current starts flowing through N11, and hence through P11, which mirrors same current to P12. Current flowing through P12 charges the load at OUT1. Since the size of N11 in FIG. 4 is smaller than the size of N11 in FIG. 1, it reduces the charging current of the load, hence increase the rising delay.

[0019] When IN1=1, IN2=0, OUT1=1, transistor N3 is on. When IN1 falls from 1 to 0, conduction of N11 decreases and conduction of N12 increases. Load at OUT1 discharges through N12. In FIG. 1, when IN1 starts falling from 1 to 0, voltage at net1 increases, hence turns P12 off. But in FIG. 4, an increment in voltage level of net1 is slower because of the transistor N3 that is on. So the discharging of load at OUT1 will be slower as compared to the case of FIG. 1.

[0020] FIGURES and 6 show the dc analysis and transient analysis results of FIG. 4 that depict an increase in the propagation delay of the receiver of the prior art.

[0021] So, the receiver with hysteresis increases the noise immunity but sufficiently increases the delays, which restricts the high frequency operation of receiver.

[0022] Therefore, there is a need for a circuit with improved noise protection for differential signaling applications as in a differential receiver, while providing minimized power dissipation.

[0023] Embodiments of the present invention provide an improved differential receiver having noise protection. Feedback control circuitry for the differential receiver functions to reduce bandwidth and current in the differential receiver. The receiver further provides minimum power dissipation by noise protection.

[0024] An embodiment of the improved differential receiver comprises a feedback circuit connected between an output node and one common node of the differential receiver to reduce the bandwidth and reject noise for a specific interval of time. The feedback circuit comprises a pulse generator circuit whose output is connected to a control bias current circuit and to a current mirror circuit in series. The pulse generator circuit may comprise a delay element and an Ex-OR gate, and the output of said delay element is connected to the input of said Ex-OR gate.

[0025] The Ex-OR gate may comprise an inverter.

[0026] The delay element may comprise a chain of inverters or a complex delay circuitry.

[0027] The control bias current circuit may comprise at least one transistor powered by a higher voltage supply.

[0028] The current mirror circuit may comprise at least two transistors connected to each other at their control nodes. The transistors may comprise P-MOS or N-MOS transistors.

[0029] An embodiment of the present invention further provides a method for rejecting noise signals by differential receiver comprising detecting an input signal by the differential receiver and outputting a response from the differential receiver to a feedback circuit for reducing the current flowing through the differential receiver and thereby reducing the bandwidth of the differential receiver to reject noise for a specific interval of time. This accordingly provides an improved differential receiver that minimizes power dissipation by reducing noise.

[0030] In accordance with an embodiment of the invention, a differential receiver comprises a pair of differential input transistors coupled at a common node and whose gates receive a differential input signal and a pair of load transistors coupled to the pair of differential input transistors to form at least one output node. A current sink circuit is coupled to the common node of the pair of differential input transistors, and a feedback circuit is coupled between the output node and the current sink circuit.

[0031] In accordance with another embodiment, a method comprises differentially receiving a signal to generate an output signal, and controlling differential receiver bias current responsive to the output signal.

[0044] FIG. 7 shows a circuit diagram of a differential receiver according to an embodiment of the present invention. The differential receiver shown here differs from the prior art in the way that it uses a feedback control circuitry for generating Ibias. The rest of the circuitry is similar to the prior art discussed above. Output OUT3 of the differential receiver is fed to an edge triggered pulse generator 301. The output of the generator 301 controls the gate of PMOS P33, which in turn controls the current Ibias of the current mirror circuitry. The edge triggered pulse generator 301 has a delay element D31, providing delay of time ‘Td’ and an EX-OR gate G31. Edge triggered pulse generator 301 generates a pulse of width ‘Td’ for every transition at its input.

[0045] It is apparent that delay element D31 can be realized in number of ways, from simple chain of inverters to more complex delay circuitry with process, temperature and voltage compensations.

[0046] Differential receiver in FIG. 7 propagates any change in logic state of differential input signals at PAD/PADbar to the output OUT3. Transition at OUT3 triggers edge triggered pulse generator 301 to generate a positive pulse of period ‘Td’. The positive pulse will switch off the PMOS P33, blocking current Ibias. The blocking of current Ibias extremely reduces the current through current sink NMOS N33 and therefore extremely reduces the bandwidth of the differential receiver. (NMOS N33 will operate in its sub-threshold region, allowing current of the order of few microamperes).

[0047] So, once the receiver detects the input transitions, the bandwidth of the receiver is extremely reduced for predetermined period of time ‘Td’. For this period of time the receiver rejects any noise disturbances at the input and power dissipation is lowered. Also, since during input transitions its current and bandwidth remains high, the high speed performance of the receiver remains unaffected.

[0048] It is to be noted that pulse period ‘Td’ is kept less than the minimum time interval between two consecutive transitions which possibly could arrive at the input of receiver. Also, output OUT3 of the receiver can be latched for the time when Ibias is blocked.

[0049] In the circuit according to a first embodiment of the invention shown in FIG. 7, ideally it is desired that current Ibias should be blocked immediately after the differential input transitions appears at the output OUT3. But in this case the delay of the edge triggered pulse generator 301 is added between a transition at output OUT3 and blocking of current Ibias.

[0050] FIG. 8 shows circuit diagram of a differential receiver according to a second embodiment of the invention. In this embodiment the delay between the transition at the output of the receiver and the blocking of the current Ibias is reduced by removing the edge triggered pulse generator circuitry 301 and instead implementing substantially the same functionality within the current mirror itself. In FIG. 8, adding series connected PMOS P44 and NMOS N45 in parallel with another series connected PMOS P43 and NMOS N46 in the Ibias current path modifies the current mirror circuitry. The gates of PMOS P44 and NMOS N46 are controlled directly by the output OUT4 of the differential receiver, while the gates of PMOS P43 and NMOS N45 are controlled by a delayed-inverted version of signal at OUT4. The delay element D41 provides a delay of time ‘Td’. The rest of the circuitry is similar to that of the first embodiment.

[0051] In FIG. 8 it can be seen that there are two parallel paths for bias current Ibias. One through PMOS P44 and NMOS N45 and other through PMOS P43 and NMOS N46. In steady state one of the two parallel paths is open and other is closed, thus allowing current Ibias to flow. During transitions at output OUT4 both the paths get open for predetermined period of time ‘Td’. So, for this period of time the current Ibias is blocked. It is to be noted that the resistance provided by both of the paths, when conducting, is exactly equal.

[0052] To understand the operation of the circuitry shown in FIG. 8, we consider a steady state condition with output OUT4 at logic low. For this state PMOS P44 and NMOS N45 will be on and PMOS P43 and NMOS N46 will be off. So in this case the path through P44 and N45 will be closed allowing current Ibias. The path through P43 and N46 will remain open. Now when output OUT4 transitions from logic low to logic high, P44 turns off immediately, opening the path through P44 and N45. As PMOS P43 remains off because of the delay ‘Td’, current Ibias is blocked. At the same time NMOS N46 is switched ‘on’ for low to high transition at OUT4. After a time delay ‘Td’ PMOS P43 also turns on, closing the path through P43 and N46. Thus current Ibais, which is blocked for predetermined period of time ‘Td’, is again allowed to flow. Now when output OUT4 again transitions from logic high to logic low, the same process is followed with NMOS N46 turning off immediately and NMOS N45 turning on after a delay of ‘Td’.

[0053] FIG. 9 shows HSpice post-layout simulation results of the differential receiver in FIG. 7 with the same package parasitic as that in FIG. 1 (package parasitic not shown in FIG. 7 but connected exactly similar to that in FIG. 1). In FIG. 9, IN and INbar are the differential input signals applied having time period of 4 ns and signal swing of 0.4V (pins IN and INbar are not shown in FIG. 4). PAD and PADbar are the actual input waveforms looked by the differential receiver. OUT4 is the output of the differential receiver according to second embodiment of the invention. Waveform of current Ibais is also shown in FIG. 9. It can be seen that although noise still persist there are no more glitches on output signal OUT4. It is also to be observed that current Ibias is blocked for time ‘Td’ immediately after the output OUT4 transitions and remains high during the transitions of differential input signals.

[0054] It seems that the solution is time dependent and will effectively lock noise and glitches for some predetermined period of time and when this period of time is exhausted, any additional noise or glitch will still be propagated through. But in the case of differential signals, as explained earlier in background of invention section, the majority of noise problem comes at the time of transitions.

[0055] The cause of false transitions and glitches in differential receivers is differential noise, which is mainly in aftermath of transitions and remains for sometime after the transitions. In the present invention this differential noise is locked for predetermined period of time Td by blocking current Ibias just after the input transitions. When this period of time ‘Td’ is exhausted and inputs are stable (not transitioning), noise coupling to the differential pair will be common mode noise which may have a differential residue due to unequal distance from aggressor, unbalance in the differential transmission line, unbalance source and load impedance. But with good up front floor planning of board layouts, this differential residue can be made very small and thus harmless.

[0056] FIG. 10 shows simulation results of differential receiver of FIG. 7, applied with differential input signals with larger time-period. Here 18 ns time period is taken in order to explain that differential noise remains only for sometimes after the transitions. After that even if time ‘Td’ is exhausted and receiver regains high bandwidth and current, chances of any glitch to appear are negligible because now whatever noise couples will be mainly weak differential residue of common mode noise.

[0057] It can be seen in FIG. 10 that differential noise, caused due to transitions, is blocked for the time it is dangerous, and until the time ‘Td’ is exhausted, it has effectively damped out.

[0058] FIG. 11 shows the output waveform of the present invention of FIG. 7. It can be observed from the figure that the same amount of noise is rejected at the output OUT4, without increasing the propagation delay of the receiver.

[0059] The table given below illustrates the comparative simulation results of the prior art and the present invention.

[0060]

				Whether
	Rising	Falling	Power	glitch
Circuit implemented	delay	delay	Dissipation	comes
FIG. 1 (Without	0.207 ns	0.183 ns	19.38 mW	YES
any noise
protection circuit)
FIG. 4 (With	0.354 ns	0.254 ns	18.32 mW	NO
hysteresis)
FIG. 7 (Present	0.207 ns	0.183 ns	15.70 mW	NO
Invention)

[0061] Thus we can say that the instant invention provides a noise protection circuit with reduced delays and minimized power dissipation.

[0062] Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.