Abstract:
Embodiments of the invention describe a method and apparatus for detecting valid differential signals with half the number of differential amplifiers required by conventional methods. By purposely mismatching an otherwise matched differential pair, a self-induced DC offset voltage is created and the additional circuitry required to generate external reference voltages according to conventional methods is eliminated. Embodiments of the invention also have improved noise rejection characteristics and enhanced high-speed capability compared to conventional circuits.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This disclosure pertains generally to a detection circuit and, in particular, to a method and apparatus for detecting differential signal levels using a circuit having a self-induced DC offset. 
     2. Description of the Related Art 
     FIG. 1 is a circuit diagram of a conventional differential signal detection circuit  100  described in U.S. Pat. No. 6,194,965 B1 issued to Kruczkowski et al. on Feb. 27, 2001 (hereafter the &#39;965 patent) and assigned to Cypress Semiconductor Corp. 
     The circuit  100  includes a differential amplifier stage  102  and an output circuit  104 . The output circuit  104  includes a differential buffer  142 . The differential amplifier stage  102  includes a logic circuit  130  composed of a differential OR gate  132 . 
     The differential amplifier stage  102  of circuit  100  uses four matched differential amplifiers. One of the four matched differential amplifiers is composed of resistors R 1 , R 2 , and R 5 ; field-effect transistors (FETs) Q 1  and Q 2 , and a current source  12  to bias the transistors Q 1  and Q 2 . The other three matched differential amplifiers in differential amplifier stage  102  have the same structure but are made from other components. The top two of the matched differential amplifiers make up a pair circuit  106  and the other two make up a pair circuit  108 . 
     The circuit  100  requires external references VCM_HI and VCM_LO, to perform the signal detect process. The current source  11  and the resistors R 5 , R 6  create common mode voltage VCM_HI in the pair circuit  106 , while the current source  15  and the resistors R 11 , R 12  create another common mode voltage VCM_LO in the pair circuit  108 . Four comparisons are made to perform the signal detect process in circuit  100 , one for each matched differential amplifier. Signals taken from nodes Z_N, Z_P, Y_N, and Y_P from the pair circuits  106  and  108  form the inputs to the differential OR gate  132 . 
     Embodiments of the invention improve upon the conventional circuit by eliminating the need for the common mode voltages VCM_HI and VCM_LO, which decreases noise susceptibility. Embodiments of the invention also improve upon the conventional circuit by reducing circuit area and power consumption. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention provide a differential signal detection device and an indication circuit. A particular embodiment includes at least two differential amplifiers that are purposely mismatched so that one leg of each amplifier is different than the other leg in the same amplifier. This gives the differential amplifiers a non-symmetric characteristic. 
     Other embodiments of the invention provide a data bus signal level detection system. The system includes first and second input terminals coupled to first and second signal lines of a data bus, and first and second differential amplifiers coupled to both the first and second input terminals. An indication circuit produces signals based upon the output from the first and second differential amplifiers. The differential amplifiers each have a first leg with a first set of components and second leg with a second set of components, but the first and second set of components do not match. 
     A method for detecting differential signals is also provided. First and second differential input signals are applied to purposely mismatched differential amplifiers. The differential amplifiers are mismatched because the leg of one branch of the amplifier is non-symmetric when compared to the other leg of the amplifier. The output signals from the differential amplifiers are then compared in a logic circuit that produces an output based on that comparison. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional differential signal detection circuit. 
     FIG. 2 is a circuit diagram for a differential signal detection circuit according to an embodiment of the invention. 
     FIG. 3 is an equivalent circuit model for the circuit shown in FIG.  2 . 
     FIG. 4 is a timing diagram illustrating the relationship between input and output voltage levels for the circuit shown in FIG.  2 . 
     FIG. 5 is a block diagram that illustrates a system incorporating the differential signal detection circuit shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention provide at least two purposely non-symmetric differential amplifier circuits, each an offset that unbalances the symmetry of an otherwise matched differential amplifier. A pair of signals with a differential-mode component is respectively applied to the differential amplifier inputs. By comparing the output of the differential amplifier circuits with a differential logic circuit, embodiments of the invention detect the amount that the input signals oppose one another. In some applications, such as for USB circuits, data is validated only if the signals oppose one another by a predetermined amount. By setting the amount of offset in the pair of differential circuits, any amount of signal opposition can be effectively measured. Embodiments of the invention additionally allow for the differential circuits to be tuned, providing a high degree of precision. 
     FIG. 2 is a circuit diagram for a differential signal detection circuit  210  according to an embodiment of the invention. Although embodiments of the invention can be tailored to measure a differential signal from data lines of a Universal Serial Bus (USB) line, for instance, the invention has a broader application than only that application. 
     The differential signal detection circuit  210  shown in FIG. 2 includes unmatched differential pair sets  212  and  214 , a logic circuit  218 , and a differential buffer  220 . In this embodiment, the logic circuit  218  is a differential OR gate  216 . Other differential logic gates or combinations of gates may be used, depending on the desired application. 
     The unmatched differential amplifier  212  includes resistances R 1  and R 2 , a source resistance Rs 1 , a current source I 1 , and transistors m 1  and m 2 . The unmatched differential amplifier  214  similarly includes resistances R 3  and R 4 , a source resistance Rs 2 , a current source  12 , and transistors m 3  and m 4 . The transistors m 1 , m 2  and m 3 , m 4  implement matched differential pairs. Although in FIG. 2 the differential pairs m 1 , m 2  and m 3 , m 4  are formed of NMOS Field Effect Transistors (FETs), any type of transconductance device can be used. For example, some other devices that might be used include PMOS FETs, NPN BJTs (Bi-Polar Junction Transistors), or PNP BJTs, etc. In this embodiment, the resistances R 1 , R 2 , R 3 , and R 4  are illustrated as passive loads, but alternative embodiments may utilize active loads. For example, resistances R 1 , R 2 , R 3 , and R 4  may be implemented by transistors operating in their resistive zone. This would allow dynamic control of the resistances during circuit operation, if desired. In this embodiment, the unmatched differential amplifiers  212 ,  214  would otherwise be matched differential amplifiers if not for the addition of the source resistances Rs 1 , Rs 2  in one of their branches. Adding the source resistances Rs 1 , Rs 2  gives the mismatched differential amplifiers  212 ,  214  a non-symmetric characteristic. Similar to the other resistances, the source resistances Rs 1  and Rs 2  can be made from active devices. 
     The drains of the FETs m 1 , m 2 , m 3 , and m 4  are coupled to an operating voltage Vcc through resistances R 1 , R 2 , R 3 , and R 4 , respectively. The current sources I 1 , I 2  are respectively coupled between the branches of unmatched differential amplifiers  212 ,  214  and a ground reference voltage Vgnd. A nominal input signal INn is coupled to the gates of FETs m 2  and m 4 , while an opposite phase input signal INp is coupled to the gates of FETs m 1  and m 3 . A set of circuit nodes Zn, Zp, Yn, and Yp function as inputs to the differential OR gate  216 . 
     In an example embodiment of the detection circuit  210 , the resistances R 1 , R 2 , R 3 , R 4  are substantially equal and have values between 3-5 kΩ. The current sources I 1  and I 2  produce substantially equivalent currents between 200 and 500 μA. The source resistances Rs 1 , Rs 2  are substantially equal and have values between 100-250 Ω. However, in other embodiments of the invention the source resistances Rs 1 , Rs 2  may be unequal, and, of course, the other values may be different. 
     The addition of a source resistance Rs 1  and Rs 2  to otherwise matched differential amplifiers to form unmatched differential amplifiers  212 ,  214  creates a self-induced DC offset, or “internal” reference voltage. With a self-induced DC offset there is no need to generate external reference voltages, as the differential amplifiers are self-compensating. For example, a comparison of FIGS. 1 and 2 reveals that the source resistance Rs 1  in FIG. 2 eliminates the need for current source I 1  and the voltage bias resistors R 5  and R 6  in the pair circuit  106  of FIG.  1 . Similarly, the presence of source resistance Rs 2  in the unmatched differential amplifier  214  of the detection circuit  210  eliminates the need for the current source  15  and bias resistors R 11  and R 12  in the pair circuit  108  (FIG.  1 ). The self-induced DC offset voltage of the detection circuit  210  of FIG. 2 also eliminates half of the number of differential amplifiers required by previous detection circuits, and in turn, reduces the power supply current and circuit area required. 
     The difference between the input signals INn and INp is called the differential-mode of the input. The common-mode of the input signals INn and INp is the average value of the two signals. Pure differential-mode signals have equal magnitude but opposite polarity at all times. In general, differential amplifiers respond in different ways to the differential-mode and common-mode components of its input signals. 
     In operation, the detection circuit  210  of FIG. 2 detects when the applied differential input signals INp and INn have a pre-determined amount of offset. If the difference between the input signals INp and INn is greater than the pre-determined offset, then one pair of differential amplifier outputs Zn, Zp or Yp, Yn will also have a difference greater than the pre-determined offset. Differential OR gate  216  checks this condition, and returns OUTp at a high voltage level if that condition is met. Differential buffer  220  buffers the output from differential OR gate  216 . 
     For instance, with reference to FIG. 2, if the input signal INp is HIGH and the signal INn is LOW, then, in the differential amplifier  212 , the transistor m 1  tends to be turned ON while the transistor m 2  remains OFF. However, the input signal INp must be high enough to overcome the built in mismatch caused by the resistance Rs 1 . If the signal INp is high enough, the transistor m 1  turns ON and node Zn is pulled LOW while node Zp remains HIGH, because it is coupled to Vcc through the resistor R 2 . Similarly, in the differential amplifier  214 , the HIGH signal INp turns on transistor m 3  while the LOW signal INn turns the transistor m 4  OFF. In the differential amplifier  214 , the mismatch caused by the resistance Rs 2  does not prevent the transistor m 4  from remaining OFF. Thus, the node Yp is LOW and the node Yn is HIGH. These signals from the nodes Zn, Zp, Yp, and Yn are routed through the differential OR gate  216 , which generates the proper output signal. In general, if the input signal INp is higher than the input signal INn by the threshold amount caused by Rs 1 , or if the input signal INn is higher than the input signal INp by the threshold amount caused by Rs 2 , then the detection circuit  210  generates a signal that indicates the input signals are valid. 
     There are many other ways to induce an internal DC offset besides using source resistances Rs 1 , Rs 2  as shown in FIG.  2 . The same non-symmetric effect may be achieved by mismatching the FET or BJT differential pairs (m 1 ≠m 2 , m 3 ≠m 4 ). A self-induced internal DC offset may alternatively be accomplished by mismatching the passive loads (R 1 ≠R 2 , R 3 ≠R 4 ), or, in other embodiments, mismatching the active loads. Alternatively, the gate voltages for the matched differential pairs could be mismatched. 
     In the embodiment illustrated in FIG. 2, the sources of the otherwise matched differential pairs are mismatched by adding a source resistance Rs 1 , Rs 2 . However, nonsymmetry may also be achieved by mismatching the drain side of the FETs m 1 , m 2  and FETs m 3 , m 4 . In embodiments that utilize BJTs, mismatching the bases, collectors, or emitters may create a self-induced DC offset as well. 
     Detection circuits according to embodiments of the invention, such as the detection circuit  210  shown in FIG. 2, improve upon the conventional circuit in other ways besides a reduction in power supply current and circuit area. For example, parasitic loading is reduced and input signals can propagate through the detection circuit  210  of FIG. 2 quicker than previous detection circuits. Thus, the detection circuit  210  has an improved bandwidth and is well-suited for high speed applications. Inducing an internal DC offset voltage, rather than using external voltage references, also allows the inherently beneficial properties of the differential amplifiers to be utilized—properties such as high power supply noise rejection ratios and high common mode noise rejection characteristics. As a result, fewer false triggers occur. An additional benefit achieved by reducing the number of differential pairs compared to conventional circuits is that it simplifies the task of matching differential amplifiers. 
     Uses for embodiments of the invention are numerous. For example, the embodiment shown in FIG. 2 can detect a valid/invalid signal level for a transmitted differential signal. When the voltage amplitude of the differential signal exceeds the self-induced offset voltage of the detection circuit  210 , the output circuit produces an output signal to indicate that the input data is valid. Conversely, if the voltage amplitude of the differential signal does not exceed the self-induced offset voltage, the output circuit produces an output signal that indicates the input data is not valid. Thus, the detection circuit  210  indicates valid signal activity from the differential inputs or differential signal loss over a period of time. 
     By implementing an opposite phase differential pair and a post-detection OR function, data may be peak detected. The signal detect indication is produced when the OR&#39;d input signals exceed a differential voltage level threshold of a transmitted differential signal. The differential circuit  210  can also be used to indicate that data is invalid when the amplitude of a USB differential signal at a receiver&#39;s inputs falls below a squelch threshold. 
     FIG. 3 is an equivalent circuit model  222  of the detection circuit  210  shown in FIG. 2, and helps to further explain the operation of the detection circuit. The equivalent circuit  222  includes the same elements as circuit  10 , except that the source resistances Rs 1  and Rs 2  of FIG. 2 have been replaced by equivalent DC offset voltages Vos 1  and Vos 2 , respectively. These DC offset voltages Vos 1  and Vos 2  are illustrated as being coupled to the gales of the FETs m 1  and m 4  in FIG.  3 . In FIG. 2, when the voltages at the gates of the differential pairs  212 ,  214  are equal, the anti-symmetric source resistances Rs 1  and Rs 2  force unequal currents through the differential amplifiers  212 ,  214 . Likewise, in FIG. 3, the DC offset voltages Vos 1 , Vos 2  force unequal currents through the differential pairs  212 ,  214 . If the source resistances Rs 1 , Rs 2  in FIG. 2 are equivalent to one another, then the DC offset voltages Vos 1 , Vos 2  in FIG. 3 are equivalent to each other as well. A threshold is reached when the voltages of the signal inputs INn and INp differ by the amount of the offset voltage introduced by Vos 1  and Vos 2 , because at that point the currents through the branches of one of the differential amplifiers  212 ,  214  are equal. 
     FIG. 4 is a timing diagram illustrating the voltage relationships between the signals INn, INp, Zn, Zp, Yn, Yp, OUTn, and OUTp for the detection circuit  210  shown in FIG. 2, and the equivalent detection circuit  222  shown in FIG.  3 . In FIG. 2, the source resistances Rs 1 , Rs 2  are equal, so the DC offset voltages Vos 1 , Vos 2  of FIG. 3 are equivalent as well. Thus, the DC offset voltages Vos 1 , Vos 2  of FIG. 3 are illustrated in FIG. 4 as a single offset voltage Vos. 
     The applied differential input signals INn, INp are illustrated at the top of FIG.  4 . Between time A and time B, the differential-mode of the input signals INn, INp is zero, because the difference between INn and INp is zero. Between time B and time F, the input signals INn, INp switch polarity several times, but their differential-mode (except for quick transitions at times C, D, and E) is at the same level as the DC offset voltage Vos. At time F, the differential-mode of the two input signals INn, INp exceeds the DC offset voltage Vos, and the differential-mode remains higher than Vos (except for quick transistions at times G, H, I, and J) through time K. 
     Zn, Zp and Yn, Yp are the outputs of the mismatched differential amplifiers  212 ,  214  in FIG.  2 . Between times A and B, the inputs INn, INp are at the same voltage level so the outputs Zn, Zp and Yn, Yp differ by an amount equal to the voltage across the source resistances Rs 1  and Rs 2 . 
     Between times B and F, the differential-mode of the input signals INn, INp is equal to the DC offset voltage Vos. The DC offset voltage Vos is the voltage difference applied to the transistors of a differential amplifiers  212 ,  214  that is required to force equal currents through each side of one of the differential amplifiers. The particular differential amplifier  212 ,  214  where this occurs depends on the polarity of the input signals INn, INp. When INp is positive with respect to FNn in an amount equal to the Vos, equal currents flow through the differential amplifier  212 , so the output voltages of Zn, Zp are be the same as well (FIG. 4, time B to time C). On the other hand, this only increases the voltage difference between outputs Yn, Yp of differential amplifier  214 . Conversely, when INn is positive with respect to INp in an amount equal to the Vos, differential amplifier  214  has equal currents and, as a result, no voltage difference exists between the outputs Yn, Yp (time C to time D). 
     After time F, the differential-mode of the inputs INn, INp exceeds the DC offset voltage Vos, and the currents through the branches of the differential amplifiers  212 ,  214  are no longer equal like they were between time B and time F. The differential-mode of the inputs INn, INp are now more than enough to overcome the offset voltage Vos so the additional voltage difference drives the differential amplifier outputs Yn, Yp and Zn, Zp further apart. This is seen by comparing the outputs for nodes Yn, Yp, Zn, and Zp for time segment B-C to the outputs in time segment F-G. In both cases, INp is positive with respect to INn. However, in time segment F-G the differential-mode of inputs INn, INp exceed Vos by an amount a. The differential amplifier outputs Zp, Zn and Yn, Yp in time segment F-G exceed the differential amplifier outputs Zp, Zn and Yn, Yp in time segment B-C by the same amount a, assuming an amplifier gain of 1. A similar result is seen when comparing time segment G-H to time segment C-D (INn is positive with respect to INp). Of course, safety margins may be built into the differential amplifiers  212 ,  214  such that they amplify the incoming signals by a larger amount than absolutely necessary, in order to ensure the correct output of the detection circuit for most operating conditions. 
     The output signals OUTn, OUTp are illustrated at the bottom of FIG.  4 . The output signals OUTn, OUTp are taken from the differential OR gate  216  that has inputs from the nodes Yn, Yp, Zn, and Zp. The differential OR gate  216  compares the difference between the signals Yn, Yp and Zn, Zp to the DC offset voltage Vos. If the difference between Yn and Yp or Zn and Zp is greater than the DC offset voltage Vos, OUTp is at a high state (times F through K). If the difference between Yn and Yp or Zn and Zp is less than the DC offset voltage, OUTp is in a LOW state (times A through F). OUTn is simply the inverse of OUTp. From time A to time F in FIG. 4, neither Yn, Yp or Zn, Zp have a difference greater than the DC offset voltage Vos. Thus, OUTp remains low. Then, at time F through time K, the difference of either Yn, Yp or Zn, Zp remain greater than the DC offset voltage Vos. In response, OUTp transitions to a high state at time F and remains there through time K. 
     FIG. 5 illustrates an example system that includes several detection circuits  210 . A USB hub microcontroller  320  receives data from four different USB peripheral devices: a keyboard  240 , a mouse  260 , a joystick  280 , and a printer  300 . Each device is connected to a respective port  340  of the USB hub microcontroller via a USB connection cable  380 . As is well known in the art (and not specifically illustrated in FIG.  5 ), a USB connection cable has four lines for carrying voltage signals—two differential signal lines used for data, a power line, and a ground line. The two USB differential signal lines from the individual cables  380  are tapped at ports  340  and become the differential inputs INn and INp for the connected detection circuits  210 . The detection circuits  210  in FIG. 5 can be the same as the circuit  210  in FIG.2, for instance. The outputs of the circuit  210  are coupled to a processor  360 , along with the original differential inputs from the ports  340 . The processor  360 , using the outputs from the circuits  210 , can then detect when valid data signals are being sent from the peripheral devices. 
     In operation, data from one of the peripheral devices is sent through its connected cable  380  to the port  340 . As described above, the data lines from the cable  380  couple directly to the processor  360 . Additionally, the data lines couple to the detection circuit  210  as inputs INp and INn. As data is sent from a peripheral device, the detection circuit  210  determines if the signal has a large enough degree of opposition to be considered valid data. If the detection circuit  210  determines that the signals are appropriate and within the predetermined specifications for acceptable data transmission, a proper signal is generated at its output, and is communicated to the processing device  360 . When the processing device receives the signal, it recognizes the incoming data as valid data and acts on it. If instead the detection circuit  210  determines that the signals are not within the specifications, another type of signal is generated and communicated to the processing device  360 . When the processing device receives this second type of signal, it does not recognize the data sent by the peripheral device as valid, and ignores such data. 
     Although a preferred embodiment and several alternative embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that the teachings of this disclosure can be extended to encompass other embodiments not previously discussed. As such, the embodiments of the invention should not be considered limited in any way except by the depth and scope of the following claims.