Abstract:
Various methods and circuits for implementing high speed peak amplitude comparison. The invention achieves higher speed of operation by eliminating the slow feedback loop commonly employed in peak detection. In one embodiment, the invention directly compares a signal that represents the peak amplitude of the input signal minus a small voltage drop, to a modified reference voltage. The modified reference voltage corresponds to the reference voltage that is adjusted to compensate for the small voltage drop in the maximum input voltage. In another embodiment, the invention implements a differential version of the peak amplitude comparator to obtain better noise rejection and reduced effective offset among other advantages.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 11/031,102, filed Jan. 6, 2005, (now U.S. Pat. No. 6,888,381) which is a continuation of U.S. patent application Ser. No. 09/969,387, filed Oct. 1, 2001, (now U.S. Pat. No. 7,0249,856) and entitled “HIGH SPEED PEAK AMPLITUDE COMPARATOR,” the entire contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates in general to integrated circuitry, and in particular to various implementations for a high speed peak amplitude comparator.  
         [0003]     There are many circuit applications wherein there is a need to detect the peak amplitude of a received signal. In data communication circuits, for example, the receiver must be able to distinguish between a noise and weak but valid signal at its input. To accomplish this, typically the peak amplitude of the input signal is first measured and then compared to a threshold voltage to determine whether the input signal is a valid signal.  FIG. 1  depicts a typical implementation for a conventional peak amplitude detector  100 . An amplifier  102  and transistor M 1  are connected in a feedback configuration with the input signal Vin being applied to one input of amplifier  102 . As Vin rises and transistor M 1  turns on, the amplitude of the signal Vc (at node  104 ) essentially follows that of Vin. When Vin drops from its peak value, transistor M 1  turns off, but capacitor C 1  maintains the charge at node  104  at the peak value of Vc. Thus, the amplitude of the signal Vc always reflects the peak amplitude of Vin. A comparator  106  is then used to compare the amplitude of Vc with the reference voltage Vref, and generates a binary signal. at its output to indicate whether the amplitude of Vc (=to peak value of Vin) is greater than or smaller than Vref. A current source I 0  is provided to allow capacitor C 1  to discharge in case of random glitches at the input. Current I 0  is made very small relative to the size of capacitor C 1 . As long as an input signal is present, Vin updates the charge stored by capacitor C 1  thus the slow discharge does not result in an appreciable reduction in Vc in the absence of a glitch.  
         [0004]     A drawback of the circuit of  FIG. 1  is that because of the feedback loop its speed of operation is limited. Thus, for very high speed applications such as data communication circuitry in the GigaHz range (e.g., SONET OC192), peak detectors with this type of feedback loop are not suitable. This has created a need for peak amplitude detection circuit techniques that are operable at very high frequencies.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The present invention provides various methods and circuits for implementing high speed peak amplitude comparison. Broadly, the invention achieves higher speed of operation by eliminating the slow feedback loop commonly employed in peak detection. In one embodiment, the invention directly compares a signal that represents the peak amplitude of the input signal minus a small voltage drop to a modified reference voltage. The modified reference voltage corresponds to the reference voltage that is adjusted to compensate for the small voltage drop in the maximum input voltage. As thus constructed, a comparison of the two voltages performs the intended function without the need for a feedback loop. In another embodiment, the invention implements a differential version of the peak amplitude comparator to obtain better noise rejection and reduced effective offset among other advantages.  
         [0006]     Accordingly, in one embodiment, the present invention provides a peak amplitude comparator including an input circuit having an input terminal coupled to receive an input signal, and configured to generate at an output terminal a first signal with an amplitude that is substantially equal to a peak amplitude of the input signal minus a predetermined voltage drop; a reference circuit having an input terminal coupled to receive a reference voltage and configured to generate at an output terminal a second signal with an amplitude that is substantially equal to the reference voltage minus the predetermined voltage drop; and a comparator having a first terminal coupled to receive the first signal and a second terminal coupled to receive the second signal.  
         [0007]     In a more specific embodiment, the input circuit includes: a transistor having a gate terminal couple to receive the input signal, a first source/drain terminal coupled to a logic high power supply and a second source/drain terminal coupled to the output terminal of the input circuit; a capacitor coupled to the second source/drain terminal of the transistor; and a current source coupled to the second source/drain terminal of the transistor, wherein the predetermined voltage drop is substantially equal to a threshold voltage of the transistor.  
         [0008]     In another embodiment, the present invention provides a differential peak amplitude comparator including an input circuit having first and second input terminals coupled to respectively receive differential first and second input signals, and configured to generate at an output terminal a first signal with an amplitude that is substantially equal to a peak amplitude of either of the first and second input signals minus a predetermined voltage drop; a reference circuit having an input terminal coupled to receive a reference voltage and configured to generate at an output terminal a second signal with an amplitude that is substantially equal to the reference voltage minus the predetermined voltage drop; and a comparator having a first terminal coupled to receive the first signal and a second terminal coupled to receive the second signal.  
         [0009]     In yet another embodiment, the present invention provides a method for comparing a peak amplitude of an input signal to a reference voltage, including storing on a first node a first signal having an amplitude that is substantially equal to a peak amplitude of the input signal minus a predetermined voltage drop; applying to a second node a second signal with an amplitude that is substantially equal to the reference voltage minus the predetermined voltage drop; and comparing a magnitude of the first signal to a magnitude of the second signal.  
         [0010]     The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the high speed peak amplitude comparator according to the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  depicts a typical circuit implementation for a conventional peak detector;  
         [0012]      FIG. 2  is a simplified circuit schematic for an exemplary implementation of a peak amplitude comparator according to one embodiment of the present invention;  
         [0013]      FIG. 3  is a simplified circuit schematic for an exemplary implementation of a differential peak amplitude comparator according to another embodiment of the present invention; and  
         [0014]      FIGS. 4A, 4B  and  4 C illustrate the offset behavior of the various embodiments of peak amplitude detectors and comparators described herein. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     To attain higher speed of operation, it is desirable to eliminate the feedback loop that is commonly employed in peak detect circuitry. Referring to  FIG. 2 , there is shown a simplified circuit schematic for an exemplary implementation of a peak amplitude comparator  200  according to one embodiment of the present invention. Peak amplitude comparator  200  includes an input circuit (or input path)  202  and a reference circuit (or reference path)  204 . Input circuit  202  includes a field effect transistor M 2  that receives the input signal Vin at its gate terminal. Transistor M 2  has its drain terminal connected to the positive power supply VDD and its source terminal connected to node  206 . A capacitor C 2  connects between node  206  and ground (or negative power supply VSS depending on the implementation). A current source device I 1  connects in parallel with capacitor C 2  and provides a discharge path for capacitor C 2  to address glitch conditions at the input. Reference circuit  204  includes a field effect transistor M 3  that receives a reference signal Vref at its gate terminal. Transistor M 3  has its drain terminal connected to VDD and its source terminal connected to node  208 . A current source device I 2 , preferably replicating current source device I 1 , connects between the source terminal of transistor M 3  and ground. A comparator  210  receives node  206  at one input and node  208  at another. The output of comparator  210  provides the output OUT of the circuit. Current source devices I 1  and I 2  may be implemented by a transistor that has its gate driven by a bias voltage. It is to be understood that the specific implementation shown in  FIG. 2  is for illustrative purposes only, and that the invention can be implemented with variations and modifications to this specific embodiment. For example, transistors M 2  and M 3  (or M 4  and M 5  in the embodiment shown in  FIG. 3 ) may be connected to another voltage, and possibly coupled to VDD via another circuit element such as a resistor. Also, some applications may include filtering such as an RC low pass filter at node  206 .  
         [0016]     In operation, Vin turns on transistor M 2  when its magnitude is one Vth greater than the signal level at its source terminal (node  206 ), where Vth is the threshold voltage of transistor M 2 . With transistor M 2  turned on, voltage V 1  at node  206  increases as Vin increases but is lower than Vin by one Vth (i.e., V 1 =Vin−Vth). However, after Vin reaches its peak amplitude, Vinmax, and starts to decrease, transistor M 2  turns off since capacitor C 2  operates to maintain the charge at node  206 . With M 2  turned off, capacitor C 2  holds voltage V 1  constant at value [Vinmax−Vth]. Signal V 1  is, therefore, not the true peak of the input signal Vin, and instead is one Vth lower than the peak. To compensate for this difference, instead of applying reference signal Vref directly to the other input of comparator  210 , the magnitude of Vref is adjusted by reference circuit  204 . Reference circuit  204  includes a circuit that essentially replicates input circuit  202 . Vref is applied to the gate terminal of transistor M 3 , and transistor M 3  is biased by current source I 2 . Signal V 2  at the source terminal of transistor M 3  is thus equal to Vref−Vth where Vth is the threshold voltage of transistor M 3 . It is preferable to use a transistor and a current source device in reference circuit  204  that is of similar size and layout as those in input circuit  202 . Comparator  210  thus compares [V 1 =Vinmax−Vth] at node  206  with [V 2 =Vref−Vth] at node  208 . In this fashion the circuit of  FIG. 2  effectively compares Vinmax with Vref without the use of any feedback loops. This circuit can operate at much higher frequencies compared to the prior art peak detectors of the type shown in  FIG. 1 .  
         [0017]     In an alternative embodiment, the present invention provides a differential implementation for a peak amplitude comparator.  FIG. 3  is a simplified circuit schematic for an exemplary implementation of a differential peak amplitude comparator  300  according to this embodiment of the present invention. Circuit  300  is similar to the single-ended circuit of  FIG. 2  in most respects except for the inclusion of a second transistor in the input path. Thus, the circuit includes a first transistor M 4  that receives the positive half Vinp of the differential input signal and a second transistor M 5  that receives the negative half Vinn of the differential input signal.  FIG. 3  also shows the use of transistors M 7  and M 8  each having its gate driven by a bias voltage Vb as the current source devices. It is to be understood that other types of implementations for current source devices are possible. For example, the current source can be implemented using resistors or cascode connected transistors, and the like.  
         [0018]     The operation of the circuit of  FIG. 3  is very similar to the single-ended peak amplitude comparator shown in  FIG. 2 , except that the differential implementation offers a number of advantages. First, due to the differential nature of the circuit, better noise rejection is obtained. Secondly, this implementation is better in handling a long stream of zeros (logic low level) at the input. With the single-ended approach, a stream of zeros at the input may cause the storage capacitor to gradually discharge through I 1  well below the peak value. With the differential implementation shown in  FIG. 3 , a stream of zeros at one input, say the positive input Vinp, means that the other input, Vinn receives a stream of ones (logic high level). Since node  302  responds to both inputs, capacitor C 3  would remain charged to the peak value of the input signal even with a stream of zeros at Vinp.  
         [0019]     Another advantage of the differential peak amplitude comparator of  FIG. 3  is a significant reduction in offset. In a typical circuit application employing the peak amplitude comparator of the present invention there are a number of sources of offset. The high speed input signal that is received from the transmission line is typically amplified before it is applied to the peak amplitude comparator. Transistor mismatch and amplifier systematic offset as well as offset inherent in the differential signal contribute to the DC offset Vos. Differences between the input path and the reference path as well as transistor mismatch in the comparator ( 304 ) also add to the DC offset Vos. The magnitude and sign of this offset signal Vos varies from chip to chip and depends on the input signal to the chip. Its distribution can be approximated by a bell shaped curve centered around zero as shown in  FIG. 4A . Hence, the peak value of Vinp (i.e., Vinpmax) is also a bell shaped curve with its center at the ideal value when the offset signal Vos equals zero as shown in  FIG. 4B . With the differential implementation, if Vos is negative, Vinpmax is reduced but Vinnmax is increased, and the peak value becomes [Viomax+Vos], where Viomax is the ideal peak value (with no offset) of both Vinp and Vinn. That is, with the differential implementation shown in  FIG. 3 , the two-sided offset distribution is rectified to only the positive side as shown in  FIG. 4C . This leads to a direct reduction in the range of the peak value that is impacted by offset.  
         [0020]     The present invention thus provides method and circuitry for implementing high speed peak amplitude comparators. Two specific embodiments, one single-ended and one differential implementations, have been described wherein peak comparison is accomplished without the need for a feedback loop. While the above provides a detailed description of certain specific embodiments of the invention, various alternatives, modifications and equivalents are possible. For example, the illustrative embodiments shown in  FIGS. 2 and 3  employ metal-oxide field effect transistor (MOSFET) technology. The present invention, however, is not limited to MOSFET technology and other technologies such as bipolar, GaAs or GaAs on silicon and the like may be used to implement the present invention. The scope of the present invention is thus not limited to the specific embodiments described, and is instead defined by the following claims and their full breadth of equivalents.