Differential Cross Coupled Peak Detector

A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by any one of the patent disclosure, as it appears in the United States Patent and Trademark office patent files or records, but otherwise reserves all copyright rights whatsoever.

This application is related to the following U.S. patent applications:

All of the related applications are filed on even date herewith, are assigned to the assignee of the present invention, and are hereby incorporated herein in their entirety by this reference thereto.

1. Field of the Invention

The present invention relates generally to electronic circuits. More particularly, the present invention related to voltage peak detector circuits.

2. Description of Related Art

Peak detectors are used in various applications to provide an output voltage equal or representative of the peak voltage of an input signal. These peak detectors are used, by way of example only, in telecommunications applications in which a digital signal is received on a single twisted wire pair. Data must be extracted from the signal. A common method used to extract the data from the signal involves detecting the peak value of the incoming signal while sampling the incoming signal at mid-range intervals.

In U.S. Pat. No. 4,866,301 and U.S. Pat. No. 4,992,674 there is disclosed a peak detector that does not suffer certain disadvantages of peak detectors that came before it. This peak detector, which is discussed in detail below, does not, for example, track errant noise peaks like detectors that came before it, nor does it fail to respond rapidly to falling input signals.

Referring now to FIG. 1, there is shown a peak detector circuit such as that disclosed in U.S. Pat. Nos. 4,866,301 and 4,992,674. This peak detector circuit, generally designated by the reference numeral 10, includes a comparator 12 in which the input signal V.sub.IN is connected to the negative input. The output of the comparator 12 is connected to the gate of a p-channel transistor 14 and also to the gate of an n-channel transistor 16. The source of the p-channel transistor 14 is connected to a charging current source 18, the other end of which is connected to VDD. The source of the n-channel transistor 16 is connected to one end of a discharge current source 20, the other end of which is connected to ground. The drains of the p-channel transistor 14 and the n-channel transistor 16 are connected together and to one end of a holding capacitor 22, the other end of which is connected to ground. The common drain connection of the p-channel transistor 14 and the n-channel transistor 16 is also connected to the gate of an n-channel buffer or isolation transistor 24. The drain of the n-channel buffer transistor 24 is connected to VDD and the source is connected to one end of a third current source 26, the other end of which is connected to ground. The source of the n-channel buffer transistor 24 is also connected to the positive input of the comparator 12 and, further, it forms the peak voltage output signal shown as V.sub.PEAK in FIG. 1.

Operation of the peak detector circuit 10 was described in U.S. Pat. Nos. 4,866,301 and 4,992,674 as follows:

1. When the input voltage V.sub.IN is greater than the peak output voltage, V.sub.PEAK, then the output of the comparator 12 will be at a low voltage level such that the p-channel transistor 14 is conductive and the n-channel transistor 16 is nonconductive. When the p-channel transistor 14 is conductive, then current from the current source 18 puts charge onto the holding capacitor 22 to cause the voltage across the holding capacitor 22 to rise. This voltage on the holding capacitor 22 is decreased by the threshold voltage of the n-channel buffer transistor 24 to form the peak output voltage V.sub.PEAK.

2. When the input signal V.sub.IN is less than the peak output voltage V.sub.PEAK, then the output of the comparator 12 is at a high voltage level such that the p-channel transistor 14 is nonconductive and the n-channel transistor 16 is conductive. Under these conditions the charge on the holding capacitor 22 is discharged through the discharge current source 20 and the voltage across the holding capacitor 22 and, therefore, the voltage at the peak voltage output signal V.sub.PEAK is decreasing.

Although the peak detector disclosed in U.S. Pat. Nos. 4,866,301 and 4,992,674 possesses a number of advantages relative to its prior art, it is not without its shortcomings. For example, it cannot react properly to differential alternate mark inversion ("AMI") coding schemes, that is schemes in which zeros are coded as no pulses and ones are coded as alternative differential positive and negative pulses. That is to say, the peak detector disclosed in U.S. Pat. Nos. 4,866,301 and 4,992,674 cannot properly react to a situation where a differential positive pulse is followed by a differential negative pulse in tracking an AMI signal.

Based upon the foregoing, it should be understood and appreciated that it is a shortcoming and deficiency of the prior art that there has not heretofore been developed a peak detector that can track a fully differential input.

The present invention overcomes the aforementioned shortcomings and deficiencies of the prior art by providing a circuit for processing an AC signal having a peak to peak envelope associated therewith, this circuit including structure for detecting the upper edge of the peak to peak envelope of the AC signal, structure for detecting the lower edge of the peak to peak envelope of the AC signal, structure for sampling the AC signal at a mid range upper point, and structure for sampling the AC signal at a mid range lower point.

Accordingly, an object of the present invention is to provide a peak detector that can track a fully differential input.

Other objects, advantages, and novel features of the present invention can be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings in which:

FIG. 1, previously discussed, is a prior act peak detector;

FIG. 2 is a peak detector circuit according to the teachings of the present invention;

FIG. 3A illustrates a positive input that could be applied to the circuit of FIG. 2; and

FIG. 3B illustrates a negative input that could be applied to the circuit of FIG. 2.

Referring now to FIG. 2 there is shown a peak detector circuit, generally designated by reference numeral 30, according to the teachings of the present invention. Although there are a number of fundamental differences between the peak detector circuit 30 and the peak detector circuit 10, which differences are discussed in detail hereinbelow, there are also a number of similarities. To describe the structure and operation of the peak detector circuit 30, it is convenient to discuss those portions of it that are similar, or identical, to portions of the peak detector circuit 10.

As a first point of similarity, it may be noted that the peak detector circuit 30 comprises two portions resembling the circuit 30. In FIG. 2, one of those two portions is depicted generally directly above the other. The uppermost portion comprises, in series, a charging current source 32, a p-channel transistor 34, an n-channel transistor 36, and a discharge current source 38. These elements correspond to the elements 18, 14, 16, and 20, respectively, in the circuit 10 depicted in FIG. 1. Furthermore, the elements 32, 34, 36, and 38 are interconnected as are the elements 18, 14, 16, and 20; that is, the source of the p-channel transistor 34 is connected to the source 32, the source of the n-channel transistor 36 is connected to the source 38, and the drains of the two transistors 34 and 36 are connected together and to one end of a holding capacitor 40 and to the gate of an n-channel transistor 42. The capacitor 40 and the transistor 42 correspond to the elements 22 and 24 in the circuit 10 depicted in FIG. 1.

The bottommost portion of the two portions referred to above is similarly configured; with elements 44, 46, 48, 50, 52, and 54 forming it; like elements 32, 34, 36, 38, 40, and 42 form the uppermost portion discussed above; and like elements 18, 14, 16, 20, 22, and 24 form the circuit 10 depicted in FIG. 1.

Now, it is appropriate to discuss a number of fundamental differences between the circuit 30 and the circuit 10. First, there is a difference in input. While the circuit 10 of FIG. 1 had a single input V.sub.IN, the circuit 30 receives two input signals, INPOS and INNEG, examples of which are depicted in FIGS. 3A and 3B, respectively. Second, while the gates of transistors 14 and 16 were connected to the output of a single comparator 12, the gates of transistors 34 and 36, and 46 and 48, are connected to the output of a NOR gate 56, and an OR gate where the inputs are inverted, rather than the outputs, as in a standard NOR gate, respectively. The inputs of the gate 56 are connected to the outputs of comparators 60 and 62; and the inputs of the gate 58 are connected to the outputs of comparators 64 and 66. INPOS and INNEG, previously mentioned, are applied to the comparators 60, 62, 64, and 66 as follows: INPOS is applied to the positive inputs of the comparators 60 and 64; and INNEG is applied to the positive inputs of comparators 62 and 66. The negative inputs of the comparators 64 and 66 have a V.sub.PEAK- signal applied to them; and the negative inputs of the comparators 60 and 62 have a V.sub.PEAK+ signal applied to them. The origins of the V.sub.PEAK- and V.sub.PEAK+ signals are discussed further below.

Whereas the end of holding capacitor 22 opposite the end connected to the drains of transistors 14 and 16 is connected to ground (see FIG. 1), the corresponding ends of holding capacitors 40 and 52 are connected together and to V.sub.REF (See FIG. 2).

Regarding isolation transistor 42 and p-channel transistor 54, while similar to transistor 24 in FIG. 1 insofar as their sources are connected back to the comparators 60, 62, 64, 66 in the form of V.sub.PEAK+ and V.sub.PEAK- signals, respectively (the precise connections discussed hereinabove and clearly depicted in FIG. 2), they differ from transistor 24 insofar as their sources are also connected via voltage dividers, i.e., a series of resistors 68, 70, 72, 74. A point between resistors 68 and 70 is further connected to the positive input of a comparator 76, and a point between resistors 72 and 74 is further connected to the negative input of a comparator 78. In FIG. 2, it may be seen that the resistors 68, 70, 72, 74 are selected so that the signal on the point between resistors 68 and 70 is 50% V.sub.PEAK+, and so that the signal on the point between resistors 72 and 74 is 50% V.sub.PEAK-. It may also be seen in FIG. 2 that INPOS and INNEG are also applied to the positive inputs of the comparators 76 and 78, respectively. The outputs of the comparators 76 and 78 are designated RTDAT and RRDA in FIG. 2.

The circuit of FIG. 2 generally operates as follows:

1) With respect to the upper portion of the circuit 30 depicted in FIG. 2, which portion is concerned with the upper edge of the peak envelope of an incoming AC signal, when V.sub.PEAK+ is less than INPOS, comparator 60 goes high. Since comparator 62 is held low, gate 56 goes low, which makes transistor 34 conductive and transistor 36 nonconductive. Source 32 then charges C.sub.HOLD 40.

2) Also with respect to the upper portion of the circuit 30, when V.sub.PEAK+ is greater than INPOS, comparators 60 and 62 are both low, causing the output of gate 56 to be high. This makes transistor 36 conductive and transistor 34 nonconductive. In this case, C.sub.HOLD 40 is drained to V.sub.REF.

3) With respect to the bottom portion of the circuit 30 depicted in FIG. 2, which portion is concerned with the lower edge of the peak envelope of an incoming AC signal, when V.sub.PEAK- is less than INNEG, comparator 66 goes high. Since comparator 64 is held high, the output of gate 58 is low, which makes transistor 46 conductive and transistor 48 nonconductive. This causes C.sub.HOLD 52 to be drained to V.sub.REF.

4) Also with respect to the bottom portion of the circuit 30, when V.sub.PEAK- is greater than INNEG, then comparator 66 is low. This causes the output of gate 58 to be high, which makes transistor 48 conductive and transistor 46 nonconductive. C.sub.HOLD is then charged.

5) The voltage dividers comprising resistors 68 and 70, and 72 and 74, operate with comparators 76 and 78, respectively, by producing high and low outputs as follows. When INPOS is less than 1/2 V.sub.PEAK+, RTDAT is low. When INPOS is greater than 1/2 V.sub.PEAK+, RTDAT is high. Similarly, when INNEG is greater than 1/2 V.sub.PEAK-, the output of comparator 78, RRDA, is high. On the other hand, when INNEG=1/2 V.sub.PEAK-, RRDA is low.

Based upon the foregoing, those skilled in the art should now fully understand and appreciate how the circuit 30 detects the peak to peak envelope of an incoming AC signal. The circuit 30 tracks the lower, as well as the upper, edge of such a signal. This capability provides enhanced ability to react properly to a differential AMI signal.

Obviously, numerous modifications and variations are possible in view of the teachings herein. Accordingly, within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.