Abstract:
A DC coupled serial data stream receiver system utilizing a switched capacitor based differencing front end which compares the instantaneous value of an analog voltage with respect to its long term minimum value to a series of reference voltages V 1  to V n  in a flash analog to digital converter style front end. The circuit is designed to interface directly to a discrete fiberoptic preamplifier. The receiver can handle multiple amplitude serial data as produced by multiple fiberoptic transmitters on the same fiber without any data loss and without any interruption in data transfer being necessary as one transmitter halts and a second one starts transmission.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to analogue tracking amplifiers, especially such as peak/valley detectors used in digital data receivers such as fiberoptic receivers. It also relates to flash analog to digital converters. 
     In a fiberoptic network, information is transferred as pulses of light across fiberoptic cable links. Optical networks typically include transfer points called “mixers” or “couplers” which accept signals from one fiberoptic link and transfer that signal to another link. Because practically all computers and data processing devices use electrical signals to operate, most origin and destination points in fiberoptic networks include converters that either detect optical signals and then produce electrical pulses, or detect electrical pulses and generate corresponding optical signals on the optical network. Converters located at destination points usually consist of a photodetector, a preamplifier and a data detection circuit. 
     Fiberoptic receivers typically comprise an optical preamplifier which takes the incident optical data and converts it to a voltage waveform whose amplitude is proportional to the intensity of the light pulses. The intensity of the light pulses tends to be poorly defined as it depends on the location and optical performance of the transmitter or transmitters. Typically multiple transmitters, each of which will transmit during different time intervals, are present. Furthermore, the voltage level at the output of the optical preamplifier, which corresponds to a dark signal or absence of an incident light pulse, is often very poorly defined and varies widely from preamplifier to preamplifier. It, however, tends to be relatively stable for any one optical preamplifier, except for temperature variations and component aging effects. The characteristics of the output signal of an optical preamplifier will have relatively time invariant dark (or signal zero) levels and varying amplitude pulses (corresponding to different active transmitters) which are superimposed on this dark level. 
     A second element of a fiberoptic receiver system takes the analog output of the preamplifier and recovers both clock and data information from the analog waveform. The first element of this clock and data recovery system is a comparator whose trigger level is set at approximately fifty percent (50%) ofthe amplitude of the signal. The comparator slices the signal into logic ones and logic zeros. The comparator evaluates the signal at a sample rate equal to or at some multiple of the data rate. When a sample rate of a multiple of the data rate is used, the signal is said to be oversampled and, in this case, it is not necessary for the clock to have a known phase relationship with the incoming data. 
     Consequently, because of the unknown dark level and the characteristics of the output signal, the data from the optical preamplifier has some characteristics that make subsequent processing of the signal difficult. FIG. 1 illustrates a waveform  11  of a received stream of logic zeros  13  and logic ones  15  with five different signal amplitudes, TX 1 , TX 2 , TX 3 , TX 4  and TX 5  all sharing a common dark level  17 . The different signal amplitudes correspond to five different transmitters on a fiberoptic network. The switch over from one transmitter to another is usually accomplished within one bit period. This means that the receiver circuitry must quickly sense the change and respond to a second output signal such as a TX 2  having a logic one represented by a signal with a different amplitude from the previous signal, TX 1 . 
     A second difficulty is that the DC average value of the signal varies with each transmitter and so it is impossible to use AC or capacitor coupling of the signal to establish an average value that can be used with a comparator to decide which level represents a logic zero and which level represents a logic one. 
     FIG. 2 illustrate the above point showing the effect of AC coupling on a multi-amplitude data stream. FIG. 2A includes a signal waveform  27  from the optical preamplifier and shows peak  23  which corresponds to the logic ones while valley  21  corresponds to logic zeros. FIG. 2B illustrates the effects of capacitance coupling. The waveform  27  now floats on a DC average signal represented by dash line  25  and consequently the peak  23  and valley  21  no longer correspond to logic zeros or logic ones (reference should be made to point  28 ). It should be noted that when the data amplitude changes, the average of the data pulses is lost for a period related to the time constant of the AC coupling system. 
     The presence of multiple amplitude signals in the data requires the use of multiple comparators or an analog to digital converter whose bit weighting is sufficiently small so as to adequately slice the smallest amplitude signal and whose full scale voltage is sufficiently large so as to adequately slice the largest signal amplitude at approximately their respective fifty percent (50%) values. In practice, the above requirements place severe restrictions on the analog to digital converter&#39;s resolution and speed. For example, in a 5V system, the dark level signal from a different optical preamplifier varies from 0.5V to 4.0V and the signal amplitude varies from 50 mV to 0.5V. The bit rate is typically in excess of 100 Mhz. An analog to digital converter that can cover this range would need a bit weighting of 25 mV and a measurement range from 0.5V to 4.25V implying a resolution in excess of 6 bits at a minimum of 100 Mhz conversion rate. 
     In U.S. Pat. No. 4,431,916 to Couch, Couch uses a signal delaying circuit to create threshold signals and detects the peak of a first delayed signal between the one-half rise point in the input signal and a one-half fall point in a second delayed signal. 
     Devices exist for detecting a predetermined voltage level and for using the detected voltage level to offset a circuit. U.S. Pat. No. 3,736,582 to Norris discloses a compensation circuit that adjusts a baseline voltage up and down in a system where that baseline voltage “gallops” or changes. 
     U.S. Pat. No. 5,381,052 issued to Kolte discusses a device which detects and tracks a peak level through a network of operational amplifiers and a capacitor which holds the peak voltage value. Kolte&#39;s device inverts a detected input voltage and drives the capacitor in two different states, depending on the polarity of the voltage between input and the inverter. In a first state, the capacitor follows the input voltage, while in a second state, the capacitor value is the average of the input voltage and the inverter voltage. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above problems by providing a dark level regeneration circuit that is fully integrated into an analog to digital converter front end. The analog to digital converter zero level, or reference level, is set to the dark level and hence the effective conversion range of the analog to digital converter is restricted to half of the maximum expected signal amplitude. In the above example, the resolution is now reduced to between 2 and 3 bits, thus, substantially reducing the complexity, power consumption and cost of the analog to digital converter. A further advantage of the invention is that the reference voltage of the analog to digital converter can be referenced to ground (or any other convenient voltage) and does not have to track the DC component of the signal. This advantage provides additional circuit simplification. 
     The present invention provides an analog to digital signal converter which has a signal input, a reference potential input, and a dark detector for providing a normalized reference signal. The digital signal converter possesses a threshold voltage generator connected to the reference potential input which produces at least one threshold voltage. 
     The analog to digital signal converter also has a system of preamps and regenerative comparators which compare a signal with respect to its dark level to a threshold voltage which is defined as a difference of two voltages. 
     The threshold voltage generator can generate a plurality of reference signals. In addition, the system of comparators and latches include a plurality of comparators for toggling between a comparison state and an autozero state, and a plurality of regenerative comparators for holding the comparator outputs for a clock cycle. Oversampling of a received signal is achieved through the use of multiple latches and multi-phase clock. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood and its numerous objects and advantages will become more apparent to those skilled in the art by referencing the following drawings, in conjunction with the accompanying specification, in which: 
     FIGS. 1 and 2 are waveform diagrams illustrating the advantages of the invention; 
     FIG. 3 is an illustrative block diagram of the invention; 
     FIG. 4 is a block diagram according to the invention; 
     FIG. 5 is a schematic diagram of the comparator of FIG. 4; 
     FIG. 6 is a timing diagram of the clock and control signals associated with the present invention; 
     FIG. 7 is a timing diagram for the dark detector of FIG. 4; 
     FIG. 8 is a schematic diagram of the preamplifier of FIG. 5; and, 
     FIG. 9 is a schematic diagram of a regenerative comparator of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 3 is an illustrative embodiment of a high speed analog to digital converter or flash analog to digital converter  10  according to the invention. V SIGNAL  is the incoming analogue signal on conductor  37  from an optical preamplifier, not shown. Reference level generator  38  generates reference voltages V 1  to V n  (where “n” is a positive number that represents the number of voltage levels plus 1) and are used as the trigger levels for comparators COMP 1  to COMPn (where COMPn represents the total number of comparators in the circuit plus 1) and are applied thereto via conductors  35  and  33  respectively. V 1  to V n  are set up to be referenced to a signal called V DARK  on conductor  63 . In the case of the disclosed embodiment, V 1  is about 25 mV in excess of V DARK  and V n  is about 250 mV in excess of V DARK . There is no restriction on the number of stages represented by “n” or on the scaling of the steps between V 1  and V n . One conventional scaling scheme is clearly a linear scaling. The zero comparator COMP 0  is used as part of the dark level regeneration circuitry. 
     The outputs of each comparator are latched by latches DL 0  to DLn (where “n” is a positive number representing the number of latches plus 1) under the control of the clock  53 . The frequency of the clock is assumed to be at a minimum equal to the bit rate of the incoming bit stream on V SIGNAL . Outputs Q 1  to Qn are provided to subsequent circuitry for processing in either a conventional flash analog to digital converter back end or in some other way. 
     The operation of the analog to digital converter is as follows. First, assume that the signal V DARK  (conductor  63 ) is an accurate representation of the dark level in the input V SIGNAL  (conductor  37 ). Comparators COMP 1  to COMPn will continuously indicate the state of V SIGNAL  with respect to the threshold levels V 1  to V n . The zero comparator COMP 0  output should be high continuously indicating that the input signal is always in excess of the dark level voltage, V DARK . There may be some noise generated by toggling of the output as V SIGNAL  comes close to V DARK . Under control of the clock (conductor  53 ), the state of comparators COMP 0  to COMPn is latched into latches DL 0  to DLn which provide Qn to Q 1  as an output. Qn to Q 1  may be applied to a decoder to indicate a logic one or applied as a bite of data to a digital processor  70  of FIG. 4 for processing. 
     A dark level regenerator  65  includes two current sources, current source  59  that provides an up current IUP and a current source  59  that provides a down current IDOWN, a capacitor C 1  and a switch SW 1  and works in the following manner. Current sources  51 ,  59  and switch SW 1  form a charge pump which charges or discharges capacitor C 1  under the control of the logic signal that through the operation of inverter  51  is the inverse of the output of latch DL 0 . The reference level for comparator COMP 0 , V DARK , is stored on capacitor C 1 . If V DARK  is less than the true minimum level in V SIGNAL , then the output of latch DL 0  will always remain low and hence switch SW 1  will remain open. Current IUP, which is typically much smaller than the current IDOWN, slowly charges capacitor C 1  and hence V DARK  rises. At some point, V DARK  will rise above the true minimum level in V SIGNAL . Comparator COMP 0  will detect this event, and a logic zero will be latched into latch DL 0 . The change in state of latch DL 0  will close switch SW 1  thus starting a discharge of capacitor C 1  and a consequent reduction in V DARK . On the next clock cycle, if V SIGNAL  is in excess of V DARK , switch SW 1  will open and capacitor C 1  slowly will charge again. The net result is that capacitor C 1  will be charged to a voltage which will be very close to the true dark level of V SIGNAL  with small voltage excursions above and below that dark level. The dark level signal is applied to the referenced level generator which floats on the V DARK  level. 
     While the foregoing illustrates the principle of the invention, it will be clear to those skilled in that art that it is very difficult to realize, particularly in CMOS technology. Because comparators COMP 0  to COMPn can detect relatively small signals in excess of 100 Mhz and V DARK  can have a wide variation in value, the comparators also have to have a very wide common mode range complicating the design task. In reality, many clock recovery systems require that the clock be a significant multiple of the bit rate which results in the sample rate of the system to be several hundred megahertz. The preferred embodiment shown below addresses these problems by using a switched capacitor based differentiating stage at the front end of a clocked comparator arrangement. As mentioned earlier, the implementation of comparators COMP 0  to COMPn is difficult given the high speed requirements of a fiberoptic receiver system. Also, depending on the clock and data recovery algorithm that is subsequently used, the clock may have to run at the bit rate or at a multiple of the bit rate. If a significant multiple is required, it is advantageous to use multiphase clocking schemes where multiple phases of the same frequency (phase shifted appropriately) are used to clock multiple comparators or portions of comparators. 
     FIGS. 4,  5 ,  6  and  7  illustrate an embodiment of a high speed analog to digital converter that utilizes a two phase clock and is implemented as a multi-level quantizer. It will be clear to those skilled in the art that this approach can be reduced to a single phase or increased to many phases as required. It will also be clear that many aspects of this system can be used in other data recovery or high speed signal analog to digital converters and are not restricted to fiberoptic receiver applications. 
     FIGS. 5 and 6 should be used in conjunction with FIG. 4 which illustrates the details of each comparator  75  that are used for comparators COMP 0  to COMPn. It comprises a fully differential preamplifier PAa and preamplifier PAb which are connected in parallel. The differential outputs of the preamplifiers PAa and PAb are connected through switch pairs  81  and  83  to a pair of Regenerative Comparators (RC) RC 1  and RC 2 . RC 1  is clocked by clock phase P 1  and RC 2  is clocked on clock phase P 2 . The inputs to the preamplifiers are connected through capacitors CA 1 , CA 2 , CB 1  and CB 2  to the voltage inputs V SIGNAL , V DARK , V n , and V REFm  via switches SA 1 , SA 2 , SA 3 , SA 4 , SB 1 , SB 2 , SB 3 , SB 4 . Switches SAZ 1  to SBZ 2  connect the preamplifier outputs to the inputs as shown and are used during an autozero operation. 
     When waveform AZ 1  (FIG. 6) is at logic one, it represents the period of time that the following switches are closed: SA 2 , SA 4 , SAZ 1  and SAZ 2 . When waveform AZ 1 n is at logic one, it represents the period of time that switches SA 1  and SA 3  are closed. When waveform AZ 2  is at logic one, it represents the period of time that switches SB 2 , SB 4 , SBZ 1  and SBZ 2  are closed. When waveform AZ 2 n is at logic one, it represents the period of time that switches SB 1  and SB 3  are closed. Waveforms SELA and SELB, when at logic one state, represent the period of time that switches  81  and  83  respectively are closed. 
     FIG. 6 is a timing diagram of a typical operating sequence for the comparator  75 . The operation of the comparator  75  of FIG. 5 is as follows: 
     First consider the situation at time t1 when differential preamplifier PAa is in autozero mode and differential preamplifier PAb is operating. During this time, the output of differential preamplifier PAb is connected through switches  83  to the inputs of RC 1  and RC 2 . RC 1  and RC 2  are clocked by clock phases P 1  and P 2  respectively; their outputs QP 1  and QP 2  indicate the state of the output of differential preamplifier PAb on the rising edges of clocks P 1  and P 2 . As mentioned earlier, at time t1 differential preamplifier PAa is in autozero mode. In this state, its outputs are connected through switches SAZ 1  and SAZ 2  to its complimentary inputs (i.e. it is connected in a negative feedback unity gain configuration). The positive terminals of capacitors CA 1  and CA 2  are therefore charged to the common mode voltage of the preamplifier PAa output. If the preamplifier has an offset voltage, this offset appears as a small differential voltage between the positive and negative inputs. In this fashion, the effect of a preamplifier offset voltage is attenuated by the open loop gain of the preamplifier. The negative side of CA 1  and CA 2  are charged to V DARK  and V REFm  through switches SA 2  and SA 4  respectively during this time. Now consider what happens at time t2 when the autozero cycle for differential preamplifier PAa ends as indicated by the falling edge of AZ 1  and the rising edge of AZ 1 n. At this time, the negative plates of capacitors CA 1  and CA 2  are disconnected from V DARK  and V REFm  and subsequently on the rising edge of P 1 , SA 1  closes and connects V SIGNAL  to the negative of capacitor CA 1  and switch SAB closes connecting Vn to the negative side of capacitor CA 2 . The feedback switches SAZ 1  and SAZ 2  are opened at the same time releasing differential preamplifier PAa from a unity gain configuration and allowing it to operate as an open loop amplifier. The voltage on the positive input to differential preamplifier PAa sees a resultant voltage change which is proportional to the quantity (V SIGNAL −V DARK ) and the negative input to differential preamplifier PAa sees a resultant voltage change proportional to the quantity (V in −V REFm ). It should be noted that these quantities now represent differences; absolute voltage levels are unimportant. Hence the V REFm  to V n  voltage range can be referenced to ground or any other convenient voltage level. The differential preamplifier PAa output now amplifies the quantity (V SIGNAL −V DARK −V n +V REFm ). Thus, V OUT  is given by: 
     
       
         V OUT =K(V SIGNAL −V DARK −V n +V REFm ); 
       
     
     where K is effective preamplifier gain referred to the negative side of capacitors CA 1 , CA 2 , CB 1  and CB 2 . Rewriting V SIGNAL −V DARK  as dV SIGNAL  and V n −V REFm  as V REFn  we have: 
     
       
         V OUT =K(dV SIGNAL −V REFn ). 
       
     
     Therefore, the preamplifier amplifies the difference between the time dependent amplitude of the input signal (with respect to its minimum or V DARK  level) and a reference level V REFn  defined as the difference between two other levels. It will be clear to those skilled in the art that these reference levels can be generated by a resistor ladder referenced to ground potential as an example. 
     At times t2 and t3 both differential preamplifier PAa and differential preamplifier PAb provide output signals which are identical, responsive to the input signal V SIGNAL . At time t3, the output of differential preamplifier PAb is disconnected from the input to the regenerative comparators RC 1  and RC 2  and the output of differential preamplifier PAa is connected instead. Differential preamplifier PAb then goes through an identical autozero cycle on the rising edge of P 1  (time t4) to close the switches identified for waveform AZ 2 . 
     The reasons for alternating the autozero cycles of differential preamplifier PAa and differential preamplifier PAb are as follows: 
     1. All switched capacitor circuits need regular recharging of the capacitors to nullify the effects of leakage. Consequently, to allow the analog to digital converter to operate continuously, two preamplifiers which autozero alternatively are required. 
     2. Having both differential preamplifier PAa and differential preamplifier PAb operate in parallel between t2 and t3 minimizes the disruption to the amplified signal presented to RC 1  and RC 2  at the crossover time t3. This means that the data stream available to subsequent processing circuitry on the outputs QP 1  and QP 2  (FIG. 5) is uninterrupted. 
     Referring to FIG. 4, the comparator COMP 0  to comparator COMPn are shown using a two phase clock P 1  and P 2  to strobe the regenerative comparators. The reference levels V 1  to V n  are generated by a resistor string  79  referenced to V REFm . It will be clear that one value of V REFm  that can be used is ground. 
     The outputs of comparators COMP 1  to COMPn indicate whether V SIGNAL  is above or below V 1  to V n  respectively as sampled by clock phases P 1  and P 2 . The outputs of the comparators are valid for one full clock cycle. 
     The comparator outputs can subsequently be processed by a standard flash analog to digital converter back-end decoder or by some other means. 
     The operation of the dark level recovery circuit  69  is very similar to that described in the illustrative embodiment of FIG.  3 . Similar to the description of the comparator operation above, there are two comparator outputs each responsive to clock phases P 1  and P 2 . The outputs of comparator COMP 0  QOP 1  and QOP 2  are combined in a NAND gate  81  whose output is provided as the drive voltage for switch SW 1 . 
     Consider the situation where V SIGNAL  is instantaneously greater than the regenerated dark level, V DARK . In this case the comparator COMP 0  outputs will be logic high when strobed by clock phases P 1  and P 2 . The outputs of the comparator COMP 0  are combined by the NAND gate  81 . The output of the NAND gate  81  will be low which results in switch SW 1  opening. In this case, the voltage on capacitor C 1  increases slowly as it is charged by current IUP. 
     Consider now what happens if V SIGNAL  is instantaneously below V DARK . If this occurs when one or both of the regenerative comparators RC 1  and RC 2  are strobed, one or both comparator outputs will be low, switching the NAND gate  81  output to logic high. This closes switch SW 1  which connects the current IDOWN to capacitor C 1  thereby causing it to discharge. If current IDOWN is substantially larger than current IUP, the resultant voltage on capacitor C 1  will exhibit a sawtooth waveform pattern centered on the true dark level of V SIGNAL . This voltage can additionally be low pass filtered using the resistor R 1 , and capacitor C 2  filter. The output of this filter is provided as the V DARK  signal to comparators COMP 0 , COMP 1  and COMPN. 
     FIG. 7 is a timing diagram of the high speed analog to digital converter such as that of FIG. 4 which is used as a dark detector. The timing diagram illustrates the flash analog to digital converter as it responds to a typical input data stream. V SIGNAL    183  is shown containing logic ones and zeros having variable time periods. For illustration purposes, the filter comprising resistor R 1  and capacitor C 2  is assumed to be removed from the circuit. The V DARK  signal level represented by waveform  185  and presented to comparator COMP 0  is now the voltage that appears on capacitor C 1 . The waveforms  185  illustrates the case where V DARK  is close to the true dark level represented by waveform  187  of the signal. 
     Clock phases P 1  (waveform  72 ) and P 2  (waveform  74 ), each a full bit position long but phase shifted by 180 degrees, are used to strobe the dual regenerative comparators RC 1  and RC 2 . The output QOP 1  (waveform  82 ) and QOP 2  (waveform  84 ) are the outputs of comparator COMP 0  responsive to the rising edge of P 1  and P 2  respectively. The resultant voltage on C 1  is a sawtooth waveform caused by the fact that current IUP is smaller than current IDOWN and that current IDOWN is switched on responsive only to the state of the outputs of comparator COMP 0 . 
     The magnitude of the deviations of the voltage on capacitor C 1  from the true dark level dVmax dotted line  89  and dVmin dotted line  91  are a function of the capacitor size C 1 , current IUP, current IDOWN, the period of the clock and the maximum number of successive logic ones that can occur in the bitstream. (For clock recovery reasons, most coding schemes limit the number of successive ones or zeros in a serial bit stream  183 ). 
     The following equations describe the relationships:          [       Δ                   V   max       =       T   max1     ·       I   UP     C1         ]                [       Δ                   V   min       =       T   clk     ·       I   DOWN     C1         ]                          
     If the filter comprising resistor R 1  and capacitor C 2  is now reinserted in the circuit, the effective deviations of V DARK  as presented to the input of comparator COMP 0  from the true dark level can be reduced to a very small number of millivolts. It will be clear that the time constant of this filter should be set at several tens of clock periods at minimum. Given the high clock rates that this circuit will operate at, this filter can be implemented on an integrated circuit easily. Finally, it will be clear that the implementation of the V DARK  regeneration fimction provides essentially a “trough” or “valley” hold function in that the circuits locks to a value representing the minimum value of V SIGNAL . Clearly, with very minor modifications, the circuit could be reconfigured to regenerate a peak value as long as the value of successive peaks varies with a much longer time constant than the clock rate. 
     FIG. 8 is a schematic diagram of the preamplifier  79 . The preamplifier  79  provides two functions: 
     1. It provides some front end gain to the signal. 
     2. It dissipates any “kickback” effects that can occur when the second stage regenerative comparator (RC) switches from one logic level to another. 
     The preamplifier  79  has two inputs, a positive input V in + and a negative input V in − and two outputs, V OUT+  and V 0UT− . A differential pair of transistors,  109  and  111  are provided with bias current by current source  129 . The differential transistors are resistively loaded by resistors  113 ,  115 . N-channel transistors  118  and  120  located between the resistor loads and the differential transistors operate as cascodes which keep the drains of the differential transistors  109  and  111  at a relatively fixed potential (independent of the small signal) and hence the time constant of the system is dominated by the resistor values and the capacitance seen at the inputs of n-channel followers, transistors  117  and  119 . Transistors  117  and  119  buffer the resistive gain node from the output load and reduce any capacitative “kickback” that may occur when successive regenerative stages switch. 
     Transistors  121  and  122  are biased such that if their sources  125  and  127  rise in voltage (caused by a saturation of the preamplifier), these transistors shunt the extra large signal current and hence prevent overload recovery delays when the large signal is removed. 
     FIG. 9, to which reference should now be made, is a schematic diagram of the regenerative comparator  106  of FIG.  5 . The outputs  105  and  107  of FIG. 8 are applied to V in + and V in −  137  which are the gates for input transistors  135  to  137  respectively. The transistors  135  and  137  are differential pair transistors loaded by cross coupled n-channel transistor loads  157  and  159 . P-channel transistors  145  and  147  act as cascodes which keep the drains of the input transistors relatively fixed. Switch  143  is responsive to clock phase P 1  and resets the latch formed by transistor loads  159  and  157 . 
     A second latching circuit is formed by nchannels transistors  161  and  163  and p-channel transistors  165 ,  167 ,  169 ,  171 . This latch is responsive to clock phase P 2 . 
     Operation of the circuits is as follows: 
     Clock phases P 1  and P 2  are non overlapping clock phases. During P 1 , the switch  143  is closed and drives nodes D and E to approximately the same voltage. Depending on the polarity of the input voltage a small differential voltage is developed between these nodes. When clock phase P 1  goes low and the switch is opened, this small initial differential voltage is amplified regeneratively causing a large signal responsive to the polarity of the input voltage to appear between nodes D and E. 
     After a short time interval, clock phase P 2  is turned on. The p-channel transistors  165 ,  167 ,  169  and  171  latch switches in response to the polarity of the differential signal between D and E. The resulting logic change is subsequently captured by the SR latch formed by Nand gates  173  and  174 . 
     N-channel transistors  149  and  151  and p-channel transistors  153  and  155  operate as limiters in the event that the comparator is subject to a large signal. They operate in such a way that any large signal curent is shunted to power or ground and hence nodes B and C so not saturate in either the positive or negative direction.