Clocked comparators have two basic modes of operation, a track mode and a regenerative mode. During the track mode, a comparator tracks the input signals and continuously changes its output based upon a comparison of the two inputs. During the transition between the track mode and regenerative mode, the comparator stops tracking and maintains the output voltage based upon a comparison between the two inputs at the time of the transition. This time is also known as the "sampling instant." During the regenerative mode, the comparator maintains the output voltage at a value indicating the result of the comparison made at the sampling instant.
Existing comparators suffer from two major problems. The first problem is "kick-back noise." During the regenerative mode in an ideal comparator, the value of the input voltages should not affect the value of the output voltage. If the input stage of the comparator is not disabled in some way during the regenerative mode, a strong change in the input signal occurring after the sampling instant can cause the output of the comparator to change during the regenerative mode. Various methods have been developed to disable the input stage during the regenerative mode. An existing solution may involve, for example, disabling an input differential pair that is normally present in existing comparators. Disabling the input circuitry, however, causes a sudden variation in the bias currents of the input devices and can cause "glitches" in the source inputs. The comparator, therefore, may disturb the source or sources it is trying to compare. A disturbance caused by disabling of the input stage is known as kick-back noise.
The second major problem with existing comparators is "aperture uncertainty". The sampling instant is normally defined by a clock edge. The comparison, however, normally does not occur simultaneously with the transition of the clock. Rather, there is normally a short delay, known as an aperture delay, before the comparator makes the comparison. In some comparators, there is a large amount of uncertainty as to the length of this delay. In many existing comparators, the aperture delay depends upon the level of the signals the comparator is trying to compare. In other words, when a signal is relatively high, it takes a specific time for the comparison to take place and when the signal is relatively low, it takes another time for the comparison to take place.
There are a number of existing techniques for dealing with aperture uncertainty and/or kick-back noise. These techniques, however, have not been completely successful in reducing aperture uncertainty and kick-back noise. In addition, many of these techniques may not be used in circuits employing a BICMOS or bipolar technology with a low voltage power supply such as a three volt power supply.