Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
1999-04-01
2001-02-06
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S055000, C327S563000
Reexamination Certificate
active
06184721
ABSTRACT:
BACKGROUND
This specification generally relates to voltage comparators. More specifically, the present specification describes method and apparatus for comparing input voltages having a small voltage difference.
Voltage comparators determine which of the two input voltages are larger or smaller. Since a voltage comparator often senses small differences between the input voltages and generates a digital output, a large amplification may be needed. The large amplification necessary for sensing small differences is constructed using a differential amplifier operating in a non-linear region. The differential amplifier has two transistors connected as a source-coupled pair with one of the transistors turned-off and the other transistor turned-on. Therefore, the amplifier has one transistor turned-on and drawing current even under static condition when the transistors are in a stable non-switching state.
As amplification circuits, comparators are susceptible to influence of noise on the input voltages. The noise on the input voltages causes erratic switching and false triggering of the comparator output. Thus, positive feedback can be applied to reduce the influence of noise on the comparison and to increase the flexibility of the differential threshold of switching. The flexibility of the threshold is increased by making the threshold less sensitive to the difference in input voltages and more sensitive to the previous levels of the input voltages. However, the feedback also tends to slow the response of the comparator and limits the lowest differential voltage which can be sensed.
The voltage comparators are used in various different applications such as in analog-to-digital converters (ADCs), signal generators, and image arrays and ADCs of complementary metal-oxide semiconductor (CMOS) active pixel sensors (APSs).
The voltage comparators used in the CMOS APSs are sensitive to a high fixed-pattern noise. The fixed-pattern noise is an unvarying display pattern resulting from the difficulty in exactly matching transistor thresholds on CMOS circuits for photocurrent amplification and transfer circuitry.
SUMMARY
The inventor noticed that by placing switches across the source-drain terminals of the cross-coupled load transistors and closing the switches during the reset mode, the load transistors are disabled. This prevents any charge or current conduction through the load transistors during the reset mode and effectively reduces the adverse effect of the load transistor threshold mismatch during the comparison mode.
In addition, this technique produces much smaller input-referred offset with a desired effect of much less erroneous comparison.
In one aspect, the present disclosure compares input voltages that are relatively close to each other. Initially, switching elements are configured in a reset mode to prevent any charge or current conduction. A strobe signal reconfigures the switching elements to transition from the reset mode to the comparison mode. Finally, a determination is made as to which of the input voltages is larger or smaller.
In some embodiments, an output result of the comparison is a positive number if a first input voltage is larger than a second input voltage, and a negative number if the second input voltage is larger than the first input voltage. In further embodiments, the output result is correctly determined with magnitude of an input voltage difference as small as 1.5 mV.
In other embodiments, the input voltages are sampled for comparison to prevent input voltages from changing for the duration of the comparison. In further embodiments, the output result of the comparison is buffered.
In another aspect, the disclosure features a CMOS active pixel image sensor system for use in detecting images through photocurrent picked up by an image pixel array. The system also includes an analog-to-digital converter which includes the improved voltage comparators.
In yet another aspect, the disclosure features a CMOS APS camera system for use in detecting and displaying images. This system also includes the improved voltage comparators.
Among the advantages of the present disclosure is the small input-referred offset. As a result, input voltages of small difference can be compared accurately.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
REFERENCES:
patent: 5274275 (1993-12-01), Colles
patent: 6018260 (2000-01-01), Gabara
Fish & Richardson P.C.
Le Dinh T.
Photobit Corporation
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