Amplifiers – With periodic switching input-output
Reexamination Certificate
1999-09-09
2001-06-26
Pascal, Robert (Department: 2817)
Amplifiers
With periodic switching input-output
C330S002000, C327S124000
Reexamination Certificate
active
06252454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multistage comparators used in analog-to-digital converters (ADCs), and more particularly relates to methods for calibrating such multistage comparators to remove the native comparator offset and charge injection offset resulting from the opening and closing of input switches during the sampling operation.
2. Description of Related Art
Comparators of the related art are well known. For example, Donald A. Kerth's U.S. Pat. No. 4,748,418 assigned to Crystal Semiconductor, discloses a quasi-autozero technique which can remove signal hysteresis in a comparator, by momentarily shorting the comparator output to ground prior to comparison. Comparators of the related art are known to use an autozero technique employing sample and hold circuits to store the comparator offset. However, when autozero and quasi-autozero techniques are combined, circuit performance is diminished.
SUMMARY OF THE INVENTION
According to one embodiment of the present invention, a multistage comparator includes a plurality of amplifiers and capacitors alternatingly coupled in series connection, each amplifier including a programmable source. A latch can be connected to the output of the last amplifier in the series, according to one embodiment of the present invention. Each stage of the multistage comparator includes a capacitor configured to perform an autozero function whereby the comparator offset voltage is reduced substantially to zero. Between each capacitor and the input of a next amplifier, there is connected an autozero switch which selectively grounds the input of the associated amplifier. Similarily, between each capacitor and the output of its preceding amplifier, there is connected a quasi-autozero switch which selectively grounds the output of the associated amplifier. According to one embodiment of the present invention, each of the amplifiers is a differential amplifier having first and second input and output connections. According to such an embodiment, each amplifier has first and second output capacitors, respectively connected to the first and second outputs of the associated differential amplifier. Further, each such differential amplifier has first and second input autozero switches connected to its respective first and second inputs; and a quasi-autozero switch connected across the first and second outputs. Further according to one embodiment of the present invention, the multistage comparator includes a latch having first and second inputs connected respectively to the first and second outputs of the last in the series of differential amplifiers. According to one embodiment of the present invention, the programmable source of a selected amplifier in a multistage comparator is calibrated by grounding the input(s) of the selected amplifier to produce an output corresponding to the amplifier offset voltage. The associated output quasi-autozero switch is closed to prevent the amplifier output voltage from being applied to the first node(s) of the associated output autozero capacitor(s), while grounding the second node or nodes of this or these capacitor(s) by closing the autozero switch(es). The resulting voltage across the autozero capacitor(s) is zero, the desired voltage after calibration. Thereafter, the autozero switch(es) are opened to sample the voltage on the autozero capacitor(s). Voltage measurements for calibrating the programmable source are made by opening the quasi-autozero switch. This allows the output of the selected amplifier to change in response to the offset voltage. The selected amplifier output is transferred to the succeeding comparator stages using AC coupling techniques via the autozero capacitor(s). According to the present invention, after the programmable source of the selected amplifier has been adjusted to accomplish calibration of said selected amplifiers's native voltage offset, another calibration operation is conducted to remove any voltage error caused by charge injection from the opening of the input autozero switch(es). The present invention accordingly improves the performance of a multistage comparator by removing undesired native voltage offset and charge injection effects. More particularly, according to the present invention, an autozeroed and quasi-autozeroed multistage comparator used for analog-to-digital conversion is calibrated to remove native voltage and charge injection offsets. The comparator is further optimized according to the present invention to enable high bandwidth and low power comparator operation by reducing offset effects in its amplifier stages. According to one embodiment of the present invention, the programmable sources are implemented as calibrable current sources for removing native amplifier and charge injection offsets. After the offset is compensated by appropriate current source calibration settings (for native amplifier offset or charge injection and native amplifier offset), at a particular stage, the next preceding stage can be calibrated. According to one embodiment of the present invention, only the first stage of the multi-stage comparator is subject to calibration. According to one embodiment of the present invention, the calibrable current sources are implemented with registers associated with particular ones of the comparator stages. According to one embodiment of the present invention, the multistage comparator is a differential system.
REFERENCES:
patent: 3810031 (1974-05-01), Poujois
patent: 4048574 (1977-09-01), Barbier et al.
patent: 4255715 (1981-03-01), Cooperman
patent: 4385286 (1983-05-01), Haque
patent: 4438354 (1984-03-01), Haque et al.
patent: 4736351 (1988-04-01), Oliver
patent: 4939516 (1990-07-01), Early
patent: 5027116 (1991-06-01), Armstrong et al.
patent: 5438274 (1995-08-01), Bitar et al.
Lee Wai Laing
Muñoz Carlos Esteban
Piasecki Douglas S.
Swanson Eric
Thompson Karl Ernesto
Choe Henry
Cirrus Logic Inc.
Lin Steven
Pascal Robert
Stewart David L.
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