Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
1999-10-01
2001-08-14
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S307000, C323S313000
Reexamination Certificate
active
06275098
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to providing a stable bandgap reference. More specifically, a technique for adjusting the offset of an op amp included in a bandgap reference circuit is disclosed.
BACKGROUND OF THE INVENTION
A stable voltage reference is required for a digital to analog converter (DAC) to output an accurate analog voltage. The accuracy of the analog voltage is particularly important for a DAC used in an ADSL line driver. The output power allowed for an ADSL system must stay within a tight range as defined by a standard (e.g. ITU 6.992.1 or 6.992.2). If the DAC in the ADSL line driver does not have a stable reference, the output power will not be well defined. The bandgap referencing technique has been widely employed for implementing a voltage reference source in bipolar integrated circuits, including circuits implemented in CMOS.
FIG. 1
is a block diagram illustrating a conventional CMOS bandgap reference circuit. The area of transistor
102
is much larger than the area of transistor
104
, usually by about a factor of
10
. The emitters of the two transistors are connected to the noninverting and inverting inputs of op amp
106
. The output of op amp
106
at node
110
is a reference voltage, V
ref
, that, ideally, is a stable bandgap reference.
In practice, V
ref
also depends on the offset voltage of op amp
106
, V
os
, and the difference between the emitter-base voltages of transistors
102
and
104
. V
ref
may be given by:
V
BG
=
V
BE
+
(
Δ
V
BE
+
V
OS
)
·
(
1
+
R
1
R
2
)
The op amp offset is a significant error source because the offset is generally not proportional to absolute temperature (PTAT). The op amp offset error and other error sources are described in more detail in “A Precision Curvature-Compensated CMOS Bandgap Reference” by Bang-Sup Song and Paul R. Gray, IEEE J. Solid State Circuits vol. SC-18, no. 6, pp. 634-643, Dec. 1983, which is herein incorporated by reference for all purposes.
Eliminating the op amp voltage offset or reducing its effect could greatly improve the stability of the bandgap reference. Different approaches have been suggested for doing that. For example, one possible solution is to use chopper stabilization to null out the op amp offset voltage. However, this technique creates undesirable switching transients and the reference voltage is valid only during a portion of the clock period. That is not preferable for a DAC used in an ADSL driver and in other applications that require a continuous and stable reference.
Another approach has been suggested for reducing the relative effect of the offset voltage by increasing the relative contribution of the bipolar transistor base-emitter voltages. By using an area-ratioed stack of three closely matched bipolar transistors, it is possible to produce a basic reference voltage that is three times the silicon bandgap voltage. That reduces the effect of the offset factor by a factor of
3
. The bandgap voltage is then given by:
V
BG
=
3
V
BE
+
(
3
Δ
V
BE
+
V
OS
)
·
(
1
+
R
1
R
2
)
Yet another approach is to trim, as with a laser, component values of certain elements within the op amp in order to reduce or eliminate the op amp offset. However, this is costly. Extra steps are required during manufacturing and testing to perform the trim function.
Although these techniques reduce variation resulting from the offset voltage of the op amp, it would be useful if a more effective technique could be developed. Specifically, a technique for compensating for the op amp offset that provides a continuous reference; that does not require removing or switching the op amp out of the bandgap reference circuit; and that does not require additional steps during manufacturing and testing, is needed.
SUMMARY OF THE INVENTION
A bandgap reference circuit that compensates for the op amp offset is disclosed. In one embodiment, a programmable current source is used to inject a current into the first stage output of a two stage op amp. An offset canceled comparator is used to determine the correct amount of current to be injected by the programmable current source. Significantly, this is accomplished without switching the op amp out of the bandgap reference circuit. In one embodiment, the bandgap reference source is used to provide a stable reference for a DAC used in an ADSL line driver. In order to maintain an accurate bandgap reference as the offset drifts over time, the current source may be reprogrammed whenever a system reset occurs.
It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Several inventive embodiments of the present invention are described below.
In one embodiment, a compensated bandgap reference circuit includes an op amp having an inverting input and a noninverting input. The op amp is configured to output a bandgap reference based on base emitter voltage differences of a plurality of transistors. A programmable current source is configured to compensate for an offset voltage between the inverting input and a noninverting input. A comparator is configured to measure the offset voltage between the inverting input and the noninverting input and control the programmable current source.
In one embodiment, a compensated bandgap reference circuit includes an op amp having an inverting input and a noninverting input. A first transistor having a first emitter is connected to the noninverting input of the op amp. A second transistor having a second emitter is connected to the inverting input of the op amp. A comparator is configured to measure a voltage offset between the inverting input and the noninverting input. A programmable current generator is configured to inject a compensating current into the op amp. The programmed current is determined as a current that causes the comparator to measure the voltage offset to be less than a threshold.
In one embodiment, a method of compensating for a voltage offset between an inverting input and a noninverting input of an op amp to provide a stable bandgap reference includes measuring the voltage offset between the inverting input and the noninverting input of the op amp and searching for a compensating current input to the op amp that compensates for the voltage offset. A programmable current source is set to output the compensating current to the op amp.
These and other features and advantages of the present invention will be presented in more detail in the following detailed description and the accompanying figures which illustrate by way of example the principles of the invention.
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Krummenacher, Francois, et al, “SA 21.3: A High-Performance Autozeroed CMOS Opamp with 50&mgr; V Offset”, ISSCC97/Session 21/Amplifiers/Paper SA 21.3, 1997 IEEE International Solid-State Circuits Conference.
Song, Bang-Sup, “A Precision Curvature-Compensated CMOS Bandgap Reference”, IEEE J. Solid-State Circuits, vol. SC-18, No. 6, pp. 634-643, Dec. 1983.
Michejda, John, et al., “A Precision CMOS Bandgap Reference”, IEEE Journal of Solid-State Circuits, vol. SC-19, No. 6, Dec. 1984, pp. 1014-1021.
Ohara et al.; A CMOS Programmable Self-Calibrating 13-bit Eight-Channel Data Acquisition Peripheral; IEEE Journal of Solid-State Circuits, vol. SC-22, No. 6, Dec. 1987, pp. 930-938.
Conroy Cormac
Sheng Samuel
Uehara Gregory T.
Callahan Timothy P.
Englund Terry L.
LSI Logic Corporation
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