Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
1998-12-23
2001-07-10
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
Reexamination Certificate
active
06259311
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to tuned filters. More particularly, it relates to a tuning method and apparatus which provides high absolute accuracy in an RC time constant of an integrated circuit filter.
2. Background of Related Art
It is known that process variation in the tolerance of the absolute value of individual components formed in an integrated circuit (e.g., resistors and/or capacitors) can be quite great, but that similar components can be matched quite closely in value because it is likely that both will be affected equally by process and/or temperature variations. Thus, individual integrated circuit resistors can typically be manufactured to have a value only within a large range or tolerance.
One method to overcome this problem is the use of matched components. However, in certain applications such as where the absolute value of an RC time constant is important, the tolerance in the resistance and/or capacitance value providing the RC time constant dictates the accuracy of any filter basing its operation thereon. The accuracy of an RC time constant and thus a filter based thereon can be improved significantly by in-circuit tuning of the resistance forming the RC time constant to compensate for fluctuations in process and/or temperature.
A continuous time filter is a traditional technique, useful for removing high frequency out-of-band power of a signal (e.g., above 2 times the Nyquist rate). A continuous time filter is particularly useful in a circuit including sampling by an analog-to-digital (A/D) converter to provide higher accuracy in the samples. The present inventors desired to provide a more accurately tuned continuous time filter than those presently available using resistor based tuning.
Some applications, e.g., a central office codec used to digitize telephone speech, will use a switched capacitor filter switched at a high speed, e.g., 1 megahertz (MHz) to achieve a desired band pass (e.g., 4 kilohertz (kHz) band pass) together with a continuous time filter (e.g., a smoothing filter) having a wide tolerance due to the large tolerance of certain components such as resistors and/or capacitors forming an RC time constant therein. Such applications typically use both a band pass filter and a continuous time filter having low tolerance requirements, e.g., in the neighborhood of 35 kHz.
FIGS. 6
to
10
show a conventional technique for tuning the resistive portion of an RC time constant in a continuous time filter based on the variances of formed resistors only.
In particular,
FIG. 6
shows a block diagram of an embodiment of a conventionally tuned continuous time filter operating on an input signal
410
to provide an output signal
420
. A filter
600
, e.g., a continuous time filter, filters an input signal
410
to provide an appropriately filtered output signal
420
. The response of the filter
600
is controlled by one or more RC time constant(s). To provide the desired accuracy, the filter
600
is tuned by hand with a trimmed current source resistor tuning control circuit
400
.
FIG. 7
is a schematic depiction of an embodiment of the conventionally tuned filter
600
shown in FIG.
6
.
In particular, the trimmed current source resistor tuning control circuit
400
includes a trimmed current portion
502
, and a comparators and latches portion
520
. A relevant segment of the filter
600
is also shown in
FIG. 7
, as are the input signal
410
and the output signal
420
.
The trimmed current portion
502
of the trimmed current source resistor tuning control circuit
400
includes a current mirror formed by two p-channel metal oxide semiconductor field effect transistors (PMOSFETs)
532
,
534
. A first side of the current mirror is trimmed with a current trimming element
536
(e.g., a variable resistor) to provide a desired current through the MOSFET
532
. The current value set in this first side of the current mirror is then duplicated in the other side of the current mirror circuit, i.e., through MOSFET
534
. The duplicated current is driven through a plurality of series connected resistors, e.g., four resistors
504
,
506
,
508
and
510
.
Three comparators
522
,
524
and
526
are fed on their respective positive inputs by nodes between each of the respective resistors
508
and
506
,
506
and
504
, and above resistor
504
. The negative input of each of the three comparators
522
,
524
and
526
is tied to a desired reference voltage VREF. The reference voltage VREF may be either internally generated on the integrated circuit or externally provided to the trimmed current source resistor tuning control circuit
400
from a source external to the integrated circuit.
The outputs of the comparators
522
,
524
and
526
are respectively latched by latches
542
,
544
and
546
. The outputs of the comparators
522
,
524
and
526
control the switching in or out of individual resistor in a tunable resistor element in the filter
600
.
For example, tunable resistor components
613
,
615
in the segment of the filter
600
shown in
FIG. 7
are adjusted or ‘tuned’ in accordance with the state of the outputs of the latches
542
,
544
and
546
. For instance, if the voltage reference VREF is at a level such that comparator
526
is saturated (i.e., the voltage level of the node between resistors
508
and
506
is greater than that of the voltage reference VREF), then latch
546
would have an active output thus turning on respective MOSFET switches
653
a
and
653
b
in the tunable filter
600
, and accordingly short resistor
634
in the first resistive component
613
and resistor
644
in the balanced resistive component
615
. Accordingly, the resistive components
613
and
615
(which are the balanced resistive portions of an RC time constant in the filter
600
) are tuned within the allowable tolerance to the resistance of formed resistors
631
-
633
and
641
-
643
, respectively, based on the performance of resistors
504
-
510
formed in the trimmed current portion
502
.
Similarly, if the resistors
504
-
510
in the trimmed current portion
502
are such that a voltage level between resistors
506
and
504
exceeds that of the voltage reference VREF, then latches
544
and
546
have active outputs to cause closure of MOSFET switches
653
a
,
653
b
,
652
a
and
652
b
, to tune the resistive components
613
and
615
to the values of resistors
631
-
632
and
641
-
642
, respectively. If the formed resistors
504
-
510
are such that the voltage level above the resistor
504
exceeds that of the voltage reference, then all three comparators
522
-
526
will become saturated and all three latches
542
-
546
will have active outputs when enabled, thus shorting all resistors except for resistor
631
in the first resistive element
613
and except for resistor
641
in the second resistive element
615
.
For completeness, more detailed schematics of an embodiment of the conventional tunable continuous time filter shown in
FIG. 6
are shown in
FIGS. 8
to
10
. In particular,
FIG. 8
is a schematic diagram of the trimmed current portion
502
of the embodiment of the trimmed current source resistor tuning control circuit
400
shown in
FIGS. 6 and 7
.
FIG. 9
is a schematic diagram of the comparators and latches portion
520
of the trimmed current source resistor tuning control circuit
400
shown in
FIGS. 6
to
8
.
FIG. 10
is a schematic diagram of the filter
600
shown in FIG.
6
.
Accordingly,
FIGS. 6-10
show a conventional technique wherein a tunable resistor portion of an RC time constant in a filter
600
can be tuned based on formed resistors and a trimmed current. However, such conventional resistance-only based designs do not provide the desired precision or tolerance in the tuned filter, largely because variances in the other portion of the RC time constant, i.e., in the capacitive portions still leave a significant amount of error in the absolute value of the RC time constant. Moreover, many integrated circuit technologies do not allow
Azimi Kouros
Nelson Dale Harvey
Agere Systems Guardian Corp.
Bollman William H.
Le Dinh T.
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