Low power active filter and method

• US 6,741,120 B1
• Filed: 08/06/2002
• Issued: 05/25/2004
• Est. Priority Date: 08/07/2001
• Status: Expired due to Term

• Current Assignee: Ikanos Communications Incorporated (Qualcomm, Inc.)
• Original Assignee: GlobespanVirata, Inc. (Synaptics Incorporated)
• Inventors: Tan, Nianxiong
• Primary Examiner(s): Cunningham, Terry D.
• Application Number:
  US10/213,483
• Time in Patent Office: 658 Days
• Field of Search: 327/552, 327/553, 327/554, 327/555, 327/559
• US Class Current: 327/555
• CPC Class Codes: H03H 11/1291 Current or voltage controlled filters

Abstract

Devices and method for effectively filtering a signal, particularly for a communication system, are disclosed. In this regard, an exemplary embodiment of the present invention may be construed as an AFE that includes a high-pass receive filter for a communication system. The filter includes an AC-coupled capacitive input and a plurality of RC integrators. At least one of the plurality of RC integrators includes a damping resistor in parallel with a feedback capacitor and a switch for enabling the damping resistor, such that when the damping resistor is enabled, the at least one RC integrator is damped so as to reduce DC instability.

Claims

1. An active filter comprising:

a summing amplifier having an input and an output, wherein the output of the summing amplifier provides the output of the filter;

a first RC integrator having an input and an output and configured to emulate a first capacitance, wherein the output of the first RC integrator is coupled to the input of the summing amplifier;

a second RC integrator having an input and an output and configured to emulate a second capacitance;

a third RC integrator having an input and an output and configured to emulate a third capacitance, wherein the output of the third RC integrator is coupled to the input of the summing amplifier;

a fourth RC integrator having an input and an output and configured to emulate an inductance, wherein the input of the fourth RC integrator is coupled to the outputs of the first and second RC integrators and the output of the fourth RC integrator is coupled to the inputs of the first and second RC integrators; and

a selectable input stage having a first state and a second state, wherein in the first state the input stage is configured to provide a DC-coupled non-capacitive input to at least one RC integrator and in the second state the input stage is configured to provide an AC-coupled capacitive input to at least two RC integrators.

2. The filter of claim 1, wherein the input stage is coupled to the input of the fourth RC integrator and the input of the summing amplifier during the first state.

3. The filter of claim 1, wherein the input stage is coupled to the input of the first RC integrator during the second state.

4. The filter of claim 1, wherein the damping coefficient of at least one of the first, second, or third RC integrators is controlled to reduce DC instability in the filter.

5. The filter of claim 4, wherein the damping coefficients of any of the RC integrators is controlled by selectively switching in a damping resistor in the respective RC integrator.

6. An analog front end (AFE) comprising:

a high-pass receive filter for a communication system, the filter comprising;

a plurality of RC integrators, wherein at least one of the plurality of RC integrators comprises;

a damping resistor in parallel with a feedback capacitor; and

a switch for enabling the damping resistor, such that when the damping resistor is enabled, the at least one RC integrator is damped so as to reduce DC instability;

an AC-coupled capacitive input electronically coupled to the inputs of at least one of the plurality of RC integrators; and

a summing amplifier electrically coupled to the plurality of RC integrators.

7. The AFE of claim 6, further comprising:

means for controlling the switch of the at least one RC integrator.

8. The AFE of claim 6, further comprising:

means for programmably configuring the RC integrators to provide effective damping so as to reduce DC instability.

9. A DSL interface card comprising:

an active high-pass filter located in an AFE of the DSL interface card, the filter comprising;

a summing amplifier having an input and an output, wherein the output of the summing amplifier provides the output of the filter;

a first RC integrator having an input and an output and configured to emulate a first capacitance, wherein the output of the first RC integrator is coupled to the input of the summing amplifier;

a second RC integrator having an input and an output and configured to emulate a second capacitance;

a third RC integrator having an output and configured to emulate a third capacitance, wherein the output of the third RC integrator is coupled to the input of the summing amplifier;

a fourth RC integrator having an input and an output and configured to emulate an inductance, wherein the input of the fourth RC integrator is coupled to the outputs of the first and second RC integrators and the output of the fourth RC integrator is coupled to the inputs of the first and second RC integrators; and

a capacitive AC-coupled input stage coupled to an input of at least two of the RC integrators.

10. The DSL interface card of claim 9, wherein the input stage of the filter is coupled to the input of the first RC integrator.

11. The DSL interface card of claim 9, wherein each of the first and second RC integrators comprise:

a damping resistor configured to reduce the DC gain of the respective integrators such a DC offset passing through the integrators is reduced.

12. The DSL interface card of claim 11, wherein each of the first and second RC integrators further comprise:

a switch configured to enable the damping resistor so as to control the effect of the damping resistor on the respective integrators.

13. The DSL interface card of claim 12, further comprising:

a control system configured to control the switches of the first and second integrators.

14. An active high-pass filter comprising:

a summing amplifier having an input and an output, wherein the output of the summing amplifier provides the output of the filter;

an inductive RC integrator configured to emulate an inductor;

a plurality of capacitive RC integrators configured to emulate capacitors of various capacitances, wherein each of the RC integrators has an input coupled to an output of the summing amplifier and each of the plurality of RC integrators has an output that is selectively coupled to an input of the summing amplifier and to an input of the inductive RC integrator so as to produce a desired transfer function; and

wherein the damping coefficients of the plurality of capacitive RC integrators are selectively controlled to reduce DC instability in the filter.

15. The filter of claim 14, further comprising:

a selectable input stage having a first state and a second state, wherein in the first state the input stage is configured to provide a DC-coupled non-capacitive input and in the second state the input stage is configured to provide an AC-coupled capacitive input.