DC cancellation circuit

• US 7,917,114 B2
• Filed: 03/30/2006
• Issued: 03/29/2011
• Est. Priority Date: 03/30/2005
• Status: Expired due to Fees

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Bagchi, Amit, Gaikwad, Rohit
• Primary Examiner(s): NGUYEN, SIMON
• Application Number:
  US11/392,868
• Publication Number:
  US 20060223472A1
• Time in Patent Office: 1,825 Days
• Field of Search: 455/136, 455/138, 4552321-2532, 455/296, 455/333, 375317-319, 375/345, 330/83, 330/86, 330259-260, 330/282
• US Class Current: 455/232.1
• CPC Class Codes: H03D 3/008 Compensating DC offsets
• CPC Class Codes: H03F 1/26 Modifications of amplifiers to reduce influence of noise generated by amplifying elements
• CPC Class Codes: H03F 1/34 Negative-feedback-circuit arrangements with or without positive feedback H03F1/02 - H03F1/30, H03F1/38 - H03F1/50, H03F3/50 take precedence ; for rejection of common mode signals H03F3/45479
• CPC Class Codes: H03F 2200/294 the amplifier being a low noise amplifier [LNA]
• CPC Class Codes: H03F 2200/372 Noise reduction and elimination in amplifier
• CPC Class Codes: H03F 3/24 of transmitter output stages
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit
• CPC Class Codes: H03F 3/45968 by offset reduction
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22

Abstract

The present invention relates to a DC offset canceling circuit. In one aspect of the invention, a DC offset canceling circuit with independently configurable gain and roll-off frequency is provided. In one embodiment of the present invention, the DC offset canceling circuit is used in the receive path of a down-conversion wireless receiver. In another aspect of the invention, a method for independently varying the gain and the roll-off frequency of the DC offset canceling circuit is provided. In one embodiment, the method is used to independently operate a gain control scheme and a DC offset cancellation strategy in a DC canceling circuit.

Claims

1. A circuit, comprising:

an input terminal for receiving an input signal;

a forward gain element, coupled to said input terminal, including a first operational amplifier, a first impedance, and a second impedance, wherein a gain of the forward gain element is responsive to a ratio of the first impedance and the second impedance;

an output terminal taken from an output of the first operational amplifier;

a feedback gain element, coupled to the output of the forward gain element, including a second operational amplifier, a third impedance, and a capacitance; and

a scaling impedance coupled between an output of the feedback gain element and the input terminal, wherein the scaling impedance scales a DC signal at the output of the feedback gain element to reduce a DC component of the input signal, and wherein a value of the scaling impedance is variable.

2. The circuit of claim 1, wherein a sum of the first and second impedance is a fixed value.

3. The circuit of claim 1, wherein a gain frequency response of the feedback gain element is responsive to the ratio of the third impedance and the capacitance, and is inversely proportional to frequency, thereby attenuating high frequency input signals.

4. The circuit of claim 1, wherein a value of the third impedance is variable.

5. The circuit of claim 1, wherein a gain frequency response of the circuit tends towards the gain of the forward gain element for high frequency input signals.

6. The circuit of claim 5, wherein a gain frequency response of the circuit tends towards zero for low frequency input signals.

7. The circuit of claim 6, wherein a high-pass roll-off frequency of the gain frequency response of the circuit is responsive to the second impedance, the third impedance, the scaling impedance and the capacitance.

8. A circuit, comprising:

an input terminal for receiving an input signal;

a forward gain element, coupled to said input terminal, including a first operational amplifier, a first impedance, and a second impedance, wherein a gain of the forward gain element is responsive to a ratio of the first impedance and the second impedance;

an output terminal taken from an output of the first operational amplifier;

a feedback gain element, coupled to the output of the forward gain element, including a second operational amplifier, a third impedance, and a capacitance; and

a scaling impedance coupled between an output of the feedback gain element and the input terminal, wherein the scaling impedance scales a DC signal at the output of the feedback gain element to reduce a DC component of the input signal;

wherein a gain of the circuit is set by fixing the ratio of the first and second impedances; and

wherein a high-pass roll-off frequency of the gain frequency response of the circuit is set by adjusting the third impedance and the scaling impedance.

9. The circuit of claim 8, wherein the gain and the high-pass roll-off frequency are set independently of each other.

10. The circuit of claim 9, wherein the circuit operates in a plurality of modes, each of the plurality of modes defined by a value of the high-pass roll-off frequency.

11. The circuit of claim 10, wherein the circuit operates in a programmable slow mode for information demodulation, the value of the high-pass roll-off frequency in said mode set according to a lowest frequency component of a received information signal.

12. The circuit of claim 11, wherein the circuit operates in a programmable fast mode for DC offset elimination, the value of the high-pass roll-off frequency in said mode set high enough to eliminate any DC offset in the received information signal within a pre-determined time duration.

13. The circuit of claim 12, wherein the circuit is toggled from the slow mode to the fast mode after the received information signal is demodulated.

14. The circuit of claim 12, wherein the circuit is toggled from the fast mode to the slow mode after the DC offset is eliminated.

15. The circuit of claim 8, wherein a change in the high-pass roll-off frequency due to a change in the gain is canceled by adjusting the third or the scaling impedance.

16. A circuit comprising:

a first gain element having a positive input terminal, a negative input terminal coupled to a ground reference voltage, and an output terminal coupled to an output terminal of the circuit;

a first impedance coupled between an input terminal of the circuit and the positive input terminal of the first gain element;

a second impedance coupled between the positive input terminal and the output terminal of the first gain element;

a second gain element having a positive input terminal, a negative input terminal coupled to a ground reference voltage, and an output terminal;

a third impedance element coupled between the output of the first gain element and the positive input terminal of the second gain element;

a capacitive element coupled between the positive input terminal and the output terminal of the second gain element; and

a fourth impedance coupled between the output terminal of the second gain element and the positive input terminal of the first gain element.