High performance switched-capacitor filter for oversampling sigma-delta digital to analog converters
First Claim
1. A filter for oversampling Sigma-Delta digital to analog converter circuits capable of accommodating different digital subscriber line applications, comprising:
- a SINC filter that decimates a digital data input signal by a predetermined factor of an over-sampling ratio utilized by a Sigma-Delta modulator and further uses positive and negative reference voltages in combination with bottom-plate switched-capacitors to avoid a signal dependent load on reference buffers;
a switched-capacitor biquad filter having a programmable cut-off frequency which tracks the over-sampling ratio; and
a sample and hold buffer.
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Abstract
An improved SC filter for OSDACs for asynchronous digital subscriber line (ADSL) applications. More specifically, an improved switched-capacitance (SC) filter for an over-sampled digital to analog converter intended for use in a multi-mode analog front end for a host of ADSL applications. The SC filter is configured for low power consumption and for a low-ambient noise floor across the frequencies of interest. The SC filter is further configured to drive a resistive load of a few kohms and for low signal distortion. The SC filter lowers power consumption by decimating the input waveform and reducing the number of operational amplifiers in the circuit.
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Citations
14 Claims
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1. A filter for oversampling Sigma-Delta digital to analog converter circuits capable of accommodating different digital subscriber line applications, comprising:
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a SINC filter that decimates a digital data input signal by a predetermined factor of an over-sampling ratio utilized by a Sigma-Delta modulator and further uses positive and negative reference voltages in combination with bottom-plate switched-capacitors to avoid a signal dependent load on reference buffers;
a switched-capacitor biquad filter having a programmable cut-off frequency which tracks the over-sampling ratio; and
a sample and hold buffer. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method of implementing a switched-capacitor (SC) filter for an over sampling digital to analog converter (OSDAC), comprising the steps of:
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decimating a digital signal by a predetermined factor in a SINC filter wherein the SINC filter uses a combination of reference voltages and bottom-plate controlled switched-capacitors to increase signal swing while avoiding signal dependent loads;
applying the decimated digital signal to a switched-capacitor biquad filter; and
further applying the output of the switched-capacitor biquad to a sample and hold buffer. - View Dependent Claims (13, 14)
changing the input data rate of the digital to analog data converter and a sampling rate used to decimate the input data rate in response to different digital subscriber line application modes.
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14. The method of claim 13, further comprising the step of:
tracking an over-sampling ratio of a digital Sigma-Delta modulator, thereby providing optimum performance over multiple XDSL modes.
Specification