Second order Sigma-Delta based analog to digital converter having superior analog components and having a programmable comb filter coupled to the digital signal processor
First Claim
1. An oversampling analog to digital converter, for converting an input signal having a baseband frequency into an output set of high resolution samples having an output frequency and representing said input signal, comprising:
- (a) a Sigma-Delta modulator for (i) receiving, from a clock pulse generator, an oversampling clock signal having an oversampling frequency, (ii) receiving said input signal, and (iii) sampling said input signal at said oversampling frequency and producing a first set of coarsely quantized samples having said oversampling frequency and representing said input signal, wherein said oversampling attenuates quantization noise in the frequency range from DC to said basedband frequency while increasing quantization noise for frequencies greater than said baseband frequency; and
(b) a digital filter for (i) receiving said oversampling clock signal, (ii) receiving said first set of samples from said modulator, and (iii) computing a first weighted average of an adjustable number of samples in said first set of samples, and thereby producing a second set of samples representing said input signal and having a second sampling frequency equal to said oversampling frequency divided by said adjustable number of samples used in computing said first weighted average, wherein said weighted averaging attenuates quantization noise for frequencies greater than said baseband frequency.
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Abstract
A "true" 16-bit second order Sigma-Delta based converter that has superior analog components and has a programmable comb filter which is coupled to the digital signal processor. This converter comprises a second order Sigma-Delta modulator and a programmable comb filter. The second order Sigma-Delta modulator dramatically attenuates the baseband quantization noise energy (which in turn increases the resolution of the converter), since its superior amplifiers and comparators enable it to oversample and coarsely quantize the analog input signal at a very high sampling frequency of 12 MHz. The amplifiers are class AB OTAs, which have cross coupled NMOS driven input stages, and cascoded output stages. Also, the common mode voltages are the optimal biasing points, and these voltages are kept constant by a differential input stage, by a PV independent temperature dependent current generator, by optimal device size, and by a common mode feedback circuitry. The programmable comb filter receives the coarsely digitized 1-bit output of the modulator at oversampling frequency FS, and provides a more accurate representation of the input signal to the DSP at slower sampling rate of FS /N. In addition, the comb filter uses a 20-bit data path, in order to enable the decimator (which is formed by the comb filter and by the FIR filter) to provide 16 bits of resolution to the DSP. The output of the programmable comb filter is then supplied to an FIR filter which is realized in the DSP, and this filter removes the remaining out-of-baseband noise.
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Citations
16 Claims
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1. An oversampling analog to digital converter, for converting an input signal having a baseband frequency into an output set of high resolution samples having an output frequency and representing said input signal, comprising:
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(a) a Sigma-Delta modulator for (i) receiving, from a clock pulse generator, an oversampling clock signal having an oversampling frequency, (ii) receiving said input signal, and (iii) sampling said input signal at said oversampling frequency and producing a first set of coarsely quantized samples having said oversampling frequency and representing said input signal, wherein said oversampling attenuates quantization noise in the frequency range from DC to said basedband frequency while increasing quantization noise for frequencies greater than said baseband frequency; and (b) a digital filter for (i) receiving said oversampling clock signal, (ii) receiving said first set of samples from said modulator, and (iii) computing a first weighted average of an adjustable number of samples in said first set of samples, and thereby producing a second set of samples representing said input signal and having a second sampling frequency equal to said oversampling frequency divided by said adjustable number of samples used in computing said first weighted average, wherein said weighted averaging attenuates quantization noise for frequencies greater than said baseband frequency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A Sigma-Delta modulator, for oversampling and coarsely quantizing an input signal, comprising:
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at least one sampler operating at an oversampling frequency for converting said input signal into a first set of digital signals; at least one transforming circuit for transforming said first set of digital signals into a second set of digital signals; and at least one integrator receiving said second set of digital signals and having a differential class AB operational transconductance amplifier (OTA) capable of processing digital signals at said oversampling frequency; wherein said OTA includes a differential cross-coupled input stage, driven by N-channel MOS transistors and having a plurality of input currents, for receiving said second set of digital signals, a first biasing stage having a plurality of biasing currents for establishing and maintaining the optimal biasing points of said OTA, wherein said optimal biasing maximize the linear range of operation of said OTA, a differential output stage driven by said input stage, wherein said input currents are mirrored into said output stage to produce a plurality of output currents, a dynamic biasing stage, coupled to said input and output stages, for dynamically biasing said output stage, a common mode feedback stage, coupled to said differential output stage and using switch capacitor sampling techniques, for establishing and maintaining the optimal biasing points of said OTA, and common mode feedback disabling circuitry for disabling said common mode feedback stage, thereby enabling said OTA to operate continuously. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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Specification