Coded data generation or conversion – Analog to or from digital conversion – Differential encoder and/or decoder
Utility Patent
1998-08-17
2001-01-02
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Differential encoder and/or decoder
C341S155000
Utility Patent
active
06169506
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention addresses the rejection of unwanted frequency components present in a given real world analog signal when such a signal is converted to a digital representation. This problem is of practical importance in such fields as data acquisition, test and measurement equipment, industrial control, etc. In particular, the invention describes the implementation of an oversampled data converter which maintains very good unwanted signal rejection while using a relatively imprecise internal clock reference.
In many applications a real world signal is converted into a stream of digital samples through the use of an analog-to-digital converter (ADC). A common implementation may include an analog domain filter, the ADC, a master clock, and a digital filter. The input signal passes through the analog domain filter which eliminates a portion of the unwanted frequency components and creates an analog filtered signal. The analog filtered signal is then sampled under the control of a clock signal from the master clock and each sample is converted into a digital representation by the ADC. The ADC output is a stream of digital samples that form a digital domain representation of the filtered signal. Afterwards, the stream is processed by the digital filter which then produces the output signal. The objective of the digital filter is to further remove from the stream unwanted frequency components. The digital filter operation also is controlled by a clock signal which is synchronous with the ADC clock signal. Due to excessive complexity, in many early implementations, the digital filter is absent or performs only a simple averaging function.
However, such applications of an analog-to-digital converter, even while using very accurate, and therefore expensive components, display only moderate performance capabilities. Thus, new methods of converting analog signals to digital signals are sought after.
One method of improving analog-to-digital conversion is by using an oversampling type converter. By oversampling (taking samples of the analog signal at a rate higher than the Nyquist rate [two times the frequency of the signal]) many of the operations of the converter, such as antialiasing, quantizing and filtering may be improved. In addition, oversampling by the converter enables the transfer of some of the analog filter functions to the digital filter where they can be implemented with greater accuracy and increased integration (The word “integrated” and “integration” in this application are used to describe internal, or “on-the-chip,” components). However, the prior art has not considered combining an oversampling converter with a high order digital filter in order to be able to use a low accuracy clock signal. The previous implementations have always relied upon clock signals with relatively high frequency accuracy whenever the rejection of certain frequency components of the input signal is important.
In addition, prior art implementations have not considered that a minimal increase in the digital filter complexity restores the converter rejection capabilities even when a low accuracy frequency clock reference is used.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an oversampling data converter utilizing a low accuracy internally generated clock signal.
It is a further object of this invention to provide an oversampling data converter implementing a slight increase in the digital filter complexity which allows the converter to utilize a low accuracy internally generated clock signal.
In accordance with this invention, an oversampling analog-to-digital converter is provided. The analog-to-digital converter includes a delta-sigma modulator for sampling the analog signal and producing a digital output signal, a high order digital filter for removing unwanted frequency components from the digital output signal, and a low accuracy internal clock generator for controlling the delta-sigma modulator and the high order digital filter.
REFERENCES:
patent: 5134401 (1992-07-01), McCartney et al.
patent: 5349352 (1994-09-01), Saleh
patent: 5757867 (1998-05-01), Caulfield et al.
Principles of Oversampling A/D Conversion, Max Hauser, J. Audio Eng. Soc., vol. 39, No. ½, 1991.
On the Use of Modulo Arithmetic Comb Filters in Sigma Delta Modulators, IEEE Proc. ICASSP '88, pp. 2001-2004, Apr. 1988.
Oprescu Florin A.
Rempfer William C.
Fish & Neave
Jean-Pierre Peguy
Linear Technology Corp.
Morris Robert W.
Weiss Joel
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