Coded data generation or conversion – Quantizer
Patent
1986-09-15
1990-02-06
Shoop, Jr., William M.
Coded data generation or conversion
Quantizer
341155, H03M 112
Patent
active
048991554
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to interpolating analogue to digital converters and a method for effecting analogue to digital conversion.
Known interpolating analogue to digital converters usually comprise a relatively coarse resolution analogue to digital (A/D) converter arranged in a feedback loop in combination with a digital to analogue (D/A) converter; the D/A converter having much finer resolution than the coarse A/D converter. The coarse A/D converter, typically 6 bits resolution, is sampled at a high sample rate, typically 10 MHz, with subsequent averaging of the output samples obtained from the course resolution A/D converter.
The quantised signal appearing at the output of the coarse A/D converter is an approximation of the analogue signal fed to it, the approximation being dependent upon the quantum jump or step size of the quantisation and the sample rate used in the quantisation process. At any point in time, the difference between the analogue input signal and the output sample from the coarse A/D converter is known as the quantisation error; generally termed Q.
The output from the coarse A/D converter is fed to the finer resolution D/A converter, the output of which is fed back and combined with the incoming analogue input signal to be digitised. However the D/A converter utilised also gives rise to errors; generally termed T, the resolution of the D/A converter.
Such interpolating A/D converters also include an analogue controller, usually in the form of a high gain amplifier of gain A, in the feed path to the coarse A/D converter.
The digital output of this type of interpolating A/D converter can be expressed as ##EQU1## where
D.sub.OUT is the digital output signal
V.sub.I is the analogue input signal
Q is the quantisation error of the coarse resolution A/D converter
T is the error in resolution of the D/A converter and
A is the open loop gain of the analogue controller.
It can be seen from this expression that the term in the quantisation error Q is minimised by maximising the open loop gain A of the analogue controller. However, an increase in the open loop gain A does not improve the linearity of the converter, that is the relationship between the digital output and the analogue input, as the term in the resolution T of the D/A converter remains constant for all but small values of open loop gain A. Furthermore, by maximising the open loop gain A, it becomes extremely difficult to maintain loop stability in the converter. In practice, the resolution of such converters is limited by the value of the term in the quantisation error Q and in modern interpolating A/D converters is usually limited to about 14 bits.
It is an object of the present invention to provide apparatus and a method for analogue to digital conversion which enable greater resolution and linearity than that attainable with known converters.
According to the present invention there is provided an interpolating converter for digitising an analogue signal, the converter comprising combiner means for receiving an analogue signal, one step digital quantiser means for receiving an input signal dependent upon an output signal from the combiner means, means for identifying the quantisation error of the one step digital quantiser means, and feedback means for affording a signal dependent upon the quantisation error of the one step digital quantiser means to the combiner means.
Preferably the means for identifying the quantisation error of the one step digital quantiser means comprises further combiner means for receiving a signal dependent upon an output signal from the one step digital quantiser means and means for affording a signal dependent upon the input signal to the one step digital quantiser means to the further combiner means.
Advantageously, the one step digital quantiser means comprises a D-type flip flop.
According to another aspect of the present invention there is also provided a method of analogue to digital conversion, the method comprising affording an analogue signal to one step digital quantiser means, identifyin
REFERENCES:
patent: 2927962 (1960-03-01), Cutler
patent: 4301446 (1981-11-01), Petit
Sep. 1988--"Linear IC Handbook", Plessey Semiconductors, pp. 103-110.
Plessey Overseas Limited
Shoop Jr. William M.
Young Brian K.
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