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Coarse/fine A-D converter using ramp waveform to generate fine digital signal
| Details |
Inventors: Cake, Brian V.; Sippach, Frederick W.;
Assignee: LeCroy Research Systems Corporation (Spring Valley, NY)
Primary Examiner: Sloyan; T. J.
Assistant Examiner:
Attorney, Agent or Firm: Curtis, Morris & Safford
Analog-to-digital conversion in which a coarse digital representation of an input analog signal is converted to a coarse analog representation, and the difference between the original analog signal and the coarse analog representation is determined. A ramp waveform signal is generated, and a change in that waveform by an amount substantially equal to the difference between the original analog signal and the coarse analog representation is sensed. A plural bit fine digital signal corresponding to the sensed change of the ramp waveform is produced. The combination of the coarse and fine digital signals constitute the digital representation of the input analog signal. In one embodiment, the ramp waveform is increased until it is equal to the difference between the analog signal and the coarse analog representation, whereupon the ramp waveform is digitized. In another embodiment, the ramp waveform is used to reduce the level of a sample of the analog signal until that sample becomes equal to the coarse analog representation, at which time a counter that had been initiated at the commencement of sample reduction is stopped, the count being equal to the fine digital signal. |
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DETAILED DESCRIPTION In accordance with this invention, a method and apparatus for converting an input analog signal to a plural-bit digital signal are provided.
A coarse digital representation of the input signal is produced, and this coarse digital representation is converted to a coarse analog signal.
A ramp waveform is generated, and when this ramp waveform has undergone a change in magnitude by an amount substantially equal to the difference between the input analog signal and the coarse analog signal, a fine digital signal corresponding to that change is produced.
The coarse and fine digital signals constitute the bits of greater significance and lesser significance, respectively, in the output digital signal.
In accordance with one embodiment of this invention, the ramp waveform is compared to the difference between the input analog and coarse analog signals, and when the magnitude of the ramp waveform is substantially equal to that difference, the ramp waveform is converted to a digital signal by, for example, a "flash" A/D converter.
In this embodiment, the ramp waveform may be increasing or decreasing, as desired.
In accordance with another embodiment of the invention, the input analog signal is sampled and stored, and the ramp waveform is used to change the stored sample until it becomes substantially equal to the coarse analog signal.
A counter, such as a clock pulse counter, is turned on at the commencement of the change in the stored analog sample and is turned off when that sample reaches the level of the coarse analog signal.
The resultant count constitutes the fine digital signal.
In this embodiment, the analog sample is stored on a capacitor, and the ramp waveform is a negative waveform which discharges the capacitor.
The number of pulses which are counted while the capacitor is discharged to the coarse analog signal level is used to produce the fine bits (i.
e.
the bits of lesser significance) of the output digital signal.
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