This invention relates to electronic circuits for digital signal processing and more particularly to the use of a single reference voltage for both positive and negative voltage swings with such circuits.
In many different circuits, such as a CODEC (Coder-Decoder), an analog signal is compared to a reference voltage where the amplitude of the analog signal can be less than or equal to the reference voltage. A CODEC essentially comprises a nonlinear analog to digital (A/D) converter and a nonlinear digital to analog (D/A) converter. These A/D and D/A functions involve both positive and negative signal voltages. Heretofore, this required the use of two reference voltages of equal magnitude, one for positive and another for negative signals. This often created problems or disadvantages because it required the two reference voltages to be matched--a condition that is difficult and expensive to provide.
Various approaches have been suggested for overcoming the two reference requirements. For example, in a paper entitled "A Two-Chip PCM CODEC for Per Channel Applications", James B. Cecil, Edwin M. W. Chow, John A. Flink and James E. Solomon, IEEE International Solid State Circuits Conference, February 1978, pp. 176-177, an arrangement was described wherein two references were generated from one reference. However, this had the disadvantage that (a) the two reference values generated may not be matched; and (b) it requires extra circuitry to generate the other reference. In another approach, as described in "A Two-Chip CMOS CODEC", George F. Landsburg, George Smarandoui, IEEE International Solid State Circuit Conference, February 1978, pp. 180-181, the authors used two separate off-chip references which had the same disadvantages as the first approach. In still another arrangement described in "A PCM Voice CODEC With On-Chip Filters", J. Terry, T. Caves, Chong H. Chan, Stanley D. Rosenbaum, Lester Sellars, John B. Terry, IEEE International Solid State Circuits Conference, February 1978, pp. 182-183, the authors essentially generated two reference voltages using resistive dividers. Thus, any errors present in the resistance values of the divider inevitably showed up as errors in the reference voltage.
It is, therefore, a general object of the present invention to provide a method for using a single reference voltage with A/D or D/A circuits that previously required two voltage references and which overcomes the disadvantages of prior attempts to solve the problem.
Another more specific object of the invention is to provide a single reference voltage circuit that may be made as an integrated circuit semiconductor device for implementing digital to analog or analog to digital conversion of positive or negative signals.
Further objects of the invention are to provide D/A and/or A/D electronic circuits using a single reference voltage that are more accurate, that require less silicon area on an integrated circuit chip; and are more economical to manufacture.