Of Books and Papers in the Computer Field

ranges are overlapping. These two features prevent the instabilities which are normally encountered when the input oscillates about a subrange boundary. Furthermore, an automatic "step size" selection feature adjusts the length of the bidirectional counter in inverse proportion to the rate of change of the input signal, resulting in a constant bandwidth-accuracy product. While the cost of these features is by no means negligible, the resulting improvement in performance is considerable. The PHENO ADC also provides, at no added cost, the analog equivalent of the low-order 10 bits, and a "change" pulse indicating a transition between subranges. Both are useful in hybrid function generation. It should be noted that the ability of the PHENO ADC to "fend for itself" in the presence of an oscillating argument removes one of the major objections to the use of analog breakpoint identification in hybrid function generation; namely, the danger of overloading the interrupt facilities of the digital computer. The function generator (termed FGDAC for "function generating DAC") is based on the well-known fact that under certain circumstances, the digital representation of a function argument contains the correct lower breakpoint number in the most significant bits and the correct interpolating ratio (xxi)/Ax in the least significant portion. The authors use potentiometers and an FET switching matrix driven by the breakpoint portion of the argument value to obtain the function value fi at the lower breakpoint, as well as the difference Af=fi+1-fi. These quantities are fed into a summing MDAC whose digital input is the interpolating ratio. The output of this MDAC is the interpolated function value