| 摘要 |
A digital-to-analog converter suitable for use in a television tuning system for converting bits of digital word to a corresponding tuning voltage includes coarse and fine digital-to-duty factor converters (816, 820) responsive respectively to and least significant bits of the digital word. The amplitudes of the resulting pulse signals are differentially weighted, e.g., by standard tolerance resistors (826, 828) (+/-10%), such that the maximum average DC potential of the fine signal is greater than a single increment of the average DC potential of the coarse signal. The weighted signals are combined and filtered by a lowpass filter (824) which derives the tuning voltage. This deliberately results in non- monotonicity but without a significant reduction in resolution. To substantially reduce the effects of the non-monotonic increments in disturbing the analog output signal, the value of the digital word in counter (810) is rapidly incremented during the occurrence of each of said non-monotonic increments. The imminence of such increments is detected (832) from the LSB values, whereupon flip-flop (834) causes the output voltage to be stored (836) and the counter (810) switched over to a fast clock (840). Resetting occurs when comparator (838) detects that VOUT has regained its stored value. <IMAGE> |
