DIGITAL-TO-ANALOG CONVERSION BY PULSE-COUNT MODULATION METHODS

Three low-cost digital-to-analog converters (DAC's) are described and compared. These designs can easily be implemented in an integrated cir cuit: the conventional pulse-width modulation (PWM) DAC, the new pulse -count modulation (PCM) DAC and the first-order noise shaping (FONS) D AC. All three methods control the ratio of the sum of all pulse durati ons to the constant total period, As the pulse durations are integral multiples of a unit pulse, all three can be classified as pulse-count modulation methods. Block diagrams of all three DAC's consisting of a simple digital circuit and a low-pass filter are presented. For a cons tant digital input value the worst case ripple of the filter output is used to calculate the cutoff frequency of the low-pass filter. Approx imations for the 3 dB cutoff frequency of first-order, second-order an d fourth-order Butterworth low-pass filters are given, The dynamic pro perties are analyzed in the time domain (settling time) and in the fre quency domain (unfiltered output spectrum of a full-scale sine wave in put). The main influences on the static accuracy are analyzed, A case study demonstrates the abilities of PCM and FONS.