Digital-to-analog converter

Claim 1

A digital-to-analogue converter comprising: (a) input means for receiving a digital word to be converted to analogue form; (b) a plurality of weighted analogue signal sources; (c) first selection means for selecting analogue signals from said sources in accordance with a digital word applied to said input means; (d) summing means for summing the signals from said selection means to provide an analogue output signal representing said digital word; (e) means associated with each of a number of said sources for providing a measurable signal representative of the magnitude of the analogue signal thereof independently of selection of said associated source by said first selection means; (f) second selection means for causing said measurable signal providing means to output said measurable signal, said first selection means and at least one of said sources being operable for providing a weighted analogue signal independently of operation of said second selection means; (g) a measurement terminal to which said second selection means applies the selected measurable signals; and (h) a calibration device arranged to operate said second selection means to select a sequence of combinations of said measurable signals which produces at said terminal a sum signal of staircase waveform, to subtract a ramp signal from said staircase waveform signal to produce a signal of sawtooth waveform and to obtain measurement data by measurement of time related to the length of the cycles of said sawtooth waveform.