Analogue Signal Handling

Abstract

We have already noted that most values we might wish to measure with our computer are analogue in nature, in that they vary smoothly between an upper and a lower limit. But the computer itself can handle only digital values, and we therefore have to convert these variables from analogue to digital form. One of the simplest methods of conversion makes use of an accurately controlled ramping voltage, from an integrator circuit, which is fed to one input of a comparator circuit. The other input carries the analogue voltage that is to be converted. As long as the ramping voltage is smaller than the analogue voltage, clock pulses are allowed to increment a digital counter. At the start of a conversion both the ramp voltage and the counter are at zero, but the count increases as the ramp voltage grows until the comparator detects that the two voltages are equal. The output of the comparator is digital, and when it switches low the clock pulses are stopped so that the counter contains a digital value which is directly proportional to the analogue voltage. When a new conversion is required, the integrator capacitor must be discharged and the counter reset to zero. The conversion process has counted the number of regular clock pulses in a time interval that has been made exactly proportional to the analogue voltage, if the voltage ramp can be controlled accurately enough. The accuracy does rely very heavily on the stability of the clocking pulse rate and also on the linearity of the ramp voltage waveform. The linearity in turn depends on the characteristics of the capacitor used in the integrator and the stability of the reference voltage used with it.