Working principle of ADC:

The process of analog to digital conversion always involves comparing two analogue signals: an input signal and some reference signal. The comparison is carried out with a circuit called a comparator as shown in figure.

A comparator circuit is essentially a high-gain differential amplifier. When Vin > Vref the output of the comparator swings to the positive supply rail, and so the output is “1”. On the other hand, when Vin < Vref the output voltage swings to the earth rail, “0”. Thus, the comparator gives a clear indication of which of two voltages is the larger one.

Successive Approximation Analog-to-Digital Converter

Fig. Successive Approximation ADC

In this method of analog-to-digital conversion successive comparisons are made between the input signal and the half of the input range. This input range is reduced in each comparison. The number of comparisons is equal the number of bits of the analog-to-digital converter output.

An example is shows in figure above. An analog input voltage of 6.81 V was converted to a digital value. The processes to do this were comparing the input voltage to the half of the input range (5 V). Then a digital value 1 was attributed to the most significant bit (msb) because the input voltage was lower than the half of the input range.

Then the input range was reduced to its half, and a new comparison was made. The next bit received the digital value 0 because in this comparison the input voltage was bigger than the half of the new input range. These steps were repeated eight times in this case, approximating the digital output value to the analog input voltage value.