Question

List the types of ADCs. Explain working principle of any one type of ADC.

Answer 1

Types : 1. Successive approximation type A/D converter or Potentiometric type 2.Voltage to time or ramp type A/D converter 3.Voltage to frequency or integrating type A/D converter 4.Dual slope integrating type A/D converter

Working Principle of: 1.Successive approximation type A/D converter: It uses an efficient code search strategy to complete n bit conversion in n clock periods. It consists of a DAC, an output register, a comparator and control circuit. Here the comparator compares the analog input with DAC reference voltage that is successively divided in half. The reference voltage is repeatedly divided for successive approximation till the divided voltage is almost equal to the unknown input voltage level. When each bit of the DAC is enabled one at a time starting from MSB, the comparator produces an output that indicates whether the analog input voltage is greater or less than the output of the DAC. If DAC output is greater than the analog input voltage, comparator output is LOW, so bit in the control register is reset. If DAC output is less than the analog input voltage, comparator output is HIGH, so bit is retained in the control register. After all the bits of the DAC are tried, the conversion process is complete and the register indicates the end of conversion.

2.Voltage to time A/D converter: This type of ADC utilizes digital counter techniques to measure the time required for a voltage ramp to rise from zero to the input voltage level. The ramp can be either positive going or negative going. The ramp voltage is applied to a comparator where it is compared with the analog voltage from the sensor. The time consumed by the ramp voltage to increase to the value of sensor voltage depends upon the size of the sampled analog voltage. When the ramp voltage starts a gate is opened which starts a binary counter counting the regular pulses from a clock. When both voltages are equal, the gate closes and the word indicated by the counter is the digital representation of the sampled voltage

3. Voltage to frequency or integrating type A/D converter : An analog voltage can be converted to digital form, by producing pulses whose frequency is proportional to the analog voltage. These pulses are counted by a counter for a fixed duration and the reading of the counter will be proportional to the frequency of the pulses and hence, to the analog voltage.

Dual slope Integration A/D converter 

In this ADC , an unknown analog voltage and a known reference voltage are converted into equivalent time periods using an integrator, These time periods are measured by the counter. This circuit is called dual slope ADC because the analog voltage and reference voltage are converted to ramp signals of different slopes by the integrator.