Types of Analog to Digital Converters
An analog to digital converter (ADC) is an electronic device used to convert input analog current to a digital number based on the current’s magnitude. ADCs are either linear or non-linear according to response, and are further categorized into five different types, namely: successive approximation, dual slope integrating, flash, charge balancing and sigma-delta converter.
Successive Approximation Converter
Successive approximation converters provide a relatively speedy conversion of the input signal. They function by comparing the input with a voltage that is half its input range. When the input signal is over this level, the converter compares it with three quarters of the range. During comparison, the signal is frozen and stored in a sample and hold circuit. Once the conversion is finished, the data is transferred and placed to the computer’s buffer memory, in which it is read and processed in convenient batches.
Dual Slope Integrating Converter
Relatively slower than the successive approximation converter, the dual slope type is known to reduce noise and allows the input signal to charge a capacitor for a certain time frame to be measured later after the capacitor discharges at a fixed rate.
Charge Balancing Converter
The charge balancing converter works similar to the dual slope integrating converter in terms of using the input signal to charge a capacitor, but this time the capacitor is discharged in units of charge packets. If the capacitor is charged more than the required packet size, the converter will release a packet; if not, a packet will not be produced. The input voltage or current is then determined by measuring the pulses coming out of the capacitor. In this type of converter, noise effects are reduced as the input signal is integrated along with the capacitor charging time.
The fastest type of ADC is the flash converter. It functions similar to the successive approximation converter; it compares the input signals to a certain reference voltage or current. A flash converter however has many different comparators, which correspond to the steps of comparison. Thus, an 8-bit converter has a total of 256, or 2 to the power of 8 comparators.
Finally, a sigma-delta converter transforms analog signal under very low resolution and a very high sampling rate. By means of digital filtering and oversampling, the resolution can be accelerated to as much as 20 bits or more. Sigma-delta converters are used in high-resolution conversion of low-distortion and low-frequency audio signals. They are known for their high accuracy and good linearity.