A/D Conversion Methods

There are several methods for analog to digital conversion. Below we cover a few examples.

Flash Conversion

When an ADC (analog to digital converter) has a comparator that fires for each decoded voltage range, it is not as a direct conversion ADC or flash ADC. A code for each voltage is generated from the comparator bank to a logic circuit for each voltage range. Direct conversion is fast, typically only 8 bits of resolution (256bit comparators) or less, A large, expensive circuit is usually needed. ADCs of this type have a high input capacitance, a large die size, and are prone to produce glitches on the output (out-of-sequence code being output can cause glitches). They are often used for fast signals like video.

Successive Approximation Conversion

A successive-approximation ADC settles on a final voltage range, by using a comparator to reject ranges of voltages. The way successive approximation functions is by comparing the input voltage constantly to a known reference voltage until the best approximation is achieved. At each step in this process, a binary value of the approximation is stored in a successive approximation register (SAR). A reference voltage is used for the SAR. If the input voltage is 150V and the reference voltage is 100V, then in the 1st clock cycle the voltage out is negative (because 100V is less than 150V). The voltage might increase by say 30V in the 2nd clock cycle, to 130V. This value still a negative value. The 3rd clock cycle results in 160V. Because of this, the output exceeds the input voltage and is positive. The binary form 110 is the result.

When the number of bit cycles is increased and the increment rise decreases, it is possible to construct an accurate A/D converter. The type of ADC is more complex than other designs.

Delta-Encoded Conversion

A delta-encoded ADC has an up-down counter. This counter that feeds a digital to analog converter (DAC). The input signal and the DAC both go to a comparator. The comparator controls the counter. The comparator uses negative feedback from the circuit to adjust the counter until the DAC's output is close to the input signal. From there, the number is read from the counter. Delta converters have wide ranges, as well as high resolution, but the conversion time depends on the input signal level.

Delta converters are good choices when it comes to reading real-world signals. Most signals from physical systems are not sporadic. Some converters combine the delta and successive approximation approaches which works well when high frequencies are known to be small in magnitude.

An integrated ADC creates a saw-tooth signal that ramps up, then falls to zero. When the ramp starts, a timer starts counting. When the ramp voltage and the input match, a comparator fires. At this point, the timer's value is recorded.

The ramp time is typically sensitive to temperature because the circuit generating the ramp is often a simple oscillator. Calibrating the timed ramp, or using a clocked counter driving a DAC and then use the comparator to preserve the counter's value are two solutions for this.