ad_what3(A/D Converter Configuration 1)
ADC <Basic A/D Converter Configurations>
Numerous configurations exist for ADC (A/D converter). We cover the basic configurations below.
Flash Method
This type of A/D converter utilizes 2N-1 comparators (for an N bit converter) to compare the analog signal with successive reference voltages. The results are then converted into digital format using an encoder.
Features:
- Analog signals are converted into digital signals directly (since the comparators themselves are the sampling devices), making a Sample and Hold circuit unnecessary.
- This allows for extremely fast conversion (with sampling frequencies above 1GHz possible).
- However, the relatively larger size and power consumption (due to the number of comparators are required: 2N-1) limit resolution to around 8bits.

Pipeline Method
In the case of a 1.5bit/stage configuration, the following processes are repeated in order from Stage 1 that determines MSB via pipeline operation (VREF: Reference Voltage).
- Analog input is sampled (using an S&H circuit)
- At the same time the analog input is converted by an A/D converter into a 3-value digital format (1.5bit). (Here the digital output stage is defined.)
- Analog input ≦ -VREF/4 → D="00"
- -VREF/4 < Analog input ≦ +VREF/4 → D="01"
- +VREF/4 < Analog input →D="10"
- These digital values are then converted into analog values using a digital to analog converter (DAC).
- D="00" → DAC output:-VREF/2
- D="01" → DAC output:0
- D="10" → DAC output:+VREF/2
- The negative DAC output voltage is amplified (x2) and output to the next stage.
Once processing of Stage N that determines LSB is completed, the delay between each stage is corrected then digital conversion completed by adding the respective digital output.

Characteristics:
- High resolution enabled (up to 16bit)
- High-speed conversion possible (200MHz max. sampling frequency)
- There is a necessary wait time until the digital signal is output (based on bipolar operation), making this impractical for applications requiring real-time processing (i.e. control).
Approximation Method
This method compares the sampled analog input with the converter's output in succession, starting with the MSB.
- The analog input signal is sampled (S&H)
- A successive approximation register (SAR), which is designed to supply an approximate digital code to the internal DAC, is initialized so that the most significant bit (MSB) is set to '1'.
- The digital values from the SAR are converted into equivalent analog values by the internal DAC.
- The sampled input voltage is compared with the DAC output voltage.
- If the sampled voltage > DAC output voltage → MSB = 1
- If the sampled voltage < DAC output voltage → MSB = 0
The digital conversion is completed by repeating the operation up to LSB.


Characteristics:
- High resolution conversion possible (up to 18bit)
- Since a clock cycle is required (resolution + α), conversion speed is moderate (10MHz max. sampling frequency)
- Good response. Connecting a multiplexer to the the input makes it easy to switch analog signals.