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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.
A/D Converter Configuration 1(Flash Type)

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.

Basic Bipolar Type A/D Converter Configuration

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.

Basic configuration of a successive approximation type converter Successive comparison (large/small)

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.

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