Page 253 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes
12.13 Flash Converters

Also called the parallel A/D converter, this circuit is the simplest to understand. It is formed of
a series of comparators, each one comparing the input signal to a unique reference voltage. The
comparator outputs connect to the inputs of a priority encoder circuit, which then produces a
binary output. The following illustration shows a 3-bit flash ADC circuit:

Figure 12.41: 3-Bit Flash ADC Circuit

V is a stable reference voltage provided by a precision voltage regulator as part of the converter
ref
circuit, not shown in the schematic. As the analog input voltage exceeds the reference voltage at
each comparator, the comparator outputs will sequentially saturate to a high state. The priority
encoder generates a binary number based on the highest-order active input, ignoring all other
active inputs.
When operated, the flash ADC produces an output that looks something like this:

Figure 12.42: Flash ADC

For this particular application, a regular priority encoder with all its inherent complexity is
not necessary. Due to the nature of the sequential comparator output states (each comparator

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