have a voltage drop of 1/256 of the total reference voltage

(V

up the MSB Ladder. They are made up of eight groups of

four resistors connected in series. Each MSB Ladder section

MSB Ladder section, each of the MSB resistors has 8/256,

or 1/32 of the total reference voltage across it. Tap points are

found between all of the resistors in both the MSB and LSB

Ladders. Through the Comparator Multiplexer these tap

points can be connected, in groups of eight, to the eight com-

parators shown at the right of

Figure 6. This function pro-

vides the necessary reference voltages to the comparators

during each flash conversion.

The six comparators, seven-resistor string (estimator DAC),

and Estimator Decoder at the left of

Figure 6 form the Volt-

age Estimator. The estimator DAC connected between

V

six Voltage Estimator comparators. These comparators per-

form a very low resolution A/D conversion to obtain an “esti-

mate” of the input voltage. This estimate is then used to con-

trol the Comparator Multiplexer, connecting the appropriate

MSB Ladder section to the eight flash comparators. Only 14

comparators, six in the Voltage Estimator and eight in the

flash converter, are needed to achieve the full eight-bit reso-

lution, instead of 32 comparators that would be needed by

traditional half-flash methods.

A conversion begins with the Voltage Estimator comparing

the analog input signal against the six tap voltages on the es-

timator DAC. The estimator decoder then selects one of the

groups of tap points along the MSB Ladder. These eight tap

points are then connected to the eight flash comparators.

For example, if the analog input signal applied to V

tween 0 and 3/16 of V

mator decoder instructs the comparator multiplexer to select

the eight tap points between 8/256 and 2/8 of V

nects them to the eight flash comparators. The first flash

conversion is now performed, producing the five MSBs of

data.

The remaining three LSBs are generated next using the

same eight comparators that were used for the first flash

conversion. As determined by the results of the MSB flash, a

voltage from the MSB Ladder equivalent to the magnitude of

DS011086-18

FIGURE 6. Block Diagram of the ADC08061/2 Multi-Step Flash Architecture

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