Rainbow Electronics ADC08062 - Manual (Page 11)

Application Information
1 0 FUNCTIONAL DESCRIPTION
The ADC08061 and ADC08062 perform an 8-bit analog-to-
digital conversion using a multi-step flash technique The
first flash generates the five most significant bits (MSBs)
and the second flash generates the three least significant
bits (LSBs)
Figure 5 shows the major functional blocks of
the ADC08061 2’s multi-step flash converter It consists of
an over-encoded 2
-bit Voltage Estimator an internal DAC
with two different voltage spans a 3-bit half-flash converter
and a comparator multiplexer
The resistor string near the center of the block diagram in
Figure 5 forms the internal main DAC Each of the eight
resistors at the bottom of the string is equal to 1 256 of the
total string resistance These resistors form the LSB Lad-
der
and have a voltage drop of 1 256 of the total reference
voltage (V REF a b V REF b
) across them The remaining re-
sistors make up the MSB Ladder They are made up of
eight groups of four resistors connected in series Each
MSB Ladder section has
of the total reference voltage
across it Within a given MSB Ladder section each of the
MSB resistors has 8 256 or
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 comparators shown at the
right of
Figure 5 This function provides the necessary refer-
ence voltages to the comparators during each flash conver-
sion
The six comparators seven-resistor string (estimator DAC)
and Estimator Decoder at the left of
Figure 5 form the Volt-
age Estimator The estimator DAC connected between
V
REF a
and V REF b
generates the reference voltages for
the six Voltage Estimator comparators These comparators
perform a very low resolution A D conversion to obtain an
‘‘estimate’’ of the input voltage This estimate is then used
to control the Comparator Multiplexer connecting the ap-
propriate MSB Ladder section to the eight flash compara-
tors Only 14 comparators six in the Voltage Estimator and
eight in the flash converter are needed to achieve the full
eight-bit resolution instead of 32 comparators that would be
needed by traditional half-flash methods
TL H 11086 – 18
FIGURE 5 Block Diagram of the ADC08061 2 Multi-Step Flash Architecture
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