Mixed Signal Techniques - Signal Processing Systems - page 797

Vin

Vref

+

-

R

+

-

+

R

b 1

-

2 b to b
encoder

b b 3

+

R

-

R

+

b b

-

R

+

-

+

R

-

R

Fig. 6 A flash ADC

3.1.4 Sub-Ranging Converters

Sub-ranging or multi-step converters are popular in high-speed, medium-accuracy
applications. The block diagram of an 8-bit subranging ADC is shown in Fig. 7 . The
conversion is done in two steps. First, the coarse ADC converts the 4 MSBs. This
result is converted back to analog with a DAC and subtracted from the input voltage.
The difference is converted digital with a fine ADC that produces the 4 LSBs. The
subconverters are typically flash ADCs, needing altogether much less comparators
than a full flash ADC of the same overall resolution. The fine ADC may be preceded
by a gain amplifier with a gain of a power of two.