Physics and Engineering of Radiation Detection

8.7. Analog to Digital Conversion 505

very long time for high rate applications. Increasing the resolution to 16 bits at
the same clock frequency would take conversion time to about 655μs.Ofcourse
the conversion time can be shortened by increasing the clock frequency but it has
its own engineering difficulties. Nevertheless Wilkinson ADCs working at 400-500
MHzhave been manufactured and successfully operated in detection systems.

B.5 FlashADC

As the name suggests, a flash ADC performs the conversion in aflash,whichsimply
means that it is extremely fast. Such ADCs are becoming more and more popular
due to the conversion speed they offer as compared to conventional ADCs. The
downside is their resolution, which is difficult to increase beyond the 8 bit mark.

+ − + − + − + − + − + − + − + −

Vin

Decoder

R R R R R R R R

Vref

Comparators

Data Word

Figure 8.7.9: A 4-bit flash ADC

having 8 comparators. The de-

coder could, for example be a

priority encoder that generates

a binary number based on the

highest order active input.Vref

is a stable reference voltage,

which is generally provided by a

precision voltage regulator that

is a part of the converter circuit.