506 Chapter 8. Signal Processing
Fig.8.7.9 depicts the working principle of a typical flash ADC. As shown, the
ADC consists of a number of comparators in parallel, each of which compares the
input signal to a unique reference voltage. The outputs of the comparators are fed
to a decoder that produces a binary output. A flash ADC withn-bit resolution
requires 2ncomparators. This is a big disadvantage if high resolution is required.
Generally in radiation detection systems a compromised 8-bit circuitry composed
of 256 comparators is employed. The component intensity of flash ADCs can be
appreciated by noting that even a moderate 12-bit resolution system would require
4096 comparators, a task extremely difficult to accomplish, if not impossible, without
jeopardizing the small size and simplicity of the design. The disadvantages of flash
ADCs include
lower resolution (generally 8 bit),high power consumption,high input capacitance,differential non-linearity, andlarge number of components required (an 8 bit flash ADC requires 256 com-
parators).8.7.C HybridADCs...........................
As we saw in the previous sections, different A/D conversion methods have different
pros and cons. Designing a particular ADC is an optimization process of the two
competing parameters: resolution and conversion time. A conventional ADC work-
ing at high resolution suffers from longer conversion time and is therefore unsuitable
for high rate situations. Fortunately there is a practical way to achieve both of these
traits, i.e. high resolution and short conversion time, in a single ADC. Such an ADC
is calledhybrid ADC, since it is a combination of two conventional ADCs.
Suppose we require a 14 bit ADC that could perform a single conversion in less
than 1μs. It will be quite a challenge to build this ADC using one of the techniques
we visited earlier. However using hybrid technique, we can divide the resolution in
two parts: coarse (say 8 bits) and fine (the remaining 6 bits). For coarse conversion
we can use a flash ADC, which can perform the conversion in 10nsor less. For fine
conversion we can use a Wilkinson ADC, which will take about 0. 6 μsfor a conversion
if working at a clock frequency of 100MHz. Hence one full 14 bit conversion will
be completed in less than 1μswith very good integral and differential linearities.
Because of their superior performance over conventional ADCs, the hybrid ADCs
are becoming more and more popular in high resolution radiation detection systems
designed to work in high rate environments.
8.8 DigitalSignalProcessing.........................
Digital signal processing is an alternative to the analog signal processing we have
been discussing up until now. The basic idea behind it is to directly digitize the
preamplified signal and then process the data digitally. Since here the shaping and