Physics and Engineering of Radiation Detection

504 Chapter 8. Signal Processing

to completely discharge the capacitor having capacitanceC.HereVinis the

input voltage. Since this time is proportional to the digital counter therefore

the final digitized word is proportional to the input voltage (see also Fig.8.7.8).

Cm Si

Vt

Stretcher

Pulse

Register

Counter/

Clock

Output

Comparator

Digital

Analog
Input

(a)

+ +

− −

Analog

Input

s

(b)

h

r

Figure 8.7.7: (a) Schematic

of a Wilkinson ADC. Cmis a

memory capacitor, Siis a cur-

rent source, andVtis the refer-

ence voltage used by the com-

parator to make the conversion

start/stop decision. Normally

Vtis kept at the ground poten-

tial, in which case the compara-

tor is called a zero-crossing com-

parator. (b) Sketch of a simple

sample and hold circuit that can

be used as a pulse stretcher in

Wilkinson ADC.

td

Sample Hold Digitize

Vin

V(t)

t

Figure 8.7.8: Conversion cy-

cle of a typical Wilkinson ADC.

The input pulse amplitude is

sampled and held on a capacitor

and then digitized. The digiti-

zation timetdis proportional to

the input voltageVin.

A big disadvantage of Wilkinson A/D technique is its long conversion time, which
can be calculated from

Tconv=2nTclock=

2 n

fclock

, (8.7.3)

wherenis the ADC resolution in bits andTclockis the period of counter’s clock
having frequencyfclock. Hencea12bit(n= 12) Wilkinson ADC having a clock
frequency of 100MHzwill complete one conversion in about 41μs,whichisa