(4)—
A Microcomputer-based X-ray or γγ-ray Spectrometer
Modern X-ray and γ-ray spectrometers frequently employ semiconductor detectors which produce
electrical pulses with sizes proportional to the energies of the incident photons. Using a fast ADC, the
signal is digitized and a conventional spectrum generated by a dedicated microcomputer. As many as
8K data points (channels) may be sampled with facilities for perhaps four spectra stored in RAM,
together with key information such as calibration data, accumulation time, etc.
The very short duration of the pulses comprising the analogue signal from the detector, demands special
attention to ADC design. A version which has become widely used is the Wilkinson type linear ramp
converter. The popularity derives from very good linearity. Figure 13.7 illustrates the essential parts of
the ADC circuit. The detector signal is fed to a comparator circuit, which responds to the arrival of a
pulse by initiating the generation of a linearly increasing ramp voltage, and signalling the opening of
the linear gate. A continuous comparison of the signal pulse amplitude is made with the ramp voltage
until the amplitude of the pulse is matched by the ramp voltage, at which point the linear gate is closed.
Thus the linear gate is opened for a time (∆t) which is proportional to the amplitude of the signal pulse.
During this time, pulses from a constant frequency clock are admitted to the address register which
accumulates them, the number accumulated being proportional to the time the gate is opened and thus
to the signal pulse amplitude. To minimize the conversion time, and maintain accurate digitization of
the input signal, high clock frequencies are needed. Commercial ADCs are currently available with
speeds of up to 400 MHz and are often characterized by quoting the speed. It should be emphasized that
this is not the same as the conversion speed which remains in the range of 5– 30 μs.
Figure 13.7
Block diagram of a linear ramp (Wilkinson-type) ADC.