Multichannel Analyzer MCA4 / Questions and Answers

Is the MCA4 a Wilkinson or fixed time constant ADC? We are interested in the high energy end of the spectrum (~8V), but vast majority of counts are low energy ~1V. How does FAST stack up against 100 MHz Wilkinson ADC in processing those low energy pulses?

The MCA4 type ADC is much faster and more precise than a 100 MHz Wilkinson type ADC.Wilkinson: at pulse peaking, the maximum has to be detected, the value isstored (sample & hold circuit), the conversion is started, result isstored at the end of conversion.MCA4: the input signal is sampled continuously at 125 MHz and converted with 65 535 ch resolution. around the maximum, the pulse is fitted and thefitting result gives the amplitude of the signal with high resolution(64k). Also some noise is removed because of the evaluation of manysampling values.

Wilkinson rate calculation:1 / (pulse peaking time (dependent on pulse shape) + maximum detection andstorage time (some 10 ns) + conversion time (1V, 8k ch, 8V input range:1000 x 10ns = 10 µs) For a 1µs wide pulse (FWHM), the maximum rate would be around 1/ (1µs + 10ns + 80µs)  = 1/11.1µs = 90kHz. For high amplitude pulses the rate is of course lower, for 8V pulses it is 1/81.1µs = 12 kHz.MCA4 ADC rate calculation:Processing time for a pulse is less than 1µs. so the maximum rate isbeyond 1 MHz as long as the input pulse shape allows separated inputpulses. The sampling process does not interfere with the pulse processing.For signal shaping times below 0.25µs, the highest rate can be reached.The input signal has to go lower than the input threshold in order to be detected as a new pulse. For narrow input pulses, a rate of more than 1MHz has been reached in our lab for all 4 input channels simultaneously.The throughput does not depend on the pulse height, so it is a fixed time constant ADC. The time constant can be adapted to the pulse shape.

Multichannel Analyzers, MCA4