A time-to-amplitude converter is a productive solution for measurements on time ranges less than 10 µs when time resolutions from 10 ps to 50 ns are required. However, a TAC can measure only a single time interval for each start pulse, and this limits its utility on the longer time ranges. For example, the condition in Equation 3 restricts the input rates to <1,000 counts/s on a 10-µs time range. This is a low data acquisition rate. On a 1-ms time range, the input rate is limited to 10 counts/s, an extremely low data acquisition rate! Obviously, a time-to-amplitude converter is handicapped by low data acquisition rates on the longer time ranges when distortion of the time spectrum must be avoided.

Most measurements that require time ranges in excess of 10 µs involve a controlled, pulsed source of excitation. In such circumstances, a multichannel scaler (MCS) is advantageous because it can accept multiple stop pulses for each start pulse. The pulsed excitation source starts the time scan on the MCS, and the events caused by the excitation are counted as a function of time on the counting input of the MCS. The result is a spectrum of the number of events versus the time after excitation. With a pulse-pair resolving time of 1 ns, the ORTEC Model 9353 is able to process average "stop" rates up to 10 MHz with less than 1% dead time losses, and burst rates up to 1 GHz. Of course, the period between excitation (start) pulses must be longer than the time interval being measured.

Clearly, the MCS is the more productive instrument for measuring time ranges longer than a few microseconds. However, the performance for some MCS models on shorter time ranges is limited by the intrinsic time resolution off set by the minimum possible dwell time.

A TAC combined with the CAMAC multi-parameter ADCs is an ideal solution for measurements requiring correlated sampling of amplitude and time data from one or more detectors. The 9353 is not suited for multi-parameter measurements.

Generally, one should consider a TAC for time ranges <1 µs and multi-parameter measurements and the 9353 for time ranges from microseconds to milliseconds. For further information on the latter two instruments see the Counters, Ratemeters, and Multichannel Scalers introduction.