time to amplitude converter
A time to amplitude converter uses an analog technique to convert small time intervals to pulse amplitudes. The device generates an output signal with an amplitude proportional to the time interval between input “start” and “stop” pulses. In most measurement setups, a multi-channel analyzer (multichannel analyzer) then saves and analyzes the amplitude distribution of the output pulses. This is usually referred to as the "time spectrum" and represents a measure of the distribution of the time intervals between the start and stop pulse.
TACs are used, for example, in classic TCSPC. In these cases, the clock signals are first processed by a TAC and then transferred to an analog-to-digital converter. This provides digital values for counting photons, in such a way that the frequency of their occurrence is assigned to the corresponding histogram bins. An important aspect with regard to the measurement accuracy is the dead time that occurs, as this results in counting loss. A detector registering a photon is unable to detect further photons for such dead time. This dead time of TCSPC instruments itself is largely determined by the TACs which are used. In order to reduce the dead time when using excitation sources with a high repetition rate, the TAC is typically operated in the "reverse" start-stop mode. It is started with the signal from the detector and stopped by the correlated signal from the excitation source. In this way, the TAC is only triggered by usable events and not also by those laser trigger pulses that do not even lead to a detected photon. This minimizes dead time effects. For even better performance and shorter dead times, the tasks of the TAC and ADC can instead be taken over by a time-to-digital converter.