A time-to-digital converter (TDC), is sometimes called a Time Digitizer. It works like a stop-watch at a horse race. The horses are analogous to the molecular ions in a TOF-MS. When the horses are released from the gate, the watch begins ticking off time. As each horse crosses the finish line, the lap-time button is pushed to record the arrival time for that horse. In the horse-race application it is important to be able to resolve the arrival times of horses crossing the finish line with close spacing. The same principle applies in mass spectrometry. An important parameter is the pulse-pair resolving time of the TDC. This defines the ability of the TDC to measure the arrival times of closely spaced ions.

A primary consideration is the time span that is needed to cover the minimum to maximum flight times of interest in the TOF-MS. Can the TDC cover the necessary time span while providing an adequate definition of the peaks in the spectrum? To precisely define the peak positions in the spectrum, the digital resolution of the time measurement in the TDC should typically be less than 1/5 the FWHM peak width. Better digital time resolution is usually achieved at the expense of pulse-pair resolving time. For example the Model 9353-P provides 100 ps digital time resolution with a 1 ns pulse-pair resolving time, while the Model 9308-PCI-P offers a much finer, 1.2 ps digital time resolution, but with a longer, 50 ns pulse-pair resolving time.

If the data rates are expected to be high, the maximum data rate that can be transferred to the computer and saved on hard disk may become critical. At low rates, the ability to choose the list mode (event-by-event record) versus the histogramming mode (all events sorted and accumulated by time bin) may be useful for file size reduction.

For more detailed explanations of Time Digitizers, click here

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