Hi,
I am curious to find out what are some meaningful difference between ADC and TDC detection (analog and time digital converter).
What are some scenarios where one chooses one of the detection over the other? (what are the benefits and disadvantages of the two?)
Dear Sir. Concerning your issue about the meaningful difference between ADC and TDC detection (analog and time digital converter) in mass spectrometry . Compared to a Time-to-Digital Converter*, a Digital Signal Averager enables several orders of magnitude higher ion rates to be processed in a time-of-flight mass spectrometer. A Time-to-Digital Converter is limited by a ceiling on the acceptable ion rate, because it can respond only to single-ion pulses1. A Digital Signal Averager, on the other hand, can respond linearly to any number of ions in each pulse, because it employs an ADC to sample the signal. This difference in performance is important, because mass accuracy, isotope ratio accuracy, and detection limits are inversely proportional to the square root of the number of ions counted in a peak. More ions yield better accuracy and lower detection limits. This application note develops the formulae defining the dependence of mass accuracy and peak area uncertainty on a) ion counting statistics and b) the sampling interval of the ADC in the Digital Signal Averager. For normal operating conditions, the systematic error from the sampling interval is negligible relative to the random error from ion counting statistics. Consequently, interleaved 2-GS/s sampling with a single 500-MS/s ADC yields the same statistical precision as 2-GS/s sampling with 4 parallel 500-MS/s ADCs. But, the former method is significantly more cost-effective. I think the following below links may help you in your analysis:
https://www.ortec-online.com/-/media/ametekortec/application%20notes/an61.pdf?la=en https://www.agilent.com/cs/library/technicaloverviews/Public/5989-0373EN%2011-Dec-2003.pdf
Thanks
TDC-based mass spectrometers can digitize only simples timing information, as used for TOF (time of flight).
ADC-based mass spetrometers digitize the whole signal including information as amplitude, pulse width, signal course ....so they carry more information.
Additionally to the above mentiod application Note " How Counting Statistics and the ADC Sampling Interval Control Mass Accuracy in Time-of-Flight Mass Spectrometry" by D. A. Gedcke (ORTEC) read also the Application Note of Spectrum Instrumentation:
https://spectrum-instrumentation.com/sites/default/files/download/an_mass_spectroscopy.pdf
Some citations from these ApplNote:
"A TDC provides a very cost effective way of measuring the arrival time of each TOF event and allows the creation of a histogram for the mass spectrum (showing the number of times a pulse with a specific arrival time occurs). They are well suited to ion counting systems, such as those found in ESI or LCMS, where ideally single ions hit the detector at any moment.
Available with a wide range of time resolutions (10ns....1ps) TDC’s can be used to make very accurate TOF measurements. However, TDC’s only provide timing information and other characteristics, such as pulse widths and heights,
are generally lost. In addition, TDC’s have difficulty resolving multiple pulses that are produced by ions arriving at the detector at the same or similar times. The TDC will count these sorts of multiple pulses as a single event. This can make using TDC’s in systems where multiple ions hit the detector at each peak (such as with MALDI-TOF) very problematic.
Another issue in dealing with multiple pulses is that it can cause a time skew effect (shifting the mass peak). The TDC only detects one edge of the pulse which may occur at slightly different times for pulses that are created by single and multiple events. This can become a significant problem, particularly in high ion count rate situations.
Finally, in order to create a spectrum with good dynamic range it’s necessary to make a lot of acquisitions. In practice this can be tens of thousands. This becomes extremely challenging if the system has a low repetition rate, which will be the case if it relies on a laser shot being fired several times per second.