A novel nuclear pulse digitizing scheme using time over dynamic threshold

To a high channel count detector system, the parallel multi-channel readout and digitization front-end electronics is always a challenge. The normal waveform digitizing scheme with an ADC for each channel turns to be unacceptable in most cases. We propose to convert the amplitude of nuclear pulse linearly into the time over dynamic threshold (TODT) and realize the pulse amplitude digitization by a TDC. Compare with the normal TOT scheme, the biggest advantage of TODT scheme is that it has better signal-to-noise ratio, besides strict linearity and larger measureable signal dynamic range. We have built a test board to test our TODT scheme. The test results show that an 8-bit digitizing resolution could be achieved using off the shelf components, which is adequate for high resolution PET detectors. The attractive advantage of TODT scheme is that it needs not to add any extra analog circuits to condition the analog signals for the conversion, nearly all the conversion and TDC circuits can be realized by FPGA. With the current TDC design technology using FPGA, the TODT digitization scheme even with hundreds of readout channels can be integrated into an FPGA, so that the physical size of front-end electronics can be very compact with a low cost.