Introduction

I am working toward a project we discussed in my last PhD committee meeting, which consists in the implementation of a 4D tracking system prototype using TI-LGAD devices. Such system would consist on stacking multiple layers of TI-LGADs with at least 4 pixels each and expose them to the particles from a test beam. The TI-LGADs would be mounted on analog readout boards and the signals from each channel have to be somehow digitized. One possibility is to proceed by brute force, i.e. by putting all our oscilloscopes together to reach the required number of channels. Though this is possible, it is cumbersome and expensive. Another possibility is to use a digitizer with more channels. There are not many available options in the market suitable for this, but one option es the CAEN DT5742 which has 16+1 channels and a sampling frequency of 5 GHz. So I was fortunate to get one of these devices to play with for some weeks in the lab before making the decision of buying it or not. Here I present my results and methods.

I had to develop a Python interface for the digitizer which can be found at https://github.com/SengerM/CAENpy. I want to thank the people from the Torino group which kindly provided me help for this and shared their code.

Setup and measurements

To perform the tests of the digitizer I used a modified version of our beta setup at UZH  on which time resolution as low as ~30 ps has been measured , and in some cases we have seen even smaller values down to ~26 ps. shows a schematic representation of the setup. The DUT was installed on the usual readout boards we have in our setup, which are the Chubut boards  very similar to the Santa Cruz board. The output from the readout board was connected to a 20 dB amplifier which, again, is part of our regular beta setup. For triggering and time reference the MCP-PMT which is part ouf our beta setup was used in the same way as usual, with the only difference being that for some tests an additional 20 dB amplifier was placed (Amp 2 in ). This was done because the threshold level in the CAEN digitizer is not easy to configure, and this additional amplifier made it easier. In order to compare the results with the current digitizing system we have, the same measurements were performed both with the CAEN and with our LeCroy Waverunner 9254M oscilloscope which is part of the setup.

Schematic of the setup.

Results

As of today, 27 Feb 2023, the measurements are still ongoing. It is taking me longer than expected but still I already have some results. These are shown in the plotted of in which the jitter is plot against the bias voltage for a TI-LGAD from wafer 16, one trench, pixel border V3, contact type dot, labeled TI106. Please note that the y axis is the jitter and not the time resolution. The reason I am comparing the jitter and not the time resolution is that the jitter is actually what we measure, while the time resolution comes after subtracting the contribution from the time reference to the jitter, in our case the MCP-PMT. This contribution depends on several things, not only on the device. One such thing is the trigger threshold value which is unfortunately unknown in the CAENIt can be changed but we only know the "DAC" value, not the volts. And it has a weird dependence with some other parameters, so you cannot know it. This is what they say in the user manual.. Another dependence is on the number of amplifiers, and here I am comparing measurements with one and two amplifiers. Thus, it makes more sense to compare the jitter and keep in mind that the time resolution of the DUT is going to be slightly lower.

Coming back to the plot of , each trace is a voltage scan. The color denotes if the measurement was performed using the LeCroy oscilloscope or the CAEN digitizer while the columns denote the number of amplifiers that were placed after the PMT (either one or two). As can be seen the results are in all cases in agreement within the error bands. The measurement of TI107 with 1 amp in the PMT and with the LeCroy oscilloscope was performed in the beta setup during 2022.

On an example of a signal digitized by the CAEN is shown. As can be seen the quality is quite good despite the lower sampling frequency when compared to the LeCroy Waverunner 9254M oscilloscope.

Conclusions

A CAEN DT5742 digitizer was set up and tested in our lab with our beta setup. The results show that, for this application, it is capable of measuring jitter comparable with our current LeCroy Waverunner 9254M oscilloscope. Jitter values as lo as 35 ps were measured with the CAEN digitizer, which should be enough for the application we are foreseeing.

Note

As I write this, additional measurements are being performed with the digitizer in the setup. As soon as I have these results I will update this document.