CAEN V792N ADC

This short section describes the behavior of the V792N ADC from CAEN. In the current setup a blue LED is coupled to an XP2020 PMT by an optical fiber and the light intensity is adjusted such that the PMT provides a signal between about 10mV and 40mV. The HV of the PMT is set at -1700V. The signal is connected to an amplifier of 10 and then connected to a linear FAN IN/OUT. The DC output of the FAN can be adjusted and is changed between 0 and 15mV in this test. The ADC V792N is a charge integrating ADC with a 12bit digitizer and has a sensitivity of 50fC per channel. There is a built in threshold for each channel that varies between about 50 and 60 bins. In this test channel 4 of the ADC is used and its threshold is found to be about 55 as shown in the next plot. This corresponds to a threshold of 2.75pC. The gate length is set at 63.6ns.

Not that the red histogram is when a signal from the PMT was present but the total integrated charge is below 55*50fC and hence below the threshold of the ADC channel. The pink curve is a fit to the black histogram that is the pedestal generated by a random trigger not correlated to the LED trigger.
In the following the DC offset is increased in steps to find the level where the total integrated charge from the LED trigger event is above threshold. The following plots show the ADC spectrum of the same channel at DC offsets of -3.5mV, -3.9mV, -4.0mV, -4.2mV, -5.0mV and -9.6mV: