Preamplifier Base Assemblies

The preamplifier base assemblies consist of resistive dividers formed using surface mount resistors attached to a printed circuit boards soldered directly to the wire leads of the PMTs. The total string resistance was chosen to be fairly high ($\sim 25$ M$\Omega$) so as to keep the power dissipation to a minimum ($\sim 0.25$ W per base under normal conditions). The preamplifiers are based on the MAXIM MAX4107 low-noise preamplifier chip. This device was selected for its large bandwidth (350 MHz at $A_V=10$) and its low noise ( $0.75~{\rm nV}\sqrt{\rm Hz}$ equivalent input noise). The 150 mW power dissipation, while not negligible, did not present a significant thermal management problem. The FMPMTs were sorted according to gain and matched to preamplifiers, which were fabricated with fixed voltage gains of 10, 20, 40, and 70. The outputs of the preamplifiers drive 30-m-long coaxial cables connected to the charge-to-time converter boards, which are situated in the electronics hut. Residual tube-to-tube gain variations and gain changes over time were compensated by adjusting the high voltage applied to the divider string. Each FMPMT in the system was biased by a separate channel of the LeCroy Model 1461 12-channel high voltage modules. These modules were computer controlled using special software developed for the purpose.