Low-noise, transformerless front-end electronics for moderate and high capacitance calorimeters

A charge-sensitive preamplifier showing very low power consumption was developed in two versions: the first using a JFET SNJ903L has a matching capacitance of 310 pF whereas the second one employing a SNJ3600L has 1100 pF. Both versions have a charge sensitivity of 8 μV/e−. The rise time is lower than 200 ns for both low and high capacitance versions at an input capacitance of 1000 and 3000 pF respectively. The low capacitance version has an equivalent noise charge of 220 e− rms at 0 pF for a CR-(RC)2 shaping at 10 μs shaping time. The noise at 1 μs is 510 e− rms at 0 pF and the noise slope is 1.59 e−/pF. The high capacitance version exhibits 400 e− rms at 0 pF and 10 μs shaping time. At 1 μs the equivalent noise charge is 1100 e− rms and the noise slope is 0.91 e−/pF. The second stage uses operational amplifiers of 16 MHz gain-bandwidth product and is prepared to make a differentiation with pole-zero cancellation if required. The final stage provides a differential output with 50 ω output impedance and can drive a 100 ω twisted pair cable loaded with its characteristic impedance. The total power dissipation is 190 mW for the SNJ903L version whereas the SNJ3600L version dissipates 265 mW. Both versions use ±6 V. The circuit is mounted using thick film technology onto a 20×50 mm2 ceramic substrate. The project is complemented with the design of a 3000 V, 24 pF/m coaxial cable for detector biasing and signal transport with minimum contribution to the ENC.