ABACUS : Two fast amplifiers for the readout of LGAD detectors

Abstract

: The design of a single particle counter for therapeutical proton beams based on Low Gain Avalanche Diodes optimized for very fast signals is carried on in the framework of the INFN MoveIt research project. Fast signal shaping front­end electronics is mandatory in this application in order to deal with particle rates of the order of hundreds of MHz. Two preamplifier architectures, one based on a fast Charge Sensitive Amplifier with self­reset capabilities and a second one based on a Trans­Impedance Amplifier have been developed in a commercial CMOS 0.11 μm technology and submitted to the foundry. a INFN sezione di Torino, 10125 Torino, Italy b Politecnico di Torino, 10129 Torino, Italy c Università di Torino, Dipartimento di Fisica, 10125 Torino, Italy R. Cirio, F. Fausti, G. Mazza, V. Monaco, J. Olave, R. Sacchi Low Gain Avalanche Diode (LGAD) Requirement : fast counting rate (100 MHz or better fast rise time → not sufficient,the full signal shape must be short. Two configurations under evaluation : 1. Charge Sensitive Amplifier (CSA) 2. TransImpedance Amplifier (TIA) CSA : fast rising time and lower noise than TIA, but the falling edge can be slow self reset capability → TIA : faster but also higher noise. Trade off between bandwidth, open loop gain and input capacitance f −3dB≈ 1 2π A RF CD 1. CSA preamplfier 2. OTA buffer 3. Discriminator stage 1 4. Discriminator stage 2 5. Single­ended to diff converter 6. CML driver 7. Threshold tuning DAC (6­bit) 8. Pulse generator 9. Recovery block 10. Inverters CSA­based readout channel TIA­based readout channel 1. Transimpedance preamplifier 2. Gain stage 3. Discriminator 4. CML driver 5. Discriminator differential threshold generator Transimpedance Amplifier architectures Two stages core amplifier with global resistive feedback : ✔ Higher open loop gain ✗ Sensitive to input capacitance variations ✗ Differential input Single stage core amplifier driven by a current buffer : ✔ TIA input capacitance independent from detector capacitance ✔ Single ended input ✗ No gain on the first stage Simulation result examples Two detectors under test : length 15 mm, width 80 μm, pitch 146 μm and length 30 mm, width 150 μm, pitch 216 μm Input capacitance C D ~ 3­6 pF Signal (protons) : 4­150 fC (with gain 15) Signal (carbon ions) : 30­150 fC (with gain 1) Input data obtained from Weightfield2 and GEANT simulations for the predicted environment. Average particle rate per channel are 50, 100, 150, 200 and 250 MHz. Post layout simulations detail for the CSA channel for 100 and 200 MHz and 60 MeV and 250 MeV proton energy. Post layout simulations for the TIA v1 channel at 200 MHz rate. Proton energy 250 MeV. Summary and outlook  Two 24­channels prototypes for the fast count of particles crossing a LGAD silicon detector have been designed and produced.  The first prototype is based on a charge sensitive amplier with self­reset  The second prototype is based on a transimpedance amplifier architecture. Two sub­version of this design have been realized.  Both designs have been made in a CMOS 0.11 μm technology powered at 1.2 V  The power consumption is of the order of 20 mW/channel.  Prototype tests will start on Fall 2018