PADI, a fast Preamplifier - Discriminator for Time-of-Flight measurements

We designed a general purpose PreAmplifier-DIscriminator (PADI) ASIC to be used as Front-End-Electronics (FEE) for readout of timing Resistive Plates Chamber (RPC) detectors in the CBM experiment at FAIR. Our design is derived from the NINO architecture that was the first ASIC realization of a low power FEE. The PADI-chip can be used for different types of RPC’s, with strip- or pad- anode structures (HADES, FOPI, and ALICE) or for diamond detectors. These fast timing detectors reveal signal rise times tR≪500 ps and generate a prompt charge usually in the range of 50 to 2000 fC. An intrinsic time resolution of σtD≪50 ps is demonstrated with RPC’s, which requires a preamplifier-discriminator stage of an intrinsic electronic resolution of σtE ≪15 ps. The first prototype developed is a three-channel PADI-ASIC in 0.18 μm CMOS technology. It has the following design: fully differential, with 50 Ω input impedance at a preamplifier gain of Gp≫ 100 and a bandwidth of BWp≫ 200 MHz; it shows a signal peaking time of tpk≪ 1ns and a noise of VN−IN ≪ 32 μVRMS referred to the input. The preamplifier use a DC feedback loop for the offset and threshold stabilization with a threshold range between ±100 mV. The discriminator has a gain of Gd≫100. These key parameters of the PADI-1 chip assure an excellent intrinsic electronic resolution of σtE≪ 15 ps and a low power consumption of P≪ 30 mW/ch. As an auxiliary function, PADI-1 offers an analogue signal for the slewing correction. In this article, we present the first measurements from PADI-1 discussing its gain, linearity, bandwidth, noise, and time-over-threshold (ToT)- behavior, as well as timing performance, crosstalk, common-mode-rejection and input impedance. In order to investigate its rate capability and stability in experimental conditions, we tested PADI-1 in two experiments using diamond detectors. We present also the successive designs: PADI-2, 3, which are four channels versions, optimized to maximize the preamplifier bandwidth still maintaining the 50 Ω input impedance. Both have OR-in/out pins to daisy-chain them e.g. for trigger purposes. PADI-4 is a 4 channel version with increased input impedance for the read out of diamond detectors.