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Description
This work describes the design and the testing of a charge-sensitive amplifier (CSA) mainly conceived for the readout of hybrid pixel detectors in extreme radiation environments, as in the high-luminosity upgrades of the LHC or in future experiments at the FCC. Developed in a 28 nm CMOS process, the CSA features a feedback network to compensate for detector leakage current and operates at a typical current close to 4 μA, with a nominal supply voltage of 0.9 V. The core amplifier of the CSA is implemented by means of a regulated cascode structure followed by an NMOS-input source follower stage. A metal-oxide-metal (MoM) capacitor, whose nominal value is around 4.5 fF, is integrated into the CSA feedback loop for charge-to-voltage conversion of the signal delivered by the sensor. A narrow channel NMOS transistor, in parallel with the feedback capacitance and operated in deep-subthreshold, is exploited to restore the CSA baseline upon signal arrival.
The CSA has been integrated into a prototype ASIC (in a mini@sic run) with additional circuitry to emulate the detector capacitance and leakage current. In particular, three MoM capacitors (0, 25 fF, and 50 fF) can be connected to the CSA input through different CMOS switches. On the other hand, the presence of an NMOS transistor, with the drain connected to the CSA input and the source tied to ground, can be exploited to emulate the detector leakage current by adjusting the voltage at its gate terminal. A charge injection circuit is also integrated in the prototype chip, which makes it possible to provide a test charge up to around 30000 electrons at the front-end input. The output of the CSA is fed into a two-stage source follower, which drives a PAD wire-bonded to a carrier board. This board is mounted on a mother board that includes biasing circuits for the CSA, as well as an additional buffer stage for the preamplifier output. The signal is then sensed by a digital oscilloscope for off-line analysis.
The prototype chip has been characterized before and after irradiation up to a total ionizing dose (TID) of 1 Grad, with a 10 keV X-ray source (Seifert RP149).
Experimental results show that the CSA can be operated properly in the presence of a detector leakage current up to 10 nA (which is the maximum that can be achieved in the test setup used for the prototype characterization), with a marginal increase in equivalent noise charge (ENC). Before irradiation, the measured ENC is close to 70 electrons RMS for a detector emulating capacitance of 50 fF, with an increase around 12% observed at 1 Grad. The Time-over-Threshold (ToT) performance of the CSA was evaluated by leveraging the digital scope, where the preamplifier signal was compared against a fixed threshold (500 electrons). The maximum integral non-linearity for the ToT characteristics was found to be close to 3.8%. A significant variation of the current discharging the CSA feedback capacitance was observed after irradiation. To restore a ToT behavior similar to the one achieved for the unirradiated device, a fine-tuning of the voltage controlling the feedback NMOS transistor was needed.
The details on the design and on the characterization of the charge sensitive amplifier will be given in the conference paper.
| Workshop topics | Front-end electronics and readout |
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