We present a readout and digitization ASIC featuring low-noise and low-power for time-of flight (TOF) applications using SiPMs. The circuit is designed in standard CMOS 110 nm technology, has 64 independent channels and is optimized for time-of-flight measurement in Positron Emission Tomography (TOF-PET). The input amplifier is a low impedance current conveyor based on a regulated common-gate topology. Each channel has quad-buffered analogue interpolation TDCs (time binning 20 ps) and charge integration ADCs with linear response at full scale (1500 pC). The signal amplitude can also be derived from the measurement of time-over-threshold (ToT). Simulation results show that for a single photo-electron signal with charge 200 (550) fC generated by a SiPM with 320 pF capacitance the circuit has 24 (30) dB SNR, 75 (39) ps r.m.s. resolution, and 4 (8) mW power consumption. The event rate is 600 kHz per channel, with up to 2 MHz dark counts rejection.

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We present the experimental characterization of the TOFPET2, a readout and digitization ASIC for radiation detectors using Silicon Photomultipliers. The circuit is designed in CMOS 110 nm technology, has 64 independent channels and is optimized for time-of-flight measurement in PET or other applications. The chip has quad-buffered TDCs and charge integration QDCs in each channel. The Coincidence Time Resolution (CTR) of 511 keV photon pairs from a 22Na point source measured with 2 × 2 × 3 mm3 LSO:Ce crystals co-doped with 0.2% Ca is 118 and 119 ps FWHM when using respectively the SiPMs NUVHD 40um from Fondazione Bruno Kessler (FBK) and the S14160-3050HS MPPC from Hamamatsu Photonics (HPK). The energy resolution obtained for the 511keV photopeak is 10.5 and 12% FWHM when using respectively the SiPMs PM3325-WB from KETEK and the QFBR-S4N44P164S from Broadcom Inc.

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A low noise transimpedance amplifier (TIA) is used in radiation detectors to transform the current pulse produced by a photo-sensitive device into an output voltage pulse with a specified amplitude and shape. We consider here the specifications of a PET (positron emission tomography) system. We review the traditional approach, feedback TIA, using an operational amplifier with feedback, and we investigate two alternative circuits: the common-gate TIA, and the regulated cascode TIA. We derive the transimpedance function (the poles of which determine the pulse shaping); we identify the transistor in each circuit that has the dominant noise source, and we obtain closed-form equations for the rms output noise voltage.We find that the common-gate TIA has high noise, but the regulated cascode TIA has the same dominant noise contribution as the feedback TIA, if the same maximum transconductance value is considered. A circuit prototype of a regulated cascode TIA is designed in a 0.35 m CMOS technology, to validate the theoretical results by simulation and by measurement.

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SUMMARYRelaxation RC-oscillators are notorious for their poor phase-noise performance. However, there are reasons to expect a phase-noise reduction in quadrature oscillators obtained by cross-coupling two relaxation oscillators. We present measurements on 5 GHz oscillators, which show that in RC-oscillators the coupling reduces both the phase-noise and quadrature error, whereas in LC-oscillators the coupling reduces the quadrature error, but increases the phase-noise. A comparison using standard figures of merit indicates that quadrature RC-oscillators may be a viable alternative to LC-oscillators when area and cost are to be minimized.