INFN Milano University of Milano Department of Physics

INFN - Milano University of Milano Department of Physics "Fast reset" ASIC Preamplifier Cutting-edge circuit technology able to boost the dynamic range of CMOS charge-sensitive preamplifiers far beyond their saturation limit Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 " Oct. 30, 2012

Outline § Context and goal of the research § Large volume HPGe detectors & required specs for charge preamps § The issue of saturation & dynamic range for integrated preamplifiers § Technique to handle saturated signals with low-noise CMOS preamps § Charge information recovery: extending the dynamic range beyond the saturation limit of the preamplifier § Reset-mode high-resolution spectroscopy § Conclusions Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Context and goal Context § New generation of nuclear-physics experiments with highintensity ion beams. § A technical advance for the new gamma-ray spectrometers is required. Goal of the research § New paradigm: high-resolution spectroscopy is possible even working with a deeply saturated CSP ! Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Large volume HPGe detectors High-resolution gamma-ray spectroscopy investigation of nuclear structure under very extreme conditions of stability ~ 9 cm Main features: • operated at cryogenic temperature (liquid nitrogen used as cooling medium @ 77 K) • electrodes capacitance values: 20 -100 p. F • energy of detected gamma-rays: from a few ke. V to a few tens of Me. V • excellent intrinsic energy resolution: 0. 1% - 0. 2% in the typical 1 Me. V region • segmentation of the outer electrode position-sensitivity: ~ 1 mm 3 D resolution Gamma-ray tracking emitting radionuclide Speaker: Stefano Capra Gamma-ray Compton imaging can take advantage of the excellent energy/position resolution of HPGe "GASPARD-HYDE-TRACE Workshop 2012 "

Charge-preamplifier specifications § low noise (gamma spectroscopy grade: 0. 1 -0. 2 % @ 1 Me. V) § excellent stability of the gain and of the shape of the preamplifier response (loop gain ~ 103) § wide bandwidth: rise time of ~ 20 ns (pulse shape analysis) § low power consumption (especially for the devices operated in the cryostat) § LARGE DYNAMIC RANGE: - at least ~104 : from a few ke. V to 10 -20 Me. V - up to ~30 Me. V depending on the physics experiment (i. e. giant resonances) - minimization of the dead time in a much larger energy range up to 100 -200 Me. V extremely hostile background of highly energetic charge particles in next-generation nuclear physics experiments with highintensity exotic beams Speaker: Stefano Capra γ ( 1 -10 Me V) p K ( 10 -100 Me. V) HPGe segmented detectors of AGATA (Advanced GAmma-ray Tracking Array) "GASPARD-HYDE-TRACE Workshop 2012 "

The issue of a wide dynamic range Old-style solution: hybrid DISCRETE preamplifiers • high flexibility in the design • use of high voltage power supply (ex: +/- 12 V) Modern CMOS integrated solutions: a mandatory task the high segmentation of the read-out electrodes yields a higher and higher count of read-out channels • small dimensions & low power dissipation • radio-purity and full functionality at cryogenic temperature Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

The issue of a wide dynamic range Old-style solution: hybrid DISCRETE preamplifiers • high flexibility in the design • use of high voltage power supply (ex: +/- 12 V) Modern CMOS integrated solutions: a mandatory task the high segmentation of the read-out electrodes yields a higher and higher count of read-out channels • small dimensions & low power dissipation • radio-purity and full functionality at cryogenic temperature Intrinsically low available voltage swing of scaled CMOS technologies A decrease of the preamplifier sensitivity (energy-to-voltage gain) would compromise the signal-to-noise ratio and the spectroscopic performances Saturation of a CMOS preamplifier for HPGe detectors is expected for input energies > 5 -10 Me. V Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Time-variant circuit structure*: typical-amplitude signals Charge-Sensitive Preamplifier (CSP) Mode • For “normal” amplitude signals (up to a few Me. V) the comparator keeps switch “S” in the right position • The circuit is a Low-Noise Charge-sensitive preamplifier • Allows for high-resolution energy measurements Normal CSP External discrete components: BF 862 Si JFET, RF=1 GW, CF=0. 2 p. F X X CSP Mode X X S Comparator Threshold of Comparator CSP Mode *see also Radeka, Overload Recovery Circuit for Charge Amplifiers, IEEE Trans. Nucl. Sci. , vol. 17, no. 1, p. 269, 1970 Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Time-variant circuit structure*: large signals Fast-Reset Mode • Minimizes the preamplifier dead time and prevents from the paralysis of the acquisition system in the case of extremely high background counting rates • Allows for charge information even in the saturation condition • Allows for high-resolution energy measurements extending the dynamic range of photons/particles spectroscopy External discrete components: BF 862 Si JFET, RF=1 GW, CF=0. 2 p. F Q + ++ + Fast Reset Mode S Reset Time Threshold of Comparator I const *see also Radeka, Overload Recovery Circuit for Charge Amplifiers, IEEE Trans. Nucl. Sci. , vol. 17, no. 1, p. 269, 1970 Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

The charge preamplifier Layout 0. 35 mm 5 V mid-oxide CMOS Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

The Schmitt trigger comparator Schmitt trigger configuration with a positive feedback loop lower reset threshold set at 0 V by the ground voltage reference external feedback resistances to adjust the upper reset threshold (~ preamplifier saturation voltage) Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

The current sink the comparator output signal drives a transmission gate so as to deviate the reset current to the input node of the preamplifier Reset current NMOSFET inserted into the negative feedback loop of an operational amplifier Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Realized chip Pre OUT Pre IN IRESET - Curr IN Comp_IN+ + + - VEE INHIB RSINK Comp OUT VEE Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Charge information recovery Computer simulation During saturation, the physical information, i. e. the charge released by the germanium crystal, is not lost but temporarily stored on the total capacitance at the input node of the circuit Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Charge information recovery The bounce of the input node voltage (virtual ground) is actually negligible because the capacitance of HPGe detectors (20 -50 p. F) is typically two order of magnitude larger than the feedback capacitance (0. 2 -0. 5 p. F) Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Charge information recovery Preamp response to a detector like test signal (200 ns width) Computer simulation The bounce of the input node voltage (virtual ground) is actually negligible because the capacitance of HPGe detectors (20 -50 p. F) is typically two order of magnitude larger than the feedback capacitance (0. 2 -0. 5 p. F) Speaker: Stefano Capra VG voltage bounce Computer simulation "GASPARD-HYDE-TRACE Workshop 2012 "

Charge information recovery Computer simulation The bounce of the input node voltage (virtual ground) is actually negligible because the capacitance of HPGe detectors (20 -50 p. F) is typically two order of magnitude larger than the feedback capacitance (0. 2 -0. 5 p. F) Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Remove Q and measure T The larger Q the longer the time T needed for complete removal • Constant current rate of removed Charge over Time is constant from the definition of electric current • Q and T are proportional • Nonlinearity of preamplifier working condition must not influence Q vs T linearity Precise charge measurements are possible in saturated conditions • Parasitic DC paths • Virtual ground bounces So, the issue of the intrinsically low available voltage swing of scaled CMOS technologies may be skipped through Time-over-Threshold measurements !! Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Active Fast Reset Computer simulations Preamplifier output signals of different amplitudes (negative holes signals) Digital pulses delivered by the comparator: a time-over-threshold measurement provides the reset time Proportional relation between input charge and reset time Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Reset time - Input charge Relation Charge Energy in Ge The percent error in the fitting mainly comes from the numerical approximation of the simulation program and has to be considered as a maximum upper limit Speaker: Stefano Capra 1 p. C 2 p. C 3 p. C 4 p. C 5 p. C 18. 2 Me. V 36. 4 Me. V 54. 5 Me. V 72. 7 Me. V 90. 9 Me. V "GASPARD-HYDE-TRACE Workshop 2012 "

Reset time - Input charge Relation Q = (IRESET + VSAT/RF) T + Q 0 • (VSAT/RF)T = charge removed (~0. 8%) by the current flowing on RF while the output signal is saturated IRESET = constant reset current VSAT = output saturation voltage RF = feedback resistance T = reset time Q 0 = offset term • Q 0= offset term due (i) to the mismatch between the baseline voltage and the 2 nd reset threshold (~0 V), (ii) to the constant charge removed by the current flowing in RF out of the saturation condition • second-order effects on the bias conditions of the detector and JFET pn junctions, due to the virtual ground bounce, are found to be negligible in our computer simulations Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

“Reset-mode” spectroscopy “Reset-mode” high-resolution spectroscopy has been demonstrated Hybrid charge preamplifier with second-stage active fast reset Reset-mode 241 Am+Be spectrum resolution on Ni line: 0. 21% @ 9 Me. V See: F. Zocca, A. Pullia, D. Bazzacco, G. Pascovici, “A Timeover-Threshold technique for wide dynamic range gammaray spectroscopy with the AGATA detector”, IEEE Trans. Nucl. Sci. , vol. 56, no. 4, pp. 2384 -2391, Aug. 2009 Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

“Reset-mode” spectroscopy Experimental results with “Fast reset” ASIC preamplifier Excellent linearity also in Fast Reset mode Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

“Reset-mode” spectroscopy Experimental results with “Fast reset” ASIC preamplifier ENC: equivalent noise charge with 15 p. F (higher value) and 0 p. F (lower value) detector capacitance Series 1/f noise Parallel white noise Series white noise Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "

Conclusions § Design & computer simulation of a JFET-CMOS preamplifier (0. 35 mm) for HPGe detectors equipped with a fast reset device for charge sensing stage de-saturation § Dead time minimization in the cases of high background counting rates § Charge measurements even in a condition of deep saturation → boost of useful dynamic range for highresolution energy measurements § Experimental tests performed on the realized test chip Speaker: Stefano Capra "GASPARD-HYDE-TRACE Workshop 2012 "