Description and details of the UCSC single channel board design.

UcscSingleChannel



Electrical characteristics

  • The read-out board is based on a single transistor common emitter design and acts as an inverting trans-impedance amplifier. Amplification is performed by an AC coupled silicon-germanium bipolar transistor with a bandwidth of 75 GHz. At a bandwidth of 1.9 GHz a gain of 29 dB is expected, while an integrated output noise of 260 μV is estimated. The feedback loop is designed for timing with small capacitance sensors inducing typical rise times in the order of 800 psec, with a feedback resistor of 470 Ω.

  • Sensors are attached to the boards using double sided conductive tape while the amplifier input is coupled to the front side metallization layer via multiple wire bonds to minimize inductance. An 1 ΜΩ resistor attached between input and ground serves for detector biasing, followed by a pair of low forward-resistance silicon pin diodes. The latter, with a 50 V breakdown at 5 μA functions as a protection for the amplifier input.

  • The overall trans-impedance within a 1.6 GHz bandwidth and terminated into 50 Ω is estimated with respect to the feedback resistor in the following table. Referenced values assume an external voltage amplifier of a gain of 10 while, an overall scale error of 10\% needs to be attributed.

Trans-impedance
Feedback resistor Trans-impedance
475 Ω 470
2 kΩ 2000
  • Care is taken to provide complete hermetic shielding on both sides of the board up to a bandwidth of 3 GHz, with RC filtering in both the HV and low voltage input lines.

  • The PCB design has been optimized to minimize parasitics and reduce inductance on the signal return path using at least 6 decoupling high voltage capacitors, 0201 size surface mount components and ground buried signal and power lines.

  • Supply voltage is set to 2.25V with an estimated current consumption at normal operation of 15-17 mA. Powering should be provided by an isolated (floating) linear power supply while grounding is assured via the signal readout cable at the DAQ level.

  • In the case of no external grounding, charge accumulation is susceptible to be observed on the second stage amplifier. The amplifier board should not be powered unless proper grounding is ensured either via the readout or via one of the ground mounting points on the board.

  • Connectors: 4 SMA front mounted top side right angle connectors, including high and low voltage, signal output and calibration input.

Electrical Characteristics
Nominal Supply Voltage 2.25 V
Nominal supply current 15 mA
Nominal Gian 10
Estimated noise (rms) 1.2 - 2.0 mV

Board Layout - Mechanics

  • Original design is implemented in FR4 dielectric in a four layer scheme. A tin immersion finish is applied on the top layer which does not guarantee planarity and should be replaced by Gold-Nickel immersion or Paladium - Nickel electroplating. Cooper thickness at the inner layers is in the order of 17.5μm while on the two outer layers the traces are 35μm thick to better cope with the grounding and HV planes.

Single Channel board layout

Schematics and Parts list

Complete schematics can be found here.

Item No. Qty. Reference on schematics Part Name DigiKey Part No. Notes
1 1 L4 Inductor, 120 mA, 47 μH 445-3623-1-ND Shielded Wirewound Inductor, 5.3 Ohm, 0805
2 2 D7, D8 DIODE PIN ASM 50V 100MA TSSLP2 BAR95-02LSE6327INCT-ND RF Diode PIN - Single 50V 150mW
3 1 Q4 TRANS RF BIPO NPN 35MA TSLP-3 BFR840L3RHESDE6327XTSA1CT-ND RF Transistor NPN 2.6V 35mA
4 2 S1, S2 BOARD_SHIELD 50X50MM 732-2495-ND Shield Frame
5 2 ----------- BOARD SHIELD 50X50MM COVER 732-2497-ND Lid for shield frame
6 2 BP7, BP8 BOND_PAD ----------- Amplifier input pad in layout, Ground pad
7 1 C22 CAP0201, 0.3 pF 399-8569-1-ND 0.3pF 0.05pF 25V Ceramic Capacitor C0G, 0201
8 2 C23, C24 CAP0201, 3.3 nF 490-3191-1-ND 3300pF 10% 16V Ceramic Capacitor X7R, 0201
9 1 C25 CAP0402, 3.3 nF 587-1063-1-ND 3300pF 5% 10V Ceramic Capacitor 0402
10 1 C26 CAP0402, 3 pF 490-11211-1-ND 3pF 0.05pF 50V Ceramic Capacitor C0G, 0402
11 2 C27, C28 CAP0603, 1000 pF 587-1069-1-ND 1000pF 5% 50V Ceramic Capacitor 0603
12 2 C35, C36 CAP0805, 10 μF 478-5167-6-ND 10 F, 10% 10 V Ceramic Capacitor X5R 0805
13 2 C29, C30 CAP1812, 1200 pF 399-15624-6-ND 1200pF, 5%, 1kV, Ceramic Capacitor C0G, 1812
14 1 L5 Inductor 180 mA, 22 μH 445-6145-1-ND Shielded Wirewound Inductor, 3.12 Ω, 180 mA
15 6 C31, C32, C33, C34, C37, C38 HV_CAP0805, 1000 pF 709-1288-1-ND 1000pF 10%, 1kV, Ceramic Capacitor X7R, 0805
16 4 H1, H2, H3, H4 MTHOLE#4 ----------- Mounting hole in layout
17 1 R13 RES0201, 1 MΩ, 1%, 1/20W P1.00MABCT-ND 1% 0.05W, 1/20W Thick Film Resistor
18 1 R15 RES0201, 3 kΩ, 1%, 1/20W P3.00KABCT-ND 1% 0.05W, 1/20W Thick Film Resistor
19 1 R14 RES0201, 475 Ω, 1%, 1/20W P475ABCT-ND 1% 0.05W, 1/20W Thick Film Resistor
20 1 R18 RES0402, 49.9Ω, 1%, 1/10W P49.9LCT-ND 1% 0.1W, 1/10W Thick Film Resistor
21 1 R17 RES1206, 10 kΩ, 1%, 1/4W P10.0KFCT-ND 1% 0.25W, 1/4W Thick Film Resistor
22 1 R16 RES1206, 63.4 Ω, 1%, 1/4W P63.4FCT-ND 1% 0.25W, 1/4W Thick Film Resistor
23 4 J1, J2, J3, J7 SMA_RT_ANGLE_BULKHEAD WM9352-ND Through Hole, Right Angle, SMA Female Connector Jack, 50 Ohm

Considerations

  • A low and high voltage R/C filter are integrated on the board. The series resistor of the HV filter is normally set to 10 kOhm but when working with irradiated sensors it is advisable to be replace with a 10 MOhm resistor to limit sensor leakage current. This is imperative in the case of highly irradiated devices since the reaction time of the compliance function of the high voltage power supplies is insufficient to prevent permanent damage to the sensor die when reaching breakdown. The change refers to R17 in the schematics and should e replaced by a 1206 size SMD 1% accuracy resistor.

  • The feedback resistor is normally set to 470 Ohm and is adjusting the signal to noise ratio. A higher value would degrade the rise time but increase significantly the signal amplitude, thous improving the signal to noise ratio. Therefor, for irradiated sensors, a 2kOhm feedback resistor value might be more preferable.

  • The input impedance of the amplifier is related to the feedback resistor value and a simulation is presented in the following plot. Although at a first approximation this value can be considered stable with time and frequency of the signal, this is not the case and a Fourier transform is required for more accurate measurements.


    Transimpedence.bmp

  • The amplifier response can be verified by injecting a calibration pulse though the calibration connector. The input is AC coupled to the transistor through a 0.3 pF capacitor. An appropriate calibration signal should be a negative pulse in the order of 5 mV with a fast rise time (~300psec) to be able to go through the input capacitor. A gain of 10 should be observed at the output with an inversed polarity signal.

  • A completely hermetic shielding is required for noise reduction. The aluminum lids in both sides need to be pierced with a 12mm diameter hole in order to allow for β particles to traverse. The hole needs to be covered with aluminum foil (< 50μm thick) that has to be soldered in its periphery to the metal lid.

-- VagelisG - 2017-03-28

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