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The GBTIA, a 5 Gbit/s radiation-hard optical receiver for the SLHC upgrades

This article presents the design and specifications of GBTIA, a regenerative optical receiver for data transmission up to 5 Gbit/s. The receiver architecture includes a transimpedance amplifier and a limiting amplifier. The chip is radiation-tolerant and operates in a wide temperature range.

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The GBTIA, a 5 Gbit/s radiation-hard optical receiver for the SLHC upgrades

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  1. The GBTIA, a 5 Gbit/s radiation-hard optical receiver for the SLHC upgrades Mohsine Menouni, CPPM - Marseille Gui Ping, SMU - Dallas - Texas Paulo Moreira, CERN - Genève

  2. Outline Introduction Specifications of the GBTIA chip Receiver architecture Transimpedance design Limiting amplifier design Pin diode bias and leakage current effect Measurement results Conclusion and Perspectives menouni@cppm.in2p3.fr

  3. GBTIA Specifications • The GBTIA is a regenerative optical receiver for data transmission up to 5 Gbit/s • It provides the proper signal level for the clock recovery and deserializer stages • Main specifications: • Bit rate : 5 Gbit/s (min) • Total jitter : < 40 ps p-p • Sensitivity: 20 μA p-p (-17 dBm) for BER = 10-12 • pin diode capacitance Cd ~ 400 fF • Dark current : 0 to 1 mA • Power supply : 2.5 V ± 10% • Power consumption < 250 mW • Large range of temperature : From -20 C to 80 C • Die size: 0.75 mm × 1.25 mm • Radiation tolerant (up to 200 Mrad) menouni@cppm.in2p3.fr

  4. Overview of the GBTIA design • Integrating the TIA with the LA in the same chip presents the risk to degrade performances • Propagation of the crosstalk noise trough power supplies or the substrate • A fully differential architecture • Better tolerance to the supply noise • However the input referred noise (thermal noise) is larger than for the single ended • The power consumption is higher • The photodiode is AC coupled to the TIA • No Need of an offset control circuit at the output of the TIA • A high value of the low cut off frequency • Parasitic of the coupling integrated capacitances limits the bandwidth • Transimpedance amplifier (TIA) • Define the sensitivity of the optical receiver • Wide bandwidth • Low noise • Limiting Amplifier and output buffer (LA) • Provide a clean signal to the output • High gain • Wide bandwidth • Offset level compensation • Additional features : • Internal voltage regulator (with enable/disable control) • leakage-current indicator • Carrier-detect and signal-strength indicators • Squelch function (with enable/disable control) menouni@cppm.in2p3.fr

  5. RF IIN VOUT AV CPAR Transimpedance Amplifier • Shunt feedback amplifier is widely used for high speed receiver designs • To increase the bandwidth : • Decrease the feedback resistor • Increase the amplifier open loop gain • Decrease the input node capacitance • To minimize the thermal noise : • Increase the feedback resistor • Decrease the input node capacitance • Increase the amplifier transconductance CPH menouni@cppm.in2p3.fr

  6. Bandwidth extension technique • In order to maintain a low level of noise with keeping a large bandwidth, the shunt peaking technique is used in the design • Shunt peaking • Introduction of an inductance in series with the load resistance • Enhances the bandwidth • The frequency response is characterized by the ratio m menouni@cppm.in2p3.fr

  7. Implemented TIA structure • Differential structure is adopted • Inductive peaking • The target bandwidth of 3.5 GHz is achieved for the worst case of process and temperature (simulations including parasitic) • High transimpedance gain (RF=380 W) • Low level of input referred noise • Cascode • Reduces the Miller effect • Current density is optimized • High current density needed to achieve high cut off frequency for the input transistor • Input transistor size optimized for an input capacitance of 700 fF • 2 V supply required 2 V 2 nH 2 nH 200 W 200 W Out- Out+ 380 W 380 W 7.8 pF 7.8 pF In- In+ menouni@cppm.in2p3.fr

  8. Offset cancellation Limiting Amplifier LA1 LA2 LA3 LA4 Buffer TIA 2 mA 2 mA 4 mA 8 mA 8 mA Limiting Amplifier requirements • Considering the sensitivity and the gain of the TIA : • the photocurrent is converted to a minimum voltage of 12 mV pp • This voltage is amplified by the Limiting Amplifier to reach the proper voltage necessary for the following stages • The design of the LA demonstrates a high gain to achieve the 400 mV pp • Gain is around 40 dB in typical condition (28 dB in worst-case scenario) • The minimum overall bandwidth is 3.5 GHz • The noise contribution of this stage is maintained negligible : • The input referred noise is maintained lower than 850 µV RMS (12mV/14) for a BER of 10E-12 • The input capacitance of the LA is sufficiently low so that it does not reduce the TIA bandwidth • The number of stages is set to 5 (4 LA + a buffer) • More stages introduce a high power dissipation • Offset cancellation is incorporated in LA block to prevent the mismatch in the differential amp from saturating the latter stages • In order to maintain a wide bandwidth while delivering large current to the load, the amplifiers stages in the LA are designed to have increasingly larger size and current • Minimize the load capacitance seen by the previous stage • Allow bandwidth extension • The gain of the first stage (LA1) is set to a high value to reduce the noise menouni@cppm.in2p3.fr

  9. Limiting Amplifier stage • High bandwidth topology for each stage • Cherry and Hopper structure • gm stage followed by shunt-feedback stage • Second stage uses active “inductors” • by active inductive peaking, the bandwidth is increased by 34% over a resistive loaded topology. menouni@cppm.in2p3.fr

  10. Output Buffer Stage • Needs to be able to deliver 4 mA current to a 50 W load at full speed • The output stage needs to be able to fully switch 8 mA taking into consideration double termination. • The buffer has not to present too large capacitance to the preceding stage Off chip 50 W transmission line Voutp Voutn Vinn 50 W Vinp 8 mA menouni@cppm.in2p3.fr

  11. Pin diode leakage current effect • The pin diode leakage current increases with the radiation dose and can reach a value of 1 mA for a high dose level • AC coupling is adopted for the fully differential receiver: • The AC coupling capacitance is integrated in the chip • The value is made as high as possible : 7.8 pF • In order to maintain the low cutoff frequency to a reasonable value we need a high value for the photodiode bias resistance • Since we have to maintain a voltage across the photodiode, this resistance is implemented with active device • Photodiode biasing • The voltage across the pin diode decreases to 0.6 V for Vdd = 2 V • The low cut-off frequency increases • The simulated cut off frequency is around 1 MHz for IDC = 1 mA • Still compatible with the GBT encoding • The DC level has an effect on the noise and the sensitivity • For the low level of the leakage current, the shot noise is negligible comparing to the thermal noise • When the DC level is around 1 mA, the shot noise level becomes comparable to the thermal noise • A sensitivity degradation is expected at the end of life of the SLHC • Simulations show a sensitivity loss of 3-4 dB Vdd iAC IDC CC IDC CC iAC menouni@cppm.in2p3.fr Long consecutive identical bits

  12. Outline • Introduction • Receiver architecture • measurement results • Chip photograph and test boards • Eye diagram measurements • Bit Error Rate estimate • Performances versus power supply • BER measurements with the GBT protocol and error correction • Radiation effects • Influence of the optical DC level on the BER • Summary and Perspectives Thanks to : Luis Amaral, Jan Troska and Csaba Soos for their help with the test setup menouni@cppm.in2p3.fr

  13. Chip photograph and test boards • 0.13-μm bulk CMOS process. • IBM CMOS8RF-LM technology, a standard eight-metal-layer • The n-MOSFET fT of this technology is in the range of100–120 GHz • Die size: 0.75 mm × 1.25 mm • 2 PCB boards were designed in order to evaluate the GBTIA performances • Board for optical tests • Use a pin-diode as a signal source • PDCS60T-XS : high speed photodiode Pin diode from Enablence • Top illuminated 10 Gb/s photodiode • Low capacitance: 240 fF • Responsitivity : 0.9 A/W at  = 1310 nm • The connection between the TIA and the pin diode is made very short < 200 µm • Board for electrical tests • PIN diode is replaced by an electrical network • Voltage source to adjust the input current • Input Capacitance is set to 500 fF • PCB parasitic capacitances were minimized • This board was used essentially for irradiation test output- Bandgap reference Input+ AC coupling capacitance TIA LA Pin diode biasing Input- output+ menouni@cppm.in2p3.fr

  14. Eye diagram measurements (1/2) • 12 Gbit/s Serial Pattern Generator • Commercial 10 Gbit/s Optical transmitter • 18 GHz BW serial data analyzer • Optical attenuator to adjust the optical level for the pin diode PRBS Generator Agilent N4903A clock data datab Optical Tx Optical Attenuator DC blocking connectors ckin Lecroy SDA18000 GBTIA Test Board outp ch1 ch2 outn menouni@cppm.in2p3.fr

  15. Eye diagram measurements (2/2) • Measured differential eye diagrams at 5 Gbit/s for different optical power at the input (-6 dBm and -18 dBm) • Well opened eye diagram for -6 dBm and still correct at 18 dBm • The test PRBS sequence length is 27-1 • A constant output swing of 400 mV • For a power supply of 2 V the power consumption is 90 mW (120 mW at 2.5 V) • For -6 dBm input : • Rise time = 30 ps • Total jitter = 0.15 UI @ BER = 10-12 • UI = 200 ps • For -18 dBm input : • Rise time = 60 ps • Total jitter = 0.55 UI @ BER = 10-12 Eye diagram 5 Gbit/s optical power = -6 dBm Eye diagram 5 Gbit/s optical power = -18 dBm menouni@cppm.in2p3.fr

  16. Bit Error Rate Tester (BERT) Xilinx ML421platform Clock generator ck ckb ckin ckinb dout doutb din dinb Optical Tx Optical Attenuator ckin Lecroy SDA18000 ch1 GBTIA Test Board ch2 DC blocking connectors Bit Error Rate measurements • Test set up to evaluate the BER of the optical link using the GBTIA chip as a receiver • The Bit Error Rate Tester (BERT) is the Xilinx platform ML421 • Virtex-4 Rocket-IO FPGA • Transceivers operating up to 6.5 Gbit/s • BER calculation is based on the comparison of the transmitted and the received data • Measuring a very low BER is time consuming • Low BER is determined using extrapolation from the measurements of BER versus the input optical power menouni@cppm.in2p3.fr

  17. Bit Error Rate Estimate • Vdd = 2 V and T = 25 °C • Data pattern : PRBS7 • The sensitivity for a BER of 10-12 is estimated around -19 dBm menouni@cppm.in2p3.fr

  18. Performances versus power supply menouni@cppm.in2p3.fr

  19. BER measurements with the GBT encoding • The SEU on the photodiode are likely to be the main source of errors • In the GBT chip an error correction system is implemented • Reed-Solomon error-correcting encoder/decoder • For this test set up, the GBT encoder decoder was implemented in virtex-4 FPGA used in the BERT platform • Without error correction, the sensitivity of the optical receiver still around -19 dBm • The sensitivity is improved by 2 dB if the correction encoder is enabled menouni@cppm.in2p3.fr

  20. Electrical board used for irradiation test Irradiation test done at CERN using Xray facility Only the GBTIA chip is submitted to Xray beam No degradation is observed after a dose rate of 200 Mrad Eye diagram versus the total dose Prerad eye diagram (input=500 mV ) 200 Mrad eye diagram (input=500 mV ) Prerad eye diagram (input=50 mV ) 200 Mrad eye diagram (input=50 mV ) menouni@cppm.in2p3.fr

  21. BER versus the total dose menouni@cppm.in2p3.fr

  22. Influence of the optical DC level on the BER • the leakage current of the photodiode increases to 1 mA at a high level of dose • In order to measure the influence of this leakage current, the pin diode is illuminated by an additional DC laser source • In this case we checked that the integrated bias circuit ensures a sufficient voltage across the pin diode • We don’t observe a notable degradation of the BER coming from the effect of the low cut-off frequency • The value of this frequency still compatible with the GBT encoding data • The power penalty introduced by the shot noise of the leakage current is around 4 dB Power penalty menouni@cppm.in2p3.fr

  23. Conclusion and Perspectives • Main Specifications in terms of bandwidth and sensitivity are respected • Eye diagram is well opened at 5 Gbit/s • Sensitivity of -19 dBm for a BER of 10-12 • The effect of the leakage current is estimated • The sensitivity is degraded by 4 dB • The value of the low cut off frequency still compatible with the data encoding used for the GBT • Radiation effects : • Radiation tolerance is proven up to 200 Mrad • We have to estimate the single event upset tolerance • Work has started to encapsulate the GBTIA and the photodiode in a TO Package • A final design is scheduled to implement the additional features : • Leakage-current and signal-strength indicators • Carrier-detect • Squelch function (with enable/disable control) menouni@cppm.in2p3.fr

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