ATLAS SCT powering issues

1. ATLAS SCT powering issues Nearly 2000 more cables needed in the final assembly ATLAS SCT Barrel 3 at CERN (192 cables visible) Material in radiation length • Total power is 50 kW of which 50% are dissipated in cables • Material in radiation length is dominated by power supply and cooling services • SLHC tracker will require even more channels and thus more cables, more cooling and more material Innovative powering system design is needed

2. Conventional scheme: Independent Powering N modules are powered independently by Nconstant voltage power supplies • Define efficiency η = PM/(PM + Pc) • η = 1/(1 + IMRC/VM) = 1/(1+x) • x = IM RC/Vm= voltage drop in cable/ module voltage • η decreases with increasing IM and RC and with decreasing Vm Im Im Im Pc = nIm2 Rc* PM = n ImVm • For ATLAS SCT: R = 3.5 Ω, V = 4 V, I = 1.3 A => x ≈ 1.14 • Power efficiencyη≈ 50%

3. Proposed scheme: Serial Powering N modules are powered in series by one constant current source; local regulators provide supply voltage to the modules Im Module 1 • η = 1/(1 + IR/nV) = 1/(1+x/n) • Efficiency increases if number of modules n increases Power cable I Supply Module 2 • Concept never practically implemented Power cable Module n Im Pc = Im2Rc* PM = nImVm • For ATLAS SCT: R = 3.5 Ω, V = 4 V, I = 1.3 A N = 10 => x ≈ 1.14 • Power efficiencyη≈ 90%

4. Advantages of serial powering • Much less power cables • Much less material (less cables, less cooling) • Improved power efficiency • Significant cost savings

5. Module 1 Module 2 Module n Much less cables • for a detector with N modules with local regulators, the number of cables is reduced by a factor of up to 2N (analogue and digital power no longer separated) • Reduction of detector material in the tracking volume: less multiple scattering and creation of secondary particles, leading to improved track finding efficiency and resolution • Cable volume reduction is mandatory for an SLHC tracker, where increased luminosity would require an increased detectors granularity by a factor of 5 to 10. It is even challenging to squeeze the current number of cables in the available space

6. Improved power efficiency • Future readout chips require reduced operation voltage (due to reduced feature size) x increases X = 4.5 *SLHC* • Overall efficiency increases with increasing number of modules N X = 1.14 *SCT* • For a future SLHC detector x ≈ 4.5: • Independent powering η≈ 18% • Serial powering (n = 10) η ≈ 69% • Serial powering (n = 20) η ≈ 81% Efficiency of serial powering normalized to independent powering vs. number of modules n for various x factors • Reduction of load to cooling system by tens of kW inside the tracker volume are possible

7. Cost savings • Reduced number of cables and remote power supplies results in major cost savings; electricity bill is reduced as well. • Take ATLAS SCT as an example: 4088 power supply modules cost ≈ 1.5 MCHF; Cabling cost ≈ 2 MCHF • For an SLHC tracker with independent powering, the power supplies and cables would cost tens of MCHF; a serial powering approach would reduce this by a large factor, implying a saving of many MCHF

8. Miles stones of serial powering R&D • Tests with ATLAS SCT modules (well advanced and promising) • Grounding and interference issues in a realistic densely-packed detector system (first implementation and test in July 2006) • Development of a redundancy and failure protection scheme • Serial Powering circuitry integration into ABC_Next chip Noise occupancy with 1 fC discriminator threshold for standard (left) and serial (right) power scheme. No added noise introduced by the alternative scheme is seen.

9. Step 1: Test with ATLAS SCT modules • Detailed set of reference measurements with up to 6 modules • Measure power saving and compare with predicted values • Noise spectrum study: introduce high frequency noise • Deadtime-less operation Noise performance with 4 SCT modules in series are very satisfactory. See talk M. Weber at LECC 2006. Have meanwhile rebuilt and streamlined hardware. Tests with 6 SCT modules will start in June 2006 Current source SP1 SCT1 SP2 SCT2 Photograph of test setup with 4 ATLAS SCT modules, serial powering scheme implemented on PCB. SP3 SCT3 SP4 SCT4 Noise occupancy with 1 fC discriminator threshold for standard (left) and serial (right) power scheme. No added noise introduced by the alternative scheme is seen.

10. Step 2: Grounding and interference in a densely- packed detector system • Tests with independent modules are sensitive to “pick-up” through the serial power line (conductive interference) • In an integrated detector arrangement, there are additional pick-up mechanisms e.g. capacitive and inductiveinterference between nearby components (bus cable/hybrids/sensors) • This will be investigated, understood and eliminated using a CDF Run IIb type stave built by Carl Haber at LBNL (first tests are scheduled for July 2006) • This stave is a most compact package and thus the ultimate test bed • Its electrical performance and interference mechanisms are well-understood and documented M. Weber et. al., NIM A556 (2006) 459-481 and R. Ely, M. Weber et al., IEEE Trans. Nucl. Sci NS-52 (5) (2005) in press. CDF Run IIb stave

11. Stave noise tests will be performed with bare-die commercial regulators • Final implementation requires radiation-hard ASICs • Noise and redundancy studies will, however, lead to regulator specifications for a dedicated ASIC or a silicon strip readout chip (RDIC) • output impedance of regulators, max. current • PSRR of RDIC • current sensing features of RDIC/regulators • controlled short • voltage adjustment features • Design of an RDIC with serial powering features is discussed with CERN MIC group in the context of the proposed ABC-Next chip, a 0.25 μm CMOS RDIC Step 4: Serial Powering circuitry integration

12. Appendix - connection diagram - • Serial Powering reduces the number of power cables by up to 2n, instead of n, when analogue module power is obtained from digital power. • The final number of modules n will depend on several factors e.g. maximum allowed voltage, failure probability, readout architecture and mechanical considerations. • The rapidly shrinking feature size in microelectronics, implies a decrease in x; We thus expect the number of modules powered in series to be higher than 10. The maximum voltage difference depends on the voltage required by each module. The latter is expected to be of the order of 1.2 – 2.5V maximum

13. Appendix - TX/RX diagram - • Modules are referenced to different “ground” levels than DAQ • Modules have to send data signals to DAQ and receive clock and command signals from DAQ • This is achieved by AC-coupling of LVDS signals Figure A1: Simplified TX/RX connection diagram. The connections are differential. Termination and feed-back resistors are omitted for clarity.

14. Appendix - Power consumption in power cables- Figure A5: Consumption in power cables • one-way cable length from power supply to detector: up to 160 m • cable resistance (including return): 3.5 Ω; ~1.5 Ω in active volume

15. Service gap Cables Barrel Discs Interaction point Appendix - Material overhead - Figure A6: Generic tracker layout with barrel and discs Figure A7: Material in radiation length • in ATLAS SCT, particles cross (0.1 to 0.45%) x √2 of R.L. of cables in service gap alone (dep. on polar angle) • a ten-fold increase of cables is prohibitive • Reduction of detector material in the tracking volume: less multiple scattering and creation of secondary particles, leading to improved tracking efficiency and resolution